A Rapid and Sensitive Test for Diagnosing Heparin-Associated Thrombocytopenia

Laboratory approach for vaccine-induced thrombotic thrombocytopenia diagnosis in the Netherlands

BACKGROUND AND OBJECTIVES

Vaccine-induced thrombotic thrombocytopenia (VITT) is a rare adverse effect characterized by thrombocytopenia and thrombosis occurring after COVID-19 vaccination. VITT pathophysiology is not fully unravelled but shows similarities to heparin-induced thrombocytopenia (HIT). HIT is characterized by the presence of antibodies against platelet factor 4 (PF4)/heparin complex, which can activate platelets in an FcγRIIa-dependent manner, whereas IgG-antibodies directed against PF4 play an important role in VITT.

MATERIALS AND METHODS

We characterized all clinically suspected VITT cases in the Netherlands from a diagnostic perspective and hypothesized that patients who developed both thrombocytopenia and thrombosis display underlying mechanisms similar to those in HIT. We conducted an anti-PF4 ELISA and a functional PF4-induced platelet activation assay (PIPAA) with and without blocking the platelet-FcγRIIa and found positivity in both tests, suggesting VITT with mechanisms similar to those in VITT.

RESULTS

We identified 65 patients with both thrombocytopenia and thrombosis among 275 clinically suspected VITT cases. Of these 65 patients, 14 (22%) tested positive for anti-PF4 and PF4-dependent platelet activation. The essential role of platelet-FcγRIIa in VITT with mechanisms similar to those in HIT was evident, as platelet activation was inhibited by an FcγRIIa-blocking antibody in all 14 patients.

CONCLUSION

Our study shows that only a small proportion of clinically suspected VITT patients with thrombocytopenia and thrombosis have anti-PF4-inducing, FcɣRIIa-dependent platelet activation, suggesting an HIT-like pathophysiology. This leaves the possibility for the presence of another type of pathophysiology ('non-HIT like') leading to VITT. More research on pathophysiology is warranted to improve the diagnostic algorithm and to identify novel therapeutic and preventive strategies.

Diagnostic value of antibody-induced procoagulant platelets in heparin-induced thrombocytopenia: communication from the ISTH SSC Subcommittee on Platelet Immunology

Heparin-induced thrombocytopenia (HIT) is an immune-mediated prothrombotic disorder characterized by a drop in platelet count and an increased risk of thromboembolic events. The accurate diagnosis of HIT involves clinical assessment and laboratory testing with well-characterized functional tests. Recent research has shown the potential of investigating procoagulant platelet formation induced by HIT antibodies. To successfully implement these assays in clinical laboratories, careful consideration of technical and preanalytical factors is crucial. In this communication from the SSC Platelet Immunology, we provide a consensus from experts on the use of flow cytometry in HIT diagnosis, highlighting the importance of standardized protocols.

Hematology in the post-COVID era: spotlight on vaccine-induced immune thrombotic thrombocytopenia and a conceptual framework (the 4P's) for anti-PF4 diseases

INTRODUCTION

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a life-threatening prothrombotic disorder first identified following the introduction of adenoviral vector vaccines for COVID-19. The condition is characterized by anti-PF4 antibodies and clinically presents with thrombocytopenia and thrombosis often in unusual anatomical sites.

AREAS COVERED

In this review, we discuss the clinical presentation, diagnostic testing, and treatment of VITT. We also review VITT-like syndromes that have been described in patients without previous vaccination. We propose a conceptual framework for the mechanism of anti-PF4 diseases that includes sufficiently high levels of PF4, the presence of a Polyanion that can form immune complexes with PF4, a Pro-inflammatory milieu, and an immunological Predisposition - the 4Ps.

EXPERT OPINION

Significant progress has been made in understanding the characteristics of the VITT antibody and in testing methods that can confirm that diagnosis. Future work should be directed at understanding long-term outcomes, mechanisms of thrombosis, and individual risk factors for this rare but dangerous immune-thrombotic disease.

Anti-PF4 immunothrombosis without proximate heparin or adenovirus vector vaccine exposure

ABSTRACT

Platelet-activating anti-platelet factor 4 (PF4)/heparin antibodies and anti-PF4 antibodies cause heparin-induced thrombocytopenia (HIT) and vaccine-induced immune thrombocytopenia and thrombosis (VITT), respectively. Diagnostic and treatment considerations differ somewhat between HIT and VITT. We identified patients with thrombocytopenia and thrombosis without proximate heparin exposure or adenovirus-based vaccination who tested strongly positive by PF4/polyanion enzyme-immunoassays and negative/weakly positive by heparin-induced platelet activation (HIPA) test but strongly positive by PF4-induced platelet activation (PIPA) test (ie, VITT-like profile). We tested these patients by a standard chemiluminescence assay that detects anti-PF4/heparin antibodies found in HIT (HemosIL AcuStar HIT-IgG(PF4-H)) as well as a novel chemiluminescence assay for anti-PF4 antibodies found in VITT. Representative control sera included an exploratory anti-PF4 antibody-positive but HIPA-negative/weak cohort obtained before 2020 (n = 188). We identified 9 patients with a clinical-pathological profile of a VITT-like disorder in the absence of proximate heparin or vaccination, with a high frequency of stroke (arterial, n = 3; cerebral venous sinus thrombosis, n = 4), thrombocytopenia (median platelet count nadir, 49 × 109/L), and hypercoagulability (greatly elevated D-dimer levels). VITT-like serological features included strong reactivity by PIPA (aggregation <10 minutes in 9/9 sera) and positive testing in the novel anti-PF4 chemiluminescence assay (3/9 also tested positive in the anti-PF4/heparin chemiluminescence assay). Our exploratory cohort identified 13 additional patient sera obtained before 2020 with VITT-like anti-PF4 antibodies. Platelet-activating VITT-like anti-PF4 antibodies should be considered in patients with thrombocytopenia, thrombosis, and very high D-dimer levels, even without a proximate exposure to heparin or adenovirus vector vaccines.

Transient Autoreactive PF4 and Antiphospholipid Antibodies in COVID-19 Vaccine Recipients

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare autoimmune condition associated with recombinant adenovirus (rAV)-based COVID-19 vaccines. It is thought to arise from autoantibodies targeting platelet factor 4 (aPF4), triggered by vaccine-induced inflammation and the formation of neo-antigenic complexes between PF4 and the rAV vector. To investigate the specific induction of aPF4 by rAV-based vaccines, we examined sera from rAV vaccine recipients (AZD1222, AD26.COV2.S) and messenger RNA (mRNA) based (mRNA-1273, BNT162b2) COVID-19 vaccine recipients. We compared the antibody fold change (FC) for aPF4 and for antiphospholipid antibodies (aPL) of rAV to mRNA vaccine recipients. We combined two biobanks of Dutch healthcare workers and matched rAV-vaccinated individuals to mRNA-vaccinated controls, based on age, sex and prior history of COVID-19 (AZD1222: 37, Ad26.COV2.S: 35, mRNA-1273: 47, BNT162b2: 26). We found no significant differences in aPF4 FCs after the first (0.99 vs. 1.08, mean difference (MD) = -0.11 (95% CI -0.23 to 0.057)) and second doses of AZD1222 (0.99 vs. 1.10, MD = -0.11 (95% CI -0.31 to 0.10)) and after a single dose of Ad26.COV2.S compared to mRNA-based vaccines (1.01 vs. 0.99, MD = 0.026 (95% CI -0.13 to 0.18)). The mean FCs for the aPL in rAV-based vaccine recipients were similar to those in mRNA-based vaccines. No correlation was observed between post-vaccination aPF4 levels and vaccine type (mean aPF difference -0.070 (95% CI -0.14 to 0.002) mRNA vs. rAV). In summary, our study indicates that rAV and mRNA-based COVID-19 vaccines do not substantially elevate aPF4 levels in healthy individuals.

Autoimmune Heparin-Induced Thrombocytopenia

Autoimmune thrombocytopenia (aHIT) is a severe subtype of heparin-induced thrombocytopenia (HIT) with atypical clinical features caused by highly pathological IgG antibodies ("aHIT antibodies") that activate platelets even in the absence of heparin. The clinical features of aHIT include: the onset or worsening of thrombocytopenia despite stopping heparin ("delayed-onset HIT"), thrombocytopenia persistence despite stopping heparin ("persisting" or "refractory HIT"), or triggered by small amounts of heparin (heparin "flush" HIT), most cases of fondaparinux-induced HIT, and patients with unusually severe HIT (e.g., multi-site or microvascular thrombosis, overt disseminated intravascular coagulation [DIC]). Special treatment approaches are required. For example, unlike classic HIT, heparin cessation does not result in de-escalation of antibody-induced hemostasis activation, and thus high-dose intravenous immunoglobulin (IVIG) may be indicated to interrupt aHIT-induced platelet activation; therapeutic plasma exchange may be required if high-dose IVIG is ineffective. Also, aHIT patients are at risk for treatment failure with (activated partial thromboplastin time [APTT]-adjusted) direct thrombin inhibitor (DTI) therapy (argatroban, bivalirudin), either because of APTT confounding (where aHIT-associated DIC and resulting APTT prolongation lead to systematic underdosing/interruption of DTI therapy) or because DTI inhibits thrombin-induced protein C activation. Most HIT laboratories do not test for aHIT antibodies, contributing to aHIT under-recognition.

Platelet phosphatidylserine is the critical mediator of thrombosis in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a severe immune-mediated prothrombotic disorder caused by antibodies (Ab) reactive to complexes of platelet factor 4 and heparin. Platelets (PLT) and their interaction with different immune cells contribute to prothrombotic conditions in HIT. However, the exact mechanisms and the role of different PLT subpopulations in this prothrombotic environment remain poorly understood. In this study, we observed that HIT patient Ab induce a new PLT population that is characterized by increased P-selectin expression and phosphatidylserine (PS) externalization. Formation of this procoagulant PLT subpopulation was dependent on engagement of PLT Fc-γ-RIIA by HIT Ab and resulted in a significant increase of thrombin generation on the PLT surface. Using an ex vivo thrombosis model and multi-parameter assessment of thrombus formation, we observed that HIT Ab-induced procoagulant PLT propagated formation of large PLT aggregates, leukocyte recruitment and most importantly, fibrin network generation. These prothrombotic conditions were prevented via the upregulation of PLT intracellular cAMP with Iloprost, a clinically approved prostacyclin analogue. Additionally, the functional relevance of P-selectin and PS was dissected. While inhibition of P-selectin did not affect thrombus formation, the specific blockade of PS prevented HIT Ab-mediated thrombin generation and most importantly procoagulant PLT-mediated thrombus formation ex vivo. Taken together, our findings indicate that procoagulant PLT are critical mediators of prothrombotic conditions in HIT. Specific PS targeting could be a promising therapeutic approach to prevent thromboembolic events in HIT patients.

Determination of Antibody Activity by Platelet Aggregation

Platelets play an important role in hemostasis by forming clots and stopping bleeding. In immune thrombotic conditions, platelets and leukocytes are aberrantly activated by pathogenic antibodies resulting in platelet aggregates and NETosis, leading to thrombosis and thrombocytopenia. A simple assay that assesses platelet function and antibody activity is light transmission aggregometry. This assay can be used to determine antibody activity in patients with disorders such as heparin-induced thrombocytopenia (HIT) and vaccine-induced thrombotic thrombocytopenia (VITT). Briefly, for detection of pathogenic antibody, platelet-rich plasma (PRP) is treated with a specific agent (e.g., patient sera or purified patient antibodies) with constant stirring. Upon activation, platelets undergo a shape change and adhere to each other forming aggregates. This causes a reduction in opacity allowing more light to pass through PRP. Light transmission through the cuvette is proportional to the degree of platelet aggregation and is measured by the photocell over time. The advantage of this protocol is that it is a simple, reliable assay that can be applied to assess antibody activity in thrombotic conditions. Light transmission aggregometry does not require the use of radioactive reagents and is technically less demanding compared with 14C-serotonin release assay, another common assay for detecting antibody activity. Key features • This protocol can be used to assess platelet function and to detect platelet activating antibodies in diseases such as HIT and VITT. • Does not require radioactive reagents, requires an aggregometer; based on the light transmission aggregometry protocol, adapted for detection of VITT and other platelet-activating antibodies. • Two positive controls are required for reliable detection of antibodies in diseases such as HIT/VITT, namely a weak HIT/VITT antibody and a physiological agonist. • For detection of HIT/VITT antibodies, it is essential to use donors known to have platelets reactive to these antibodies to avoid false negative results.

Laboratory Testing for Heparin-Induced Thrombocytopenia and Vaccine-Induced Immune Thrombotic Thrombocytopenia Antibodies: A Narrative Review

Heparin-induced thrombocytopenia (HIT) and vaccine-induced immune thrombotic thrombocytopenia (VITT) are highly prothrombotic (thrombosis frequency ≥50%). Both are caused by platelet-activating anti-platelet factor 4 (PF4) antibodies, forming PF4/IgG-containing immune complexes that engage platelet FcγIIa receptors, producing strong platelet activation. In HIT, heparin crosslinks several PF4 molecules, whereas in VITT, anti-PF4 antibodies alone crosslink PF4. Sufficient levels of circulating anti-PF4 antibodies are needed to create the pathogenic immune complexes on platelet surfaces; this explains why certain serum (plasma)-based assays are highly sensitive for detecting HIT/VITT antibodies. Accordingly, HIT and VITT are "clinical-pathological" disorders, that is, positive testing for such antibodies-together with a compatible clinical picture-is integral for diagnosis. Heparin (low concentrations) enhances HIT antibody-induced platelet activation, but platelet activation by VITT sera is usually inhibited by heparin. For both HIT and VITT, high sensitivity (>99% and >95%, respectively) characterizes PF4-dependent enzyme immunoassays (EIAs) and PF4-enhanced platelet activation assays; in contrast, certain rapid immunoassays have high sensitivity for HIT (>90-97%) but poor sensitivity (<25%) for VITT. HIT and VITT antibodies are directed at distinct sites on PF4: solid-phase EIAs and platelet activation assays are indifferent to these distinct antigen targets, but rapid immunoassays are not. We discuss a conceptual model where PF4 is viewed as a "globe," with the heparin-binding site the "equator"; in this model, HIT antibodies are primarily directed at antigen site(s) at the north and south "poles" of PF4 (formed when PF4 binds to heparin), whereas VITT antibodies recognize sites on the equator.

Heparin-induced thrombocytopenia: An illustrated review

Heparin-induced thrombocytopenia (HIT) is an immune-mediated adverse drug effect from unfractionated or low-molecular-weight heparin that results in thrombocytopenia and potentially catastrophic thrombosis. HIT occurs due to the development of platelet-activating antibodies against multimolecular complexes of platelet factor 4 and heparin. Given the frequency of thrombocytopenia and heparin use among hospitalized patients, calculation of the 4Ts Score is recommended to identify patients at increased likelihood of HIT and direct further evaluation. In patients with an intermediate or high probability 4Ts Score, an immunoassay and functional assay are recommended to confirm or refute the diagnosis of HIT. Heparin avoidance and initiation of nonheparin anticoagulation are the mainstays of acute HIT management. In this illustrated review, we provide visual summaries of the diagnosis and management of HIT, highlighting connections between pathophysiology and clinical care as well as summarizing efforts in quality improvement in the field. We further emphasize common pitfalls and pearls in diagnosis and management to encourage evidence-based care. We include graphical representation of the unique challenges of HIT with cardiopulmonary bypass and also delineate autoimmune HIT and its subtypes.

FcγRIIA-specific DARPins as novel tools in blood cell analysis and platelet aggregation

Fc receptors are involved in a variety of physiologically and disease relevant responses. Among them, FcγRIIA (CD32a) is known for its activating functions in pathogen recognition and platelet biology, and, as potential marker of T lymphocytes latently infected with HIV-1. The latter has not been without controversy due to technical challenges complicated by T-B cell conjugates and trogocytosis as well as a lack of antibodies distinguishing between the closely related isoforms of FcγRII. To generate high-affinity binders specific for FcγRIIA, libraries of designed ankyrin repeat proteins (DARPins) were screened for binding to its extracellular domains by ribosomal display. Counter-selection against FcγRIIB eliminated binders cross-reacting with both isoforms. The identified DARPins bound FcγRIIA with no detectable binding for FcγRIIB. Their affinities for FcγRIIA were in the low nanomolar range and could be enhanced by cleavage of the His-tag and dimerization. Interestingly, complex formation between DARPin and FcγRIIA followed a two-state reaction model and discrimination from FcγRIIB was based on a single amino acid residue. In flow cytometry, DARPin F11 detected FcγRIIA⁺ cells even when they made up less than 1% of the cell population. Image stream analysis of primary human blood cells confirmed that F11 caused dim but reliable cell surface staining of a small subpopulation of T lymphocytes. When incubated with platelets, F11 inhibited their aggregation equally efficient as antibodies unable to discriminate between both FcγRII isoforms. The selected DARPins are unique novel tools for platelet aggregation studies as well as the role of FcγRIIA for the latent HIV-1 reservoir.

How to Solve the Conundrum of Heparin-Induced Thrombocytopenia during Cardiopulmonary Bypass

Heparin-induced thrombocytopenia (HIT) is a major issue in cardiac surgery requiring cardiopulmonary bypass (CPB). HIT represents a severe adverse drug reaction after heparin administration. It consists of immune-mediated thrombocytopenia paradoxically leading to thrombotic events. Detection of antibodies against platelets factor 4/heparin (anti-PF4/H) and aggregation of platelets in the presence of heparin in functional in vitro tests confirm the diagnosis. Patients suffering from HIT and requiring cardiac surgery are at high risk of lethal complications and present specific challenges. Four distinct phases are described in the usual HIT timeline, and the anticoagulation strategy chosen for CPB depends on the phase in which the patient is categorized. In this sense, we developed an institutional protocol covering each phase. It consisted of the use of a non-heparin anticoagulant such as bivalirudin, or the association of unfractionated heparin (UFH) with a potent antiplatelet drug such as tirofiban or cangrelor. Temporary reduction of anti-PF4 with intravenous immunoglobulins (IvIg) has recently been described as a complementary strategy. In this article, we briefly described the pathophysiology of HIT and focused on the various strategies that can be applied to safely manage CPB in these patients.

Functional Testing for Heparin-Induced Thrombocytopenia Using Whole Blood Multiplate Impedance Aggregometry

Immune-mediated heparin-induced thrombocytopenia (HIT) occurs when heparin-dependent IgG antibodies bind to heparin/platelet factor 4 (H/PF4) complexes and activate platelets. There is a vast panoply of assays to investigate HIT which can be divided into two groups, antigen-based immunoassays that detect all antibodies against H/PF4 and are used as a first diagnostic step and functional assays that will identify only the antibodies capable of activating platelets and are mandatory to confirm a diagnosis of pathological HIT. The serotonin-release assay, known as SRA, has been the gold standard for decades, but in the last 10 years, other easier alternatives have been described. The current chapter will focus on whole blood multiple electrode aggregometry, a validated method for the functional diagnosis of HIT.

A machine-learning model for reducing misdiagnosis in heparin-induced thrombocytopenia: A prospective, multicenter, observational study

BACKGROUND

Diagnosing heparin-induced thrombocytopenia (HIT) at the bedside remains challenging, exposing a significant number of patients at risk of delayed diagnosis or overtreatment. We hypothesized that machine-learning algorithms could be utilized to develop a more accurate and user-friendly diagnostic tool that integrates diverse clinical and laboratory information and accounts for complex interactions.

METHODS

We conducted a prospective cohort study including 1393 patients with suspected HIT between 2018 and 2021 from 10 study centers. Detailed clinical information and laboratory data were collected, and various immunoassays were conducted. The washed platelet heparin-induced platelet activation assay (HIPA) served as the reference standard.

FINDINGS

HIPA diagnosed HIT in 119 patients (prevalence 8.5%). The feature selection process in the training dataset (75% of patients) yielded the following predictor variables: (1) immunoassay test result, (2) platelet nadir, (3) unfractionated heparin use, (4) CRP, (5) timing of thrombocytopenia, and (6) other causes of thrombocytopenia. The best performing models were a support vector machine in case of the chemiluminescent immunoassay (CLIA) and the ELISA, as well as a gradient boosting machine in particle-gel immunoassay (PaGIA). In the validation dataset (25% of patients), the AUROC of all models was 0.99 (95% CI: 0.97, 1.00). Compared to the currently recommended diagnostic algorithm (4Ts score, immunoassay), the numbers of false-negative patients were reduced from 12 to 6 (-50.0%; ELISA), 9 to 3 (-66.7%, PaGIA) and 14 to 5 (-64.3%; CLIA). The numbers of false-positive individuals were reduced from 87 to 61 (-29.8%; ELISA), 200 to 63 (-68.5%; PaGIA) and increased from 50 to 63 (+29.0%) for the CLIA.

INTERPRETATION

Our user-friendly machine-learning algorithm for the diagnosis of HIT (https://toradi-hit.org) was substantially more accurate than the currently recommended diagnostic algorithm. It has the potential to reduce delayed diagnosis and overtreatment in clinical practice. Future studies shall validate this model in wider settings.

FUNDING

Swiss National Science Foundation (SNSF), and International Society on Thrombosis and Haemostasis (ISTH).

Unique features of vaccine-induced immune thrombotic thrombocytopenia; a new anti-platelet factor 4 antibody-mediated disorder

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a highly prothrombotic disorder caused by anti-PF4 antibodies that activate platelets and neutrophils, leading to thrombosis. Heparin-induced thrombocytopenia (HIT) is a related anti-PF4 mediated disorder, with similar pathophysiology and clinical manifestations but different triggers (i.e., heparin vs adenoviral vector vaccine). Clinically, both HIT and VITT typically present with thrombocytopenia and thrombosis, although the risk of thrombosis is significantly higher in VITT, and the thromboses occur in unusual anatomical sites (e.g., cerebral venous sinus thrombosis and hepatic vein thrombosis). The diagnostic accuracy of available laboratory testing differs between HIT and VITT; for VITT, ELISAs have better specificity compared to HIT and platelet activation assays require the addition of PF4. Treatment of VITT and HIT is anticoagulation non-heparin anticoagulants; however, heparin may be considered for VITT if no other option is available.

Off-the-shelf cryopreserved platelets for the detection of HIT and VITT antibodies

Heparin-induced thrombocytopenia (HIT) is suspected much more often than it is confirmed. Technically simple platelet factor 4 (PF4)-polyanion enzyme-linked immunosorbent assays (ELISAs) are sensitive but nonspecific. In contrast, accurate functional tests such as the serotonin release assay, heparin-induced platelet activation assay, and PF4-dependent P-selectin expression assay require fresh platelets and have complex assay end points, limiting their availability to specialized reference laboratories. To enable broad deployment of functional testing, we sought to extend platelet viability significantly by optimizing storage conditions and developed a simple functional assay end point by measuring the release of a platelet α-granule protein, thrombospondin-1 (TSP1), in an ELISA format. Platelet cryopreservation conditions were optimized by freezing platelets at controlled cooling rates that preserve activatability. Several-month-old cryopreserved platelets were treated with PF4 or heparin and were evaluated for their ability to be activated by HIT and vaccine-induced immune thrombotic thrombocytopenia (VITT) antibodies in the TSP1 release assay (TRA). HIT and spontaneous HIT patient samples induced significantly higher TSP1 release using both PF4-treated (PF4-TRA) and heparin-treated cryopreserved platelets relative to samples from patients suspected of HIT who lacked platelet-activating antibodies. This latter group included several patients that tested strongly positive in PF4-polyanion ELISA but were not platelet-activating. Four VITT patient samples tested in the TRA activated PF4-treated, but not heparin-treated, cryopreserved platelets, consistent with recent data suggesting the requirement for PF4-treated platelets for VITT antibody detection. These findings have the potential to transform the testing paradigm in HIT and VITT, making decentralized, technically simple functional testing available for rapid and accurate in-hospital diagnosis.

Vaccine-induced immune thrombotic thrombocytopenia: Updates in pathobiology and diagnosis

Coronavirus disease 2019 (COVID-19) is a viral respiratory infection caused by the severe acute respiratory syndrome virus (SARS-CoV-2). Vaccines that protect against SARS-CoV-2 infection have been widely employed to reduce the incidence of symptomatic and severe disease. However, adenovirus-based SARS-CoV-2 vaccines can cause a rare, thrombotic disorder termed vaccine-induced immune thrombotic thrombocytopenia (VITT). VITT often develops in the first 5 to 30 days following vaccination and is characterized by thrombocytopenia and thrombosis in unusual locations (e.g., cerebral venous sinus thrombosis). The diagnosis is confirmed by testing for anti-PF4 antibodies, as these antibodies are capable of platelet activation without any cofactor. It can be clinically challenging to differentiate VITT from a similar disorder called heparin-induced thrombocytopenia (HIT), since heparin is commonly used in hospitalized patients. VITT and HIT have similar pathobiology and clinical manifestations but important differences in testing including the need for PF4-enhanced functional assays and the poor reliability of rapid immunoassays for the detection of anti-platelet factor 4 (PF4) antibodies. In this review we summarize the epidemiology of VITT; highlight similarities and differences between HIT and VITT; and provide an update on the clinical diagnosis of VITT.

Detection of Platelet-Activating Antibodies Associated with Vaccine-Induced Thrombotic Thrombocytopenia by Flow Cytometry: An Italian Experience

Rare cases of thrombocytopenia and thrombosis after anti-COVID-19 adenovirus-associated mRNA vaccines (VITT) due to platelet-activating anti-platelet-factor 4 (PF4)/polyanion antibodies have been reported. VITT laboratory diagnosis, similarly to heparin-induced thrombocytopenia (HIT) diagnosis, requires immunoassays for anti-PF4/polyanion antibodies identification, such as ELISA assays and platelet-activating functional tests, such as heparin-induced platelet activation test (HIPA), to confirm their pathogenicity. We compared the flow cytometry (FC) measurement of platelet p-selectin exposure to the gold standard functional test HIPA for diagnosis confirmation in 13 patients with a clinical VITT syndrome (6M/7F; median age 56 (33–78)) who resulted positive to anti-PF4/polyanion antibodies ELISA assays (12/13). FC and HIPA similarly identified three different patterns: (1) a typical non-heparin-dependent VITT pattern (seven and six patients by FC and HIPA, respectively); (2) low/no platelet activation in patients under IvIg therapy (five out of five and two out of four patients by FC and HIPA, respectively); (3) a HIT pattern. Antibodies investigated by FC became negative after 7, 17, and 24 days of therapy in three patients. FC measurement of P-selectin exposure was as sensitive as HIPA but simpler to detect anti-PF4/polyanion antibodies in VITT patients. FC could reliably discriminate VITT from HIT, thus helping for the choice of the anticoagulant.

Antibody-mediated procoagulant platelet formation in COVID-19 is AKT dependent

BACKGROUND

Thromboembolic events are frequently reported in patients infected with the SARS-CoV-2. Recently, we observed that platelets from patients with severe COVID-19 infection express procoagulant phenotype. The molecular mechanisms that induce the generation of procoagulant platelets in COVID-19 patients are not completely understood.

OBJECTIVES

In this study, we investigated the role of AKT (also known as Protein Kinase B), which is the major downstream effector of PI3K (phosphoinositid-3-kinase) (PI3K/AKT) signaling pathway in platelets from patients with COVID-19.

PATIENTS AND METHODS

Platelets, Sera and IgG from COVID-19 patients who were admitted to the intensive care unit (ICU) were analyzed by flow cytometry as well as western blot and adhesion assays.

RESULTS

Platelets from COVID-19 patients showed significantly higher levels of phosphorylated AKT, which was correlated with CD62p expression and phosphatidylserine (PS) externalization. In addition, healthy platelets incubated with sera or IgGs from ICU COVID-19 patients induced phosphorylation of PI3K and AKT and were dependent on Fc-gamma-RIIA (FcγRIIA). In contrast, ICU COVID-19 sera mediated generation of procoagulant platelets was not dependent on GPIIb/IIIa. Interestingly, the inhibition of phosphorylation of both proteins AKT and PI3K prevented the generation of procoagulant platelets.

CONCLUSIONS

Our study shows that pAKT/AKT signaling pathway is associated with the formation of procoagulant platelets in severe COVID-19 patients without integrin GPIIb/IIIa engagement. The inhibition of PI3K/AKT phosphorylation might represent a promising strategy to reduce the risk for thrombosis in patients with severe COVID-19.

Recent Advances in Anticoagulant Treatment of Immune Thrombosis: A Focus on Direct Oral Anticoagulants in Heparin-Induced Thrombocytopenia and Anti-Phospholipid Syndrome

For more than 10 years, direct oral anticoagulants (DOACs) have been increasingly prescribed for the prevention and treatment of thrombotic events. However, their use in immunothrombotic disorders, namely heparin-induced thrombocytopenia (HIT) and antiphospholipid syndrome (APS), is still under investigation. The prothrombotic state resulting from the autoimmune mechanism, multicellular activation, and platelet count decrease, constitutes similarities between HIT and APS. Moreover, they both share the complexity of the biological diagnosis. Current treatment of HIT firstly relies on parenteral non-heparin therapies, but DOACs have been included in American and French guidelines for a few years, providing the advantage of limiting the need for treatment monitoring. In APS, vitamin K antagonists are conversely the main treatment (+/− anti-platelet agents), and the use of DOACs is either subject to precautionary recommendations or is not recommended in severe APS. While some randomized controlled trials have been conducted regarding the use of DOACs in APS, only retrospective studies have examined HIT. In addition, vaccine-induced immune thrombotic thrombocytopenia (VITT) is now a part of immunothrombotic disorders, and guidelines have been created concerning an anticoagulant strategy in this case. This literature review aims to summarize available data on HIT, APS, and VITT treatments and define the use of DOACs in therapeutic strategies.

First diagnosis of thrombotic thrombocytopenic purpura after SARS-CoV-2 vaccine - case report

Background

We report a case of a 25-year-old male patient, who developed acquired thrombotic thrombocytopenic purpura (aTTP) after receiving a first dose of mRNA-based SARS-CoV-2 vaccine Spikevax (mRNA-1273, Moderna Biotech, USA). While this is the first case in literature describing a case of aTTP after receiving the Spikevax vaccine, there are two other cases after mRNA-based Covid-19 vaccine and two after adenoviral SARS-CoV-2 vaccine.

Case presentation

The patient presented with persisting malaise, fever, headache, word-finding difficulties, nausea, vomiting, petechial bleeding, and hematuria 13 days after receiving a first dose of vaccination. Laboratory testing showed low platelet count, Coombs-negative hemolytic anemia, and mild acute kidney injury. We excluded vaccine induced immune thrombotic thrombocytopenia (VITT) as another important differential diagnosis and the final diagnosis was established after ADAMTS-13 (A Disintegrin And Metalloproteinase with a ThromboSpondin type 1 motif, member 13) activity was found to be < 1% (reference range > 40%) and ADAMTS-13 antibodies being 72.2 IU/L (reference range < 12 IU/L).

We initiated empiric therapy of plasmapheresis and corticosteroids on admission and started caplacizumab the day after. The patient’s thrombocyte count normalized 3 days after admission, hemolysis and acute kidney injury resolved after 2 weeks. The patient received 2 doses of rituximab (1 g each) after the diagnosis of immune TTP was established. One month after the initial presentation, the patient is in good overall condition, but still receives daily caplacizumab due to ADAMTS-13 activity of < 1%.

Conclusions

Low platelet count after vaccination against SARS-CoV-2 has gained attraction after vaccine-induced immune thrombotic thrombocytopenia (VITT) has been described as a rare but severe complication of adenoviral-based vaccines. Thrombotic thrombocytopenic purpura (TTP) is an important differential diagnosis, but there are only few reports of TTP following SARS-CoV-2 vaccination. Despite pathophysiological and clinical differences of both entities, diagnostic uncertainty can result in the acute setting, since they share main symptoms such as headache and neurological alterations in addition to thrombocytopenia. In difference to other cases reported, this patient developed first symptoms of TTP as early as 4 days after vaccination, which suggests that vaccination merely acted as trigger for occult TTP, instead of truly inducing an autoimmunological process.

Heparin-induced Thrombocytopenia Diagnosis: A Retrospective Study Comparing Heparin-induced Platelet Activation Test to 14 C-serotonin Release Assay

Laboratory confirmation of heparin-induced thrombocytopenia (HIT) is of crucial importance and remains challenging and relies on platelet functional assays highlighting the presence of heparin-dependent platelet-activating antibodies in patient serum or plasma. Platelet functional assays using washed platelets include the ¹⁴ C-serotonin release assay (SRA), usually described as the gold standard, and the heparin-induced platelet activation assay (HIPA). Since its first comparison with SRA there has been no additional published study regarding HIPA diagnostic performances compared with SRA. Aim of our retrospective study was to compare the concordance between HIPA and SRA in HIT suspected-patients with positive anti-PF4/heparin antibodies between October 2010 and October 2015. Fifty-five HIT-suspected patients who beneficiated from both HIPA and SRA were included. Positive and negative percent agreements were 83.8% (95% CI 68.0–93.8%) and 66.7% (95% CI 41.0–86.7%), respectively. Overall percent agreement was 78.2% (95% CI 65.0–92.2%). Agreement was higher in patients who underwent cardiopulmonary bypass with extracorporeal circulation circuit for cardiac surgery. We also confirm that the use of a minimum of 2 platelet donors to establish positive HIT diagnosis and 4 platelet donors to exclude HIT diagnosis allows obtaining a good agreement with SRA. Although HIPA and SRA were performed with different platelet donors and in different laboratories, HIPA had a good positive agreement with SRA for HIT diagnosis, showing that HIPA is a useful functional assay that does not require radioactivity and could be developed worldwide to improve HIT diagnosis.

Upregulation of cAMP prevents antibody-mediated thrombus formation in COVID-19

Thromboembolic events are frequently reported in patients infected with the SARS-CoV-2 virus. The exact mechanisms of COVID-19 associated hypercoagulopathy, however, remain elusive. Recently, we observed that platelets (PLTs) from patients with severe COVID-19 infection express high levels of procoagulant markers, which were found to be associated with increased risk for thrombosis. In the current study, we investigated the time course as well as the mechanisms leading to procoagulant PLTs in COVID-19. Our study demonstrates the presence of PLT-reactive IgG antibodies that induce marked changes in PLTs in terms of increased inner-mitochondrial-transmembrane potential (Δψ) depolarization, phosphatidylserine (PS) externalization and P-selectin expression. The IgG-induced procoagulant PLTs and increased thrombus formation was mediated by ligation of PLT Fc gamma RIIA (FcγRIIA). In addition, PLTs´ contents of calcium and cyclic-adenosine-monophosphate (cAMP) were identified to play central role in antibody-induced procoagulant PLT formation. Most importantly, antibody-induced procoagulant events as well as increased thrombus formation in severe COVID-19 were inhibited by Iloprost a clinically approved therapeutic agent that increases the intracellular cAMP levels in PLTs. Our data indicate that upregulation of cAMP could be a potential therapeutic target to prevent antibody-mediated coagulopathy in COVID-19 disease.

PF4-Dependent Immunoassays in Patients with Vaccine-Induced Immune Thrombotic Thrombocytopenia: Results of an Interlaboratory Comparison

BACKGROUND

 Coronavirus disease 2019 vaccine ChAdOx1 nCov-19 may rarely lead to vaccine-induced thrombotic thrombocytopenia (VITT). Antibody-mediated, platelet factor 4 (PF4)-dependent platelet activation appears to resemble a key mechanism in VITT, partially comparable to heparin-induced thrombocytopenia. The use of PF4/heparin immunoassays has been proposed as part of a diagnostic approach, but their sensitivity has not been established.

METHODS

 Sera from 12 well-defined VITT patients were first studied by two different laboratories in functional assays. Sera where then used for an interlaboratory comparison, in which five different PF4/heparin immunoassays were used by four laboratories.

RESULTS

 Results for functional testing were highly concordant. VITT antibodies were also reliably detected by PF4/heparin enzyme-linked immunosorbent assays (ELISAs) (92-100%). In contrast, only 25% of VITT antibodies were reactive in a particle gel immunoassay (PaGIA), and 8% in a lateral flow assay (LFA). An automated chemiluminescence immunoassay (CLIA) was negative for all sera tested (0%).

CONCLUSION

 It seems feasible to establish functional antibody testing for the confirmation of VITT. For the initial screening of suspected VITT cases, PaGIA, LFA, and CLIA are useless when applied as single tests. Only ELISA-based PF4/heparin immunoassays are sensitive enough to be incorporated in the diagnostic workup. However, a combination of a positive ELISA and a negative CLIA may be useful to identify VITT antibodies in the absence of confirmatory functional assays.

Recommendations for the clinical and laboratory diagnosis of VITT against COVID-19: Communication from the ISTH SSC Subcommittee on Platelet Immunology

Vaccine administration is under way worldwide to combat the current COVID-19 pandemic. The newly developed vaccines are highly effective with minimal adverse effects. Recently, the AstraZeneca ChadOx1 nCov-19 vaccine has raised public alarm with concerns regarding the rare, but serious, development of thrombotic events, now known as vaccine-induced immune thrombotic thrombocytopenia (VITT). These thrombotic events appear similar to heparin-induced thrombocytopenia, both clinically and pathologically. In this manuscript, the ISTH SSC Subcommittee on Platelet Immunology outlines guidelines on how to recognize, diagnose and manage patients with VITT.

A comparative study of platelet factor 4-enhanced platelet activation assays for the diagnosis of heparin-induced thrombocytopenia

BACKGROUND

Functional platelet activation assays, such as the serotonin release assay (SRA), are the gold standard for the diagnosis of heparin-induced thrombocytopenia (HIT). Recently, platelet activation assays using added platelet factor 4 (PF4) have been described and suggest improved sensitivity. Direct comparisons of these assays have not been performed.

OBJECTIVE

We compare the performance characteristics of three PF4-enhanced platelet activation assays, the PF4/heparin-SRA (PF4/hep-SRA), the PF4-SRA, and the P-selectin expression assay (PEA), at a single reference laboratory.

METHODS

Serum samples from two cohorts of patients were used. The referral cohort (n = 84) included samples that had previously undergone routine diagnostic testing for HIT and tested positive or negative using the SRA. The clinical cohort (n = 101) consisted of samples from patients with clinically confirmed HIT whose serum contained platelet-activating antibodies. We simultaneously tested all samples in PF4-enhanced SRA-based assays (PF4/hep-SRA, PF4-SRA) and the flow cytometry-based PEA.

RESULTS

In the referral cohort, the three PF4-enhanced assays identified all samples that were previously determined to be positive in the SRA. However, specificity of the PF4/hep-SRA was 96.6%, the PF4-SRA was 84.7%, and the PEA was 67.8%. In the clinical cohort of samples, all SRA-based assays displayed high performance characteristics (>92.1% sensitivity, >98.4% specificity). Sensitivity and specificity of the PEA was the lowest, 65.8% and 63.5%, respectively; but improved to 92.1% and 96.8% using preselected platelet donors.

CONCLUSIONS

All PF4-enhanced assays demonstrated good performance characteristics when platelet donors were preselected. Further comparisons across multiple laboratories should be conducted for consensus on optimal HIT diagnostic testing.

Challenges in Detecting Clinically Relevant Heparin-Induced Thrombocytopenia Antibodies

Heparin-induced thrombocytopenia (HIT) is an antibody-mediated hypercoagulable state featuring high thrombosis risk and distinct pathogenesis involving immunoglobulin G-mediated platelet activation. The target of the immune response is a cationic “self” protein, platelet factor 4 (PF4), rendered antigenic by heparin. A key problem is that only a minority of anti-PF4/polyanion antibodies induced by heparin are pathogenic, i.e., capable of causing platelet activation and thereby clinical HIT. Since thrombocytopenia occurs frequently in hospitalized, heparin-treated patients, testing for “HIT antibodies” is common; thus, the problem of distinguishing between pathogenic and nonpathogenic antibodies is important. The central concept is that those antibodies that have platelet-activating properties demonstrable in vitro correlate well with pathogenicity, as shown by platelet activation tests such as the serotonin-release assay (SRA) and heparin-induced platelet activation assay. However, in most circumstances, immunoassays are used for first-line testing, and so it is important for clinicians to appreciate which immunoassay result profiles—in the appropriate clinical context—predict the presence of platelet-activating antibodies (Bayesian analysis). Clinicians with access to rapid, on-demand HIT immunoassays (e.g., particle gel immunoassay, latex immunoturbidimetric assay, chemiluminescent immunoassay) can look beyond simple dichotomous result interpretation (“negative”/“positive”) and incorporate semiquantitative interpretation, where, for example, a strong-positive immunoassay result (or even combination of two immunoassays) points to a greater probability of detecting platelet-activating antibodies, and hence supporting a diagnosis of HIT. Recent recognition of “SRA-negative HIT” has increased the importance of semiquantitative interpretation of immunoassays, given that strong immunoassay reactivity is a potential clue indicating possible HIT despite a (false) negative platelet activation assay.

Ticagrelor causes false-negative functional tests for heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) occurs in 1-2% of patients with acute coronary events or cardiac surgery. Eekels and colleagues report that ticagrelor comedication must be taken into account by laboratory physicians for interpretation of functional tests for HIT, as the presence of the drug can cause false-negative results.

Heparin Induced Thrombocytopenia for the Perioperative and Critical Care Clinician

Purpose of Review

This review will illustrate the importance of heparin-induced thrombocytopenia in the intraoperative and critical care settings.

Recent Findings

Heparin-induced thrombocytopenia (HIT) occurs more frequently in surgical patients compared with medical patients due to the inflammatory release of platelet factor 4 and perioperative heparin exposure. Recognition of this disease requires a high index of suspicion. Diagnostic tools and therapeutic strategies have been expanded and refined in recent years.

Summary

HIT is a condition where antibodies against the heparin/platelet factor 4 complex interact with platelet receptors to promote platelet activation, aggregation, and thrombus formation. Our review will focus on intraoperative and postoperative considerations related to HIT to help the clinician better manage this rare but often devastating hypercoagulable disease process.

Platelet antibodies in immune thrombocytopenia and related conditions

Platelet autoantibodies are a common finding in immune thrombocytopenia (ITP) and in rare cases of antibody-mediated platelet function (“acquired thrombasthenia”). In drug-induced immune thrombocytopenia, antibodies react with platelets only in the presence of the offending drug. Alloantibodies reacting with platelets are induced by transfusion of cellular blood products or during pregnancy. They are responsible for fetal/neonatal alloimmune thrombocytopenia (FNAIT), they are able to cause febrile, nonhemolytic transfusion reactions and they give rise to insufficient platelet increments following platelet transfusions. Two rare transfusion reactions: post-transfusion purpura (PTP) and passive alloimmune thrombocytopenia (PAT) are triggered by platelet alloantibodies. This review discusses the clinical value of tests for platelet antibodies in various clinical situations related to insufficient primary hemostasis.

Activated platelets kill Staphylococcus aureus, but not Streptococcus pneumoniae-The role of FcγRIIa and platelet factor 4/heparinantibodies

BACKGROUND

Heparin induced thrombocytopenia (HIT) is likely a misdirected bacterial host defense mechanism. Platelet factor 4 (PF4) binds to polyanions on bacterial surfaces exposing neo-epitopes to which HIT antibodies bind. Platelets are activated by the resulting immune complexes via FcγRIIA, release bactericidal substances, and kill Gram-negative Escherichia coli.

OBJECTIVES

To assess the role of PF4, anti-PF4/H antibodies and FcγRIIa in killing of Gram-positive bacteria by platelets.

METHODS

Binding of PF4 to protein-A deficient Staphylococcus aureus (SA113Δspa) and non-encapsulated Streptococcus pneumoniae (D39Δcps) and its conformational change were assessed by flow cytometry using monoclonal (KKO,5B9) and patient derived anti-PF4/H antibodies. Killing of bacteria was quantified by counting colony forming units (cfu) after incubation with platelets or platelet releasate. Using flow cytometry, platelet activation (CD62P-expression, PAC-1 binding) and phosphatidylserine (PS)-exposure were analyzed.

RESULTS

Monoclonal and patient-derived anti-PF4/H antibodies bound in the presence of PF4 to both S. aureus and S. pneumoniae (1.6-fold increased fluorescence signal for human anti-PF4/H antibodies to 24.0-fold increase for KKO). Staphylococcus aureus (5.5 × 104 cfu/mL) was efficiently killed by platelets (2.7 × 104 cfu/mL) or their releasate (2.9 × 104 cfu/mL). Killing was not further enhanced by PF4 or anti-PF4/H antibodies. Blocking FcγRIIa had no impact on killing of S. aureus by platelets. In contrast, S. pneumoniae was not killed by platelets or releasate. Instead, after incubation with pneumococci platelets were unresponsive to TRAP-6 stimulation and exposed high levels of PS.

CONCLUSIONS

Anti-PF4/H antibodies seem to have only a minor role for direct killing of Gram-positive bacteria by platelets. Staphylococcus aureus is killed by platelets or platelet releasate. In contrast, S. pneumoniae affects platelet viability.

Combination of two complementary automated rapid assays for diagnosis of heparin-induced thrombocytopenia (HIT)

BACKGROUND

HIT diagnosis typically uses complementary diagnostic assays (eg, a PF4-dependent enzyme-immunoassay [EIA] and a platelet activation assay such as the serotonin-release assay [SRA]).

OBJECTIVES

To determine whether the combination of two automated assays-a latex immunoturbidimetric assay (LIA) that evaluates competitive inhibition of a HIT-like monoclonal antibody and a chemiluminescence immunoassay (CLIA) for detecting anti-PF4/heparin IgG-optimizes diagnostic sensitivity while also yielding good specificity, particularly at high assay reactivities.

PATIENTS/METHODS

We determined operating characteristics using combined LIA/CLIA results from a HIT observational trial (n = 430; derivation cohort) and 147 consecutive patients with HIT (n = 147; supplementary derivation cohort). We also evaluated 678 consecutive samples referred for HIT testing (replication cohort). LIA/CLIA reactivities were scored individually as "negative" (<1.00 U/mL, 0 points), "weak" (1.00-4.99 U/mL, 1 point), "moderate" (5.00-15.99 U/mL, 2 points) and "strong" (≥16.00 U/mL, 3 points), thus contributing up to 6 points (maximum) when LIA/CLIA results were combined. We also examined whether higher LIA/CLIA scores predicted presence of platelet-activating antibodies by conventional and modified (PF4- or PF4/heparin-enhanced) SRA.

RESULTS

Combined LIA/CLIA testing yielded high diagnostic sensitivity (~99%) similar to EIA. Interpretation of LIA/CLIA results using the 6-point scale indicated progressively greater likelihood for the presence of platelet-activating antibodies with increasing scores (semi-quantitative reactivity). A LIA/CLIA score ≥ 4 points predicted the presence of platelet-activating antibodies by SRA or PF4-enhanced SRA with high probability (~98%).

CONCLUSION

Combined LIA/CLIA testing optimizes diagnostic sensitivity, with progressively greater probability of detecting platelet-activating antibodies with higher assay reactivity that reaches 98% when both automated assays yield moderate or strong results.

Detection of Platelet-Activating Antibodies Associated with Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a prothrombotic immune drug reaction caused by platelet-activating antibodies that in most instances recognize platelet factor 4 (PF4)/polyanion complexes. Platelet activation assays (i.e., functional assays) are more specific than immunoassays, since they are able to discern clinically relevant heparin-induced antibodies. All functional assays used for HIT diagnosis share the same principle, as they assess the ability of serum/plasma from suspected HIT patients to activate fresh platelets from healthy donors in the presence of several concentrations of heparin. Depending on the assay, donors’ platelets are stimulated either in whole blood (WB), platelet-rich plasma (PRP), or in a buffer medium (washed platelets, WP). In addition, the activation endpoint studied varies from one assay to another: platelet aggregation, membrane expression of markers of platelet activation, release of platelet granules. Tests with WP are more sensitive and serotonin release assay (SRA) is considered to be the current gold standard, but functional assays suffer from certain limitations regarding their sensitivity, specificity, complexity, and/or accessibility. However, the strict adherence to adequate preanalytical conditions, the use of selected platelet donors and the inclusion of positive and negative controls in each run are key points that ensure their performances.

Heparin-induced thrombocytopenia: Diagnostic challenges in intensive care patients especially with extracorporeal circulation

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a serious drug induced reaction that may be associated with life threatening complications. Platelet-activating antibodies directed against platelet factor 4 (PF4)/heparin complexes cause the disease. The diagnosis of HIT is challenging, as thrombocytopenia is a frequent finding in intensive care (ICU) patient population, especially during extracorporeal membrane oxygenation.

OBJECTIVE

To investigate the performance of a diagnostic algorithm for HIT in ICU patients.

METHODS

ICU patients who developed thrombocytopenia or thrombosis under heparin treatment were included in this study. The pretest probability for HIT was estimated using the 4Ts-score and patient's sera were tested using two rapid immunoassays (RA) LFI-HIT and PaGIA (from Milenia Biotec and DiaMed), and within 72 h using the IgG enzyme immunoassay (EIA) from Hyphen and the heparin induced platelet activation assay (HIPA).

RESULTS

392 consecutive ICU patients with suspected HIT were enrolled in this study, of whom 83/392 (21.2%) patients had extracorporeal circulation. Sera from 120/392 (30.6%) and 98/392 (25.0%) patients revealed positive results in RA and IgG EIA, respectively. The HIPA test revealed heparin-dependent platelet activation in a total of 15/392 (3.8%) ICU patients (3 medical and 12 surgical patients). In addition, sera from 7 patients revealed indeterminate HIPA results, of whom 2 patients had a clinical course compatible with HIT.

CONCLUSIONS

Data from our study confirm the high frequency of IgG PF4/heparin antibodies in ICU patients under unfractionated heparin and shows that the combination of 4Ts-score and RA does not reduce the laboratory overinvestigation for HIT in these patients.

Blood donor-derived buffy coat to produce platelets in vitro

BACKGROUND AND OBJECTIVES

Platelet transfusion is a standard medical therapy used to treat several bleeding disorders. However, a critical drawback is the dependency on donor-derived platelets, which leads to concerns like insufficient availability and immunological complications. In vitro platelet production from hematopoietic progenitor cells (CD34) may represent a reasonable solution.

MATERIALS AND METHODS

CD34+ cells were isolated from either buffy coat or peripheral blood and compared in terms of platelet production in vitro. The number and the quality of magnetically isolated CD34+ cells and their capability to differentiate into mature megakaryocytes were investigated using flow cytometry. Additionally, the functionality of megakaryocytes in term of in vitro platelet production was tested.

RESULTS

Similar purity and quantity of CD34+ cells was found after their isolation from both cell sources. In contrast, after 6 days of culture, enhanced number of CD34+ cells isolated from buffy coat compared with peripheral blood was observed (5·3 x 106 vs. 3·0 x 106, respectively). Interestingly, despite a comparable nuclear maturation phenotype, the yield of platelets released from buffy coat-derived megakaryocytes was significantly higher than from peripheral blood cells (platelet yield pro MK: 7·2 vs. 2·7, respectively). Importantly, platelets produced from buffy coat-derived cells could be activated by agonists.

CONCLUSION

Haematopoietic progenitor cells isolated from buffy coat have increased yield of platelets released from mature megakaryocytes and enhanced in vitro functionality, compared with peripheral blood-derived cells. Our study, suggests that buffy coat, obtained during blood donation processing, might be a promising source of megakaryocytes for in vitro platelet production.

The use of flow cytometry in the diagnosis of heparin-induced thrombocytopenia (HIT)

Heparin-induced thrombocytopenia (HIT) affects some of the patients exposed to heparin. It is mediated by antibodies that recognize neoepitopes on platelet factor 4 (PF4)/heparin complexes. A HIT diagnosis requires both clinical and laboratory evaluation and remains a challenge. Since many patients develop antibodies in response to heparin, but only a few of them generate anti-PF4/heparin antibodies capable of activating platelets which consequently cause clinical complications, the performance of serologic assays is not enough to diagnose HIT. Functional assays can identify pathogenic antibodies capable of platelet activation, but they are more demanding and their limited availability contributes to the problem of diagnosing HIT. Restricted laboratories usually collect sera of multiple patients to perform functional assays only once or twice a week; hence, a HIT diagnosis can take several days. The use of flow cytometry appears to be a promising alternative in the confirmation of pathogenic anti-PF4/heparin antibodies. Flow cytometric assays detect either activation markers on a healthy donor's platelet surfaces or platelet derived microparticles formed after platelet incubation with a patient's serum. Flow cytometers are readily available in many clinical laboratories, so this technology introduces the possibility of an earlier HIT diagnosis. The objective of this review was to collect findings on flow cytometric HIT confirmations to the present date, and to review the currently available flow cytometric assays used in the diagnosis of HIT.

Pathogenic heparin-induced thrombocytopenia and thrombosis (HIT) antibodies determined by rapid functional flow cytometry

OBJECTIVES

Reliable diagnosis of heparin-induced thrombocytopenia and thrombosis (HIT) is mandatory for patient management, yet prompt determination of pathogenic antibodies remains an unmet clinical challenge. Common immunoassays carry inherent limitations and functional assays which detect antibody-mediated platelet activation are not usually readily available to routine laboratories, especially the serotonin release assay (SRA), being technically demanding, time consuming, and requires high level expertise. To overcome some of these limitations, we have developed a practical functional flow cytometric assay (FCA) for routine clinical use.

METHODS

A simple FCA is described which avoids platelet manipulation, is highly specific and sensitive compared with SRA, and provides rapid results.

RESULTS

Of the 650 consecutive samples, from HIT-suspected patients, 99 (15.3%) were positive by the PaGIA Heparin/PF4 immunoassay and 31 (4.8%) by FCA. Average platelet activation was 11-fold higher in PaGIA+/FCA+ vs PaGIA-/FCA- samples. Of 21 SRA-positive samples, 19 were FCA-positive (relative sensitivity 90.5%), and of 42 SRA-negative samples, 40 were FCA-negative (relative specificity 95.2%). The FCA showed significantly higher correlation with the clinical presentation of HIT (4Ts score) performed on 182 patients, compared with PaGIA Heparin/PF4 (ROC-plot analysis, AUC 0.93 vs 0.63, P < 0.001). At a 92% sensitivity, the assay specificity was 96%.

CONCLUSIONS

The present FCA is practical for routine testing, providing prompt reliable results for initial diagnosis and confirmation, to effectively assist in HIT patient management.

Evaluation of a flow cytometer-based functional assay using platelet-rich plasma in the diagnosis of heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is caused by platelet-activating antibodies that recognize platelet factor 4/heparin (PF4/hep)-complexes. The in vitro demonstration of PF4/hep antibodies using functional assays is essential for an optimal management of patients suspected to have HIT. However, conventional functional assays are technically challenging and limited to specialized laboratories. In contrast, flow cytometers are commonly used in routine laboratories. The aim of this study is to investigate the performance characteristics of a commercially available, flow cytometer based assay in the diagnosis of HIT.

STUDY DESIGN

Sera of consecutive patients with suspected HIT were investigated using the Emo-test HIT Confirm® assay and compared to the standard method consisting of an IgG-specific enzyme immunoassay (EIA) for anti-PF4/hep antibodies and the heparin induced platelet aggregation (HIPA) test.

RESULTS

390 sera were included in the study, 164 sera tested IgG EIA-positive, of which 33 also tested HIPA-positive. No HIPA-positive samples were EIA-negative. In the Emo-test HIT Confirm® assay, 112 sera revealed positive results (%Hepla > 13); however, 51 (45.5%) were EIA-negative. Of the 33 HIPA-positive/EIA-positive HIT sera, 23 tested positive in the Emo-test HIT Confirm® assay, 2 gave ambiguous results, and 8 sera yielded false-negative results. Accordingly, the HIT Confirm® assay showed a sensitivity of 69.7% with a slightly better specificity of 75.4% compared to the EIA (sensitivity 100%, specificity 63.3%). An increase in diagnostic specificity for HIT to 85% was found when positive results were obtained in both the Emo-test HIT Confirm® assay and EIA.

CONCLUSION

The Emo-Test HIT Confirm® assay may improve the specificity of laboratory investigations of HIT. However, the assay can only be recommended in combination with an immunoassay due to the high rate of false negativity. Our observation indicates a need to establish external quality assessment for functional assays to avoid such clinically relevant pitfalls.

An international external quality assessment for laboratory diagnosis of heparin-induced thrombocytopenia

Essentials A pilot study for External Quality Assessment for testing of HIT is described. The qualitative accordance for the PF4/heparin IgG test was 97.6%. The qualitative accordance for functional HIT tests was considerably lower. External Quality Assessment for functional HIT tests is required. SUMMARY: Objective Heparin-induced thrombocytopenia (HIT) is a potentially life-threatening complication of heparin exposure. Diagnosis is most reliable using a combination of an enzyme immunoassay (EIA) that detects antibodies against platelet factor 4 (PF4)/heparin complexes ("antigen" assay) and a "functional" assay that detects platelet-activating properties of the pathogenic HIT antibodies. No External Quality Assessment (EQA) is available for a combination of the tests. Here we report on the results of the first international EQA. Methods The pilot EQA was organized by the Department of Transfusion Medicine, Universitätsmedizin Greifswald, Germany. Six serum samples of patients, which were referred to Greifswald for HIT diagnosis, and one negative control sample were distributed to seven participants in Germany, Canada, and Singapore. Participants were asked to report the optical density (OD) values of their local EIA test for IgG-specific antibodies against the PF4/heparin complexes and the results for a functional assay (HIPA or SRA). Consensus was defined as a minimum 70% agreement, i.e., agreement among at least five of the seven participating laboratories. Results and conclusion Six out of seven participants reported results for EIA, with a high quantitative accordance (97.6%). For the functional assay, consensus was reached for all samples except the negative control, for which some participants reported nonspecific reactivity. All HIT-negative samples were correctly diagnosed by all participants; for HIT-positive samples, consensus of 70% was reached. Although the limited availability of sample material is an obstacle to overcome, an EQA combining both EIA and functional testing is feasible.

Characterization of platelet factor 4 amino acids that bind pathogenic antibodies in heparin-induced thrombocytopenia

Essentials Many patients produce antibodies but few lead to heparin-induced thrombocytopenia (HIT). Pathogenic epitopes are difficult to identify as HIT antibodies are polyclonal and polyspecific. KKO binding to platelet factor 4 (PF4) depends on 13 amino acids, three of which are newly observed. Five amino acids in PF4 can help distinguish pathogenic from non-pathogenic antibodies. SUMMARY: Background Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction that results in thrombocytopenia and, in some patients, thrombotic complications. HIT is mediated by antibodies that bind to complexes of platelet factor 4 (PF4) and heparin. The antigenic epitopes of these anti-PF4/heparin antibodies have not yet been precisely defined, because of the polyspecific immune response that characterizes HIT. Objectives To identify PF4 amino acids essential for binding pathogenic HIT antibodies. Methods Alanine scanning mutagenesis was utilized to produce 70 single point mutations of PF4. Each PF4 mutant was used in an enzyme immunoassay (EIA) to test their capacity to bind a platelet-activating murine monoclonal anti-PF4/heparin antibody (KKO) and HIT patient sera (n = 9). Results and Conclusions We identified 13 amino acids that were essential for binding KKO because they directly affected either the binding site or the antigenic conformation of PF4. We also identified 10 amino acids that were required for the binding of HIT patient sera and five of these amino acids were required for binding both KKO and the HIT patient sera. The 10 amino acids required for binding HIT sera were further tested to differentiate pathogenic HIT antibodies (platelet activating, n = 45) and non-pathogenic antibodies (EIA-positive but not platelet activating, n = 28). We identified five mutations of PF4 that were recognized to be essential for binding pathogenic HIT antibodies. Using alanine scanning mutagenesis, we characterized possible binding sites of pathogenic HIT antibodies on PF4.

Fibronectin modulates formation of PF4/heparin complexes and is a potential factor for reducing risk of developing HIT

Heparin-induced thrombocytopenia (HIT) is caused by platelet-activating anti-platelet factor 4 (PF4)/heparin antibodies. Platelet activation assays that use "washed" platelets are more sensitive for detecting HIT antibodies than platelet-rich plasma (PRP)-based assays. Moreover, heparin-exposed patients vary considerably with respect to the risk of PF4/heparin immunization and, among antibody-positive patients, the risk of subsequent "breakthrough" of clinical HIT with manifestation of thrombocytopenia. We used washed platelets and PRP, standard laboratory HIT tests, and physicochemical methods to identify a plasma factor interfering with PF4/heparin complexes and anti-PF4/heparin antibody-platelet interaction, thus explaining differences in functional assays. To investigate a modulating risk for PF4/heparin immunization and breakthrough of HIT, we also tested 89 plasmas from 2 serosurveillance trials. Fibronectin levels were measured in 4 patient groups exhibiting different degrees of heparin-dependent immunization and expression of HIT. The heat-labile plasma protein, fibronectin, inhibited PF4 binding to platelets in a dose-dependent fashion, particularly in washed (vs PRP) systems. Fibronectin also inhibited PF4/heparin binding to platelets, anti-PF4/heparin antibody binding to PF4/heparin complexes, and anti-PF4/heparin antibody-induced platelet activation as a result of PF4/heparin complex disruption. In addition, plasma fibronectin levels increased progressively among the following 4 patient groups: enzyme-linked immunosorbent assay (ELISA)+/serotonin-release assay (SRA)+/HIT+ < ELISA+/SRA+/HIT- ∼ ELISA+/SRA-/HIT- < ELISA-/SRA-/HIT-. Altogether, these findings suggest that fibronectin interferes with PF4/heparin complex formation and anti-PF4/heparin antibody-induced platelet activation. Reduced fibronectin levels in washed platelet assays help to explain the greater sensitivity of washed platelet (vs PRP) assays for HIT. More importantly, lower plasma fibronectin levels could represent a risk factor for PF4/heparin immunization and clinical breakthrough of HIT.

Drug-associated thrombocytopenia

Many drugs have been implicated in drug-induced immune thrombocytopenia (DITP). Patients with DITP develop a drop in platelet count 5 to 10 days after drug administration with an increased risk of hemorrhage. The diagnosis of DITP is often challenging, because most hospitalized patients are taking multiple medications and have comorbidities that can also cause thrombocytopenia. Specialized laboratory diagnostic tests have been developed and are helpful to confirm the diagnosis. Treatment of DITP involves discontinuation of the offending drug. The platelet count usually starts to recover after 4 or 5 half-lives of the responsible drug or drug metabolite. High doses of intravenous immunoglobulin can be given to patients with severe thrombocytopenia and bleeding. Although in most cases, DITP is associated with bleeding, life-threatening thromboembolic complications are common in patients with heparin-induced thrombocytopenia (HIT). Binding of antiplatelet factor 4/heparin antibodies to Fc receptors on platelets and monocytes causes intravascular cellular activation, leading to an intensely prothrombotic state in HIT. The clinical symptoms include a decrease in platelet counts by >50% and/or new thromboembolic complications. Two approaches can help to confirm or rule out HIT: assessment of the clinical presentation using scoring systems and in vitro demonstration of antiplatelet factor 4/heparin antibodies. The cornerstone of HIT management is immediate discontinuation of heparin when the disease is suspected and anticoagulation using nonheparin anticoagulant. In this review, we will provide an update on the pathophysiology, diagnosis, and management of both DITP and HIT.

Targeted resequencing of a locus for heparin-induced thrombocytopenia on chromosome 5 identified in a genome-wide association study

Immune-mediated heparin-induced thrombocytopenia (HIT) is the clinically most important adverse drug reaction (ADR) in response to heparin therapy characterized by a prothrombotic state despite a decrease in platelet count. We conducted a genome-wide association study in 96 suspected HIT cases and 96 controls to explore the genetic predisposition for HIT within a case-control pharmacovigilance study followed by replication in additional 86 cases and 86 controls from the same study. One single nucleotide polymorphism (SNP, rs1433265, P = 6.5 × 10^-5, odds ratio (OR) 2.79) from 16 identified SNPs was successfully replicated (P = 1.5 × 10^-4, OR 2.77; combined data set P = 2.7 × 10^-8, OR 2.77) and remained the most strongly associated SNP after imputing locus genotypes. Fine mapping revealed a significantly associated risk-conferring haplotype (P = 4.9 × 10^-6, OR 2.41). In order to find rare variants contributing to the association signals, we applied a targeted resequencing approach in a subgroup of 73 HIT patients and 23 controls for the regions with the 16 most strongly HIT-associated SNPs. C-alpha testing was applied to test for the impact of rare variants and we detected two candidate genes, the discoidin domain receptor tyrosine kinase 1 (DDR1, P = 3.6 × 10^-2) and the multiple C2 and transmembrane domain containing 2 (MCTP2, P = 4.5 × 10^-2). For the genes interactor of little elongation complex ELL subunit 1 (ICE1) and a disintegrin-like and metalloproteinase with thrombospondin type 1 motif, 16 (ADAMTS16) nearby rs1433265, we identified several missense variants. Although replication in an independent population is warranted, these findings provide a basis for future studies aiming to identify and characterize genetic susceptibility factors for HIT. KEY MESSAGES: We identified and validated a HIT-associated locus on chromosome 5. Targeted NGS analysis for rare variants identifies DDR1 and MCTP2 as novel candidates. In addition, missense variants for ADAMTS16 and ICE1 were identified in the locus.

Identifying patients at high risk of heparin-induced thrombocytopenia-associated thrombosis with a platelet activation assay using flow cytometry

To diagnose heparin-induced thrombocytopenia (HIT), detection of platelet-activating antibodies (HIT antibodies) is crucial. However, serum platelet activation profiles vary across patients and depend on test conditions. We evaluated the association between clinical outcomes and platelet-activating profiles assessed by a platelet microparticle assay (PMA), which detects activation of washed platelets induced by HIT antibodies, in 401 consecutive patients clinically suspected of having HIT. We made modifications to the assay, such as donor selection for washed platelets that increased sensitivity. Serum that activated platelets at a therapeutic (but not high) heparin concentration was defined as positive. Of these, serum that activated platelets within 30 minutes or in the absence of heparin was defined as strongly positive. The remaining samples were considered weakly positive. As a result, 97 % and 93 % of patients who tested strongly and weakly positive had clinical profiles consistent with HIT, respectively. The incidence of thromboembolic events (TEEs) after heparin exposure in patients who tested strongly positive, weakly positive, and negative was 61 %, 40 %, and 29 %, respectively. Among patients who did not experience a TEE on the day HIT was suspected, there was no significant difference in the cumulative incidence of subsequent TEEs between patients who tested strongly and weakly positive when argatroban was initiated on the same day (19.0 % vs 7.1 %, p=0.313), but there was a significant difference when argatroban therapy was delayed by one or more days (61.1 % vs 17.6 %, p=0.007). The modified PMA is effective in diagnosing HIT and identifying patients at high risk for HIT-associated TEEs.

Supplementary Material to this article is available online at www.thrombosis-online.com.

Clinical and laboratory tests for the diagnosis of heparin-induced thrombocytopenia

A rapid diagnostic work-up is required in patients with suspected heparin-induced thrombocytopenia (HIT). However, diagnosis of HIT is challenging due to a number of practical issues and methodological limitations. Many laboratory tests and a few clinical scoring systems are available but the individual characteristics and the diagnostic accuracy of these are hard to appraise. The 4Ts score is a well evaluated clinical assessment tool with the potential to rule out HIT in many patients. Still, it requires experience and is subject to a relevant inter-observer variability. Immunoassays such as enzyme-linked immunosorbent assays or recently developed rapid assays are able to exclude HIT in a number of patients. But, accuracy of immunoassays differs depending on type of assay, threshold, antibody specificity and even manufacturer. Due to a comparatively low positive predictive value, HIT cannot be confirmed by immunoassays alone. In addition, only some of them are immediately accessible, particularly in small laboratories. While functional assays such as the serotonin release assay (SRA) and the heparin-induced platelet activation assay (HIPA) are considered as gold standard for diagnosis of HIT, they require a highly specialised laboratory. In addition, some of them are not adequately evaluated. In clinical practice, we recommend an integrated diagnostic approach combining not only clinical assessment (the 4Ts score) but immunoassays and functional assays as well. We propose a clear diagnostic algorithm supporting clinical decision-making. Furthermore, we provide an overview of all current laboratory techniques for HIT and discuss diagnostic pathways and strategies to reduce diagnostic errors, and future perspectives.