A multicenter laboratory assessment of a new automated chemiluminescent assay for ADAMTS13 activity

Immune Thrombotic Thrombocytopenic Purpura: A Review

Importance

Immune thrombotic thrombocytopenic purpura (iTTP) is a life-threatening thrombotic microangiopathy that presents with microangiopathic hemolytic anemia (MAHA) and thrombocytopenia. Worldwide annual incidence of iTTP is 2 cases per million to 6 cases per million.

Observations

Immune TTP is caused by an autoantibody to a disintegrin and metallopeptidase with thrombospondin type 1 motif 13 (ADAMTS13), an enzyme that cleaves von Willebrand factor (vWF). With severely low ADAMTS13 activity (<10%), large multimers of vWF accumulate and bind platelets, forming microvasculature thromboses that cause ischemic organ injury (eg, myocardial infarction and stroke). The incidence of iTTP is higher in adults than children (incident rate ratio [IRR], 31.62 per million person-years [95% CI, 14.68-68.10]), females than males (IRR, 3.19 [95% CI, 2.65-3.85]), and Black compared with non-Black individuals (IRR, 7.09 [95% CI, 6.05-8.31]). Common presenting symptoms are neurologic (eg, headache, confusion, or seizures [39%-80%]) and abdominal pain (35%-39%). For patients presenting with MAHA and thrombocytopenia, clinical prediction scores for iTTP using laboratory data, such as platelet count less than 30 × 109/L and creatinine level less than 2.0 mg/dL (176.8 μmol/L), can help guide empirical treatment initiation for iTTP before ADAMTS13 results are available. Prompt initiation of therapy with therapeutic plasma exchange, corticosteroids, and rituximab improves survival with iTTP from almost zero to approximately 93%. Caplacizumab, a synthetic small antibody (nanobody) that blocks platelet binding to vWF, administered concurrently with immunosuppression and therapeutic plasma exchange and continued until ADAMTS13 recovery, reduces the time to normalization of platelet count and decreases the risk of early recurrence (defined as within 30 days of completing therapeutic plasma exchange) compared with placebo (risk difference [RD], -29% [95% CI, -42 to -14%]) but increases bleeding risk (RD, 17% [95% CI, 4%-30%]). After obtaining clinical remission (defined as at least 30 days of sustained normalization of platelet count, decreased serum lactate dehydrogenase level, and absence of new or progressive ischemic organ injury without therapeutic plasma exchange or caplacizumab), 16% of patients have at least 1 relapse of iTTP. Regular monitoring of ADAMTS13 activity in remission and administration of rituximab when ADAMTS13 activity is less than 20% reduces risk of relapse (odds ratio, 0.09 [95% CI, 0.04-0.24]).

Conclusions and Relevance

Immune TTP is a rare immune-mediated disorder that presents with thrombocytopenia and MAHA and may cause life-threatening thrombosis. Treatment with therapeutic plasma exchange, corticosteroids, and rituximab is associated with 30-day survival rates of more than 90%. Addition of caplacizumab shortens time to normalization of platelet count and reduces recurrences while receiving the drug but increases bleeding risk. Monitoring ADAMTS13 activity in survivors and initiation of rituximab for those with low ADAMTS13 activity reduces the risk of clinical relapse.

A novel automated chemiluminescent enzyme immunoassay for ADAMTS-13 activity enables accompanying measurements of the inhibitory autoantibodies

BACKGROUND

Thrombotic thrombocytopenic purpura (TTP) is a fatal disease caused by severe deficiency in ADAMTS-13 (a disintegrin and metalloproteinase with thrombospondin type 1 motifs 13) activity. ADAMTS-13 activity measurement is essential for the diagnosis of TTP, but conventional standard assays are manual and time-consuming. Automated ADAMTS-13 activity assays have recently become available; however, their accuracy remains challenging.

OBJECTIVES

We here developed a novel chemiluminescent enzyme immunoassay (CLEIA) for ADAMTS-13 activity that is fully automated, highly sensitive, and has a short reaction time (17 minutes). We evaluated the utility of our fully automated CLEIA for measuring ADAMTS-13 activity and inhibitory antibodies and compared it with conventional manual assays.

METHODS

We compared our CLEIA for ADAMTS-13 activity and inhibitory antibodies with an in-house FRETS-VWF73 assay and commercial enzyme-linked immunosorbent assay (ELISA) using samples from 100 patients and 50 healthy donors. Agreement between assays was evaluated using a cutoff value of 10 international units/dL for ADAMTS-13 activity and 0.5 Bethesda units/mL for inhibitory antibodies.

RESULTS

The CLEIA and conventional assays for ADAMTS-13 activity correlated well. The CLEIA showed high agreement with the FRETS-VWF73 assay (kappa = 0.96) and ELISA (kappa = 1.0) in classifying patients with a cutoff value of 10 international units/dL for ADAMTS-13 activity. Furthermore, in classifying patients with the cutoff value of 0.5 Bethesda units/mL for inhibitory antibodies, the CLEIA agreed strongly with the FRETS-VWF73 assay (kappa = 0.95) and ELISA (kappa = 0.98). Its diagnostic performance for TTP was satisfactory.

CONCLUSION

The high-performance and fully automated CLEIA enables rapid in-hospital diagnosis and follow-up of TTP, as well as detection of inhibitory ADAMTS-13 autoantibodies.

Laboratory Testing for ADAMTS13 for Thrombotic Thrombocytopenia Purpura and Beyond

ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), also called von Willebrand factor (VWF) cleaving protease, acts as a moderator of VWF activity. ADAMTS13 cleaves VWF multimers, thereby reducing VWF activity in blood. When ADAMTS13 is absent (e.g., in patients with TTP [thrombotic thrombocytopenia purpura]), accumulation of VWF in plasma can occur, particularly as "ultra-large" VWF multimers, with this leading to adverse outcomes such as thrombosis. Relative ADAMTS13 deficiencies also occur in several other conditions, including secondary thrombotic microangiopathies (TMA), cancer, and with severe infections such as in COVID-19 (coronavirus disease 2019). These situations might therefore be accompanied with relative loss of ADAMTS13, thereby potentially also leading to pathological VWF accumulation, with this then generating a prothrombotic milieu, thus contributing to enhance the risk of thrombosis. Laboratory testing for ADAMTS13 can aid in the diagnosis of such disorders (i.e., TTP, TMA), and help guide their management, with testing now accomplished using various assays. As most presentations of TTP reflect an acquired condition due to anti-ADAMTS13 antibodies, there may also be a need to test for these, as this will also influence clinical management. We herein provide an overview of TTP, note other conditions in which low levels of ADAMTS13 may be present, and then detail laboratory testing for both ADAMTS13 and associated inhibitors.

Practical Considerations for the Use of the Rapid AcuStar® ADAMTS13 Activity Assay in the Diagnosis of Acute Thrombotic Thrombocytopenic Purpura (TTP)

Introduction: Conventional practice in the management of acute TTP entails empirical treatment of suspected cases whilst awaiting confirmatory ADAMTS13 deficiency testing. Rapid ADAMTS13 assays offer increased accessibility and rapid diagnostics. The new automated HemosIL AcuStar® ADAMTS13 assay has seen increasing use among UK TTP Specialist Centres alongside the traditional ELISA method to confirm severe ADAMTS13 deficiency. Methods: A multi-centre retrospective case-control study was performed to review patients demonstrating discrepant ADAMTS13 activity results measured using rapid (AcuStar®) and ELISA assays in parallel from September 2019 to December 2021. Cases were compared with a cohort of suspected TTP patients exhibiting no difference in assay results and in relation to their presenting characteristics and pre-test probability of a diagnosis of TTP. Results: Where the clinical index of suspicion for TTP was high at presentation, acute TTP was confirmed using the AcuStar® assay < 0.2 IU/dL and subsequently < 10 IU/dL by ELISA with zero incidence of discrepancy. For patients with low clinical suspicion of acute TTP, a discrepancy between the AcuStar® and ELISA assay results was observed in 2% of cases; 5-10 IU/dL in AcuStar®, confirmed as >20 IU/dL by ELISA. A concurrent cancer diagnosis or sepsis was observed in 40% of discrepant cases. Conclusions: Where acute TTP is strongly suspected, there is a good correlation between the rapid AcuStar® ADAMTS13 assay and the conventional ELISA assay. Where the clinical suspicion of acute TTP is low, caution should be exercised in the interpretation of the ADAMTS13 activity using the AcuStar® assay. Accurate interpretation requires robust ADAMTS13 testing algorithms to be incorporated into diagnostic pathways.

ADAMTS13 in the New Era of TTP

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening, often immune-mediated disease that affects 2-13 persons per million per year. Hemolytic anemia, thrombocytopenia, and end-organ damage due to the formation of microthrombi are characteristic of TTP. ADAMTS13 is a disintegrin, metalloproteinase, cleaving protein of von Willebrand factor (VWF) that processes the VWF multimers to prevent them from interacting with platelets and, in turn, to microvascular thrombosis. Prompt diagnosis of TTP is critical yet challenging. Thrombotic microangiopathies have similar clinical presentation. Measurement of ADAMTS13 activity helps in the differential diagnosis. Less than 10% ADAMTS13 activity is indicative of TTP. Laboratory ADAMTS13 activity assays include incubating the test plasma with the substrate (full-length VWM multimers) and detection with direct or indirect measurement of the cleavage product. The purpose of this study is to examine the diagnostic potential, advantages, and weaknesses of the ADAMTS13 potency in TTP.

A comparative study of anti-ADAMTS-13 antibody dynamics in immune-mediated thrombotic thrombocytopenic purpura

Background

Thrombotic thrombocytopenic purpura, particularly its immune-mediated variant (iTTP), necessitates accurate diagnostic approaches for effective management.

Objectives

To compare a chemiluminescence immunoassay (CLIA) and an enzyme-linked immunosorbent assay (ELISA) for testing ADAMTS-13 activity and detecting anti-ADAMTS-13 autoantibodies (AAbs) in patients with iTTP.

Methods

This study involved 31 paired samples from 12 iTTP patients. ADAMTS-13 activity was measured using the HemosIL AcuStar (Instrumentation Laboratory, CLIA) and Technozym (Technoclone) activity assay (ELISA). The presence of AAbs was assessed using Technozym ADAMTS-13-INH assay (ELISA) and HemosIL AcuStar activity (CLIA) within a Bethesda assay following mixing with normal pool plasma. von Willebrand factor (VWF) multimers were analyzed using the HYDRASYS-2 SCAN system and the HYDRAGEL 5- or 11-VW Multimer kits (Sebia). VWF activity levels were measured with the HemosIL AcuStar VWF:GPIbR on the ACL AcuStar Analyzer (IL).

Results

For ADAMTS-13 activity, a strong linear relationship and no bias between CLIA and ELISA were confirmed (slope = 1.01 [0.91, 1.11], intercept = 0.00 [-0.47, 0]). However, significant discrepancies were found in AAb detection during remission phases with ADAMTS-13 activity between 10% and 50%, with CLIA and ELISA showing significant divergence (P < .001, Cohen's g = 0.34). Consistently, VWF multimers and activity levels exhibited significantly different values between remission samples with ADAMTS-13 activity below 50% and above 50%. In longitudinal analysis of patients with multiple iTTP relapses, positivity to CLIA appears to precede ELISA in predicting exacerbations.

Conclusion

While CLIA and ELISA might be interchangeable for assessing ADAMTS-13 activity, they are not equivalent for detecting AAbs, particularly in patients in clinical remission with ADAMTS-13 activity between 10% and 50%.

Cost-effectiveness of rapid vs in-house vs send-out ADAMTS13 testing for immune thrombotic thrombocytopenic purpura

ABSTRACT

While awaiting confirmatory results, empiric therapy for patients suspected to have immune thrombotic thrombocytopenic purpura (iTTP) provides benefits and also accrues risks and costs. Rapid assays for ADAMTS13 may be able to avoid the cost and risk exposure associated with empiric treatment. We conducted, to our knowledge, the first cost-effectiveness evaluation of testing strategies with rapid vs traditional ADAMTS13 assays in patients with intermediate- to high-risk PLASMIC scores, with and without caplacizumab use. We built a Markov cohort simulation with 4 clinical base-case analyses: (1) intermediate-risk PLASMIC score with caplacizumab; (2) intermediate-risk PLASMIC score without caplacizumab; (3) high-risk PLASMIC score with caplacizumab; and (4) high-risk PLASMIC score without caplacizumab. Each of these evaluated 3 testing strategies: (1) rapid assay (<1-hour turnaround); (2) in-house fluorescence resonance energy transfer (FRET)-based assay (24-hour turnaround); and (3) send-out FRET-based assay (72-hour turnaround). The primary outcome was the incremental net monetary benefit reported over a 3-day time horizon and across accepted willingness-to-pay thresholds in US dollars per quality-adjusted life-year (QALY). While accruing the same amount of QALYs, the rapid assay strategy saved up to $46 820 (95% CI, $41 961-$52 486) per patient tested. No parameter variation changed the outcome. In probabilistic sensitivity analyses, the rapid assay strategy was favored in 100% (3 base cases and scenario analyses) and 99% (1 base-case and scenario analysis) across 100 000 Monte Carlo iterations within each. Rapid ADAMTS13 testing for patients with intermediate- or high-risk PLASMIC scores yields significant per patient cost savings, achieved by reducing the costs associated with unnecessary therapeutic plasma exchange and caplacizumab therapy in patients without iTTP.

The Role of the von Willebrand Factor Collagen-Binding Assay (VWF:CB) in the Diagnosis and Treatment of von Willebrand Disease (VWD) and Way Beyond: A Comprehensive 36-Year History

The von Willebrand factor (VWF) collagen binding (VWF:CB) assay was first reported for use in von Willebrand diagnostics in 1986, by Brown and Bosak. Since then, the VWF:CB has continued to be used to help diagnose von Willebrand disease (VWD) (correctly) and also to help assign the correct subtype, as well as to assist in the monitoring of VWD therapy, especially desmopressin (DDAVP). However, it is important to recognize that the specific value of any VWF:CB is predicated on the use of an optimized VWF:CB, and that not all VWF:CB assays are so optimized. There are some good commercial assays available, but there are also some "not-so-good" commercial assays available, and these may continue to give the VWF:CB "a bad reputation." In addition to VWD diagnosis and management, the VWF:CB found purpose in a variety of other applications, from assessing ADAMTS13 activity, to investigation into acquired von Willebrand syndrome (especially as associated with use of mechanical circulatory support or cardiac assist devices), to assessment of VWF activity in disease states in where an excess of high-molecular-weight VWF may accumulate, and lead to increased (micro)thrombosis risk (e.g., coronavirus disease 2019, thrombotic thrombocytopenic purpura). The VWF:CB turns 37 in 2023. This review is a celebration of the utility of the VWF:CB over this nearly 40-year history.

Medical consult: aHUS, TTP? How to distinguish and what to do

Immune thrombotic thrombocytopenic purpura (iTTP) caused by an autoantibody-mediated deficiency of ADAMTS13 and atypical hemolytic syndrome (aHUS) caused by alternative complement dysregulation are the most common primary thrombotic microangiopathies (TMAs). The evaluation of a patient with TMA is a medical emergency since it is critical to quickly distinguish iTTP and aHUS from other causes of TMA. Untreated iTTP is rapidly fatal, and delays in initiating complement inhibition in aHUS increase the risk of irreversible renal failure. An ADAMTS13 activity level of less than 10% is diagnostic of iTTP in the appropriate clinical setting. In settings where rapid-turnaround ADAMTS13 testing is not available, clinical features and clinical prediction tools are useful to identify patients who should receive emergent plasma exchange. We present an evidence-based approach to the initial (first 24 hours) diagnosis and management of iTTP and review the clinical and laboratory features that can be used to identify patients with aHUS who will benefit from early C5 blockade. We also discuss the potential use of complement blockade to improve outcomes in selected patients with secondary TMA.

Laboratory investigation and diagnosis of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a rare and potentially fatal disease for which rapid diagnosis is crucial for patient outcomes. Deficient activity (< 10%) of the liver enzyme, ADAMTS13, is the pathophysiological hallmark of TTP, and measurement of the enzyme activity can establish the diagnosis of TTP with high accuracy. Thus, along with the clinical history, appropriate laboratory assessment of a suspected case of TTP is essential for diagnosis and treatment. Here, we present a review of the available laboratory tests that can assist clinicians in establishing the diagnosis of TTP, with special focus on ADAMTS13 assays, including the measurement of the antigen and activity, and detection of autoantibodies to ADAMTS13.

Novel mechanism of the COVID-19 associated coagulopathy (CAC) and vascular thromboembolism

Previous studies from our laboratory revealed that SARS-CoV-2 spike protein (SP) administration to a genetically engineered model expressing the human angiotensin-converting enzyme 2; ACE2 receptor (i.e., hACE2 humanized mouse) mimicked the coronavirus disease-19 (COVID-19) pathology. In humans the cause of high morbidity, and mortality is due to 'cytokine-storm' led thromboembolism; however, the exact mechanisms of COVID-19 associated coagulopathy (CAC) have yet to be discovered. Current knowledge suggests that CAC is distinct from the standard coagulopathy, in that the intrinsic and extrinsic thrombin-dependent coagulation factors, and the pathway(s) that are common to coagulopathy, are not recruited by SARS-CoV-2. Findings from patients revealed that there is little change in their partial thromboplastin, or the prothrombin time coupled with a significant decline in platelets. Further, there appears to be an endothelial dysfunction during COVID-19 suggesting an interaction of the endothelia with immune cells including neutrophils. There are also reports that inflammatory NGAL is elevated during COVID-19. Furthermore, the levels of NPT are also increased indicating an increase in inflammatory M1 macrophage iNOS which sequesters BH4; an essential enzyme co-factor that acts as a potent antioxidant thus causing damage to endothelia. SARS-CoV-2 entry into the host cells is facilitated by a co-operative action between TMPRSS2 and the main ACE2 receptor. Interestingly, after infection ADAMTS13; a von Willebrand factor; VWF cleaving enzyme is found to be decreased. Based on these facts, we hypothesize that vascular thromboembolism is associated with serine and metalloproteinase, and in that context, we opine that inhibition of iNOS might help mitigate COVID-19 harmful effects. To test this hypothesis, we administered SP to the hACE2 mice that were subsequently treated with amino guanidine (AG; a potent inhibitor of glycoxidation, lipoxidation and oxidative vicious cycles). Our results revealed increase in TMPRSS2, and NGAL by SP but treatment with AG mitigated their levels. Similarly, levels of MMP-2, and -9 were increased; however, AG treatment normalized these levels. Our findings suggest that occurrence of CAC is influenced by TMPRSS2, ADAMTS13, NGAL and MMP- 2, and -9 factors, and an intervention with iNOS blocker helped mitigate the CAC condition in experimental settings.

Global Health Resource Utilization and Cost-Effectiveness of Therapeutics and Diagnostics in Immune Thrombotic Thrombocytopenic Purpura (TTP)

In this review, we examine the current landscape of health resource utilization and cost-effectiveness data in the care of patient populations with immune thrombotic thrombocytopenic purpura. We focus on the therapeutic (therapeutic plasma exchange, glucocorticoids, rituximab, caplacizumab) and diagnostic (ADAMTS13 assay) health technologies employed in the care of patients with this rare disease. Health resource utilization and cost-effectiveness data are limited to the high-income country context. Measurement of TTP-specific utility weights in the high-income country context and collection of health resource utilization data in the low- and middle-income country settings would enable an evaluation of country-specific quality-adjusted life expectancy and cost-effectiveness of these therapeutic and diagnostic health technologies. This quantification of value is one way to mitigate cost concerns where they exist.

Challenges in the diagnosis of thrombotic thrombocytopenic purpura

INTRODUCTION

Immune-mediated TTP (iTTP) is a rare condition without pathognomonic signs and symptoms. For this reason, the diagnosis of iTTP may be delayed or even missed, with potentially catastrophic consequences.

AREAS COVERED

The authors performed an extensive literature review on the diagnosis of iTTP and its challenges combined with their own experience in a referral center for patients with iTTP.

EXPERT OPINION

Although a definitive diagnosis of iTTP depends on the ADAMTS13 activity result, timely testing is rarely available at many centers to which patients present. If less complex tests were to become available, they would decrease the chances of late and/or missed diagnoses of iTTP throughout the world. While clinical scores to estimate the likelihood of iTTP exist, they are not well known, and can be misleading if used in the wrong context. Furthermore, the three scoring systems (PLASMIC, Bentley, and French) only moderately correlate with each other, which further complicates the landscape. The existence of these scores and how they should be used in practice is but one opportunity that can be seized through more robust programs to educate nonspecialist clinicians on how to recognize and treat patients with iTTP.

From the Discovery of ADAMTS13 to Current Understanding of Its Role in Health and Disease

ADAMTS13 (a disintegrin-like metalloprotease domain with thrombospondin type 1 motif, member 13) is a protease of crucial importance in the regulation of the size of von Willebrand factor multimers. Very low ADAMTS13 activity levels result in thrombotic thrombocytopenic purpura, a rare and life-threatening disease. The mechanisms involved can either be acquired (immune-mediated thrombotic thrombocytopenic purpura [iTTP]) or congenital (cTTP, Upshaw-Schulman syndrome) caused by the autosomal recessive inheritance of disease-causing variants (DCVs) located along the ADAMTS13 gene, which is located in chromosome 9q34. Apart from its role in TTP, and as a regulator of microthrombosis, ADAMTS13 has begun to be identified as a prognostic and/or diagnostic marker of other diseases, such as those related to inflammatory processes, liver damage, metastasis of malignancies, sepsis, and different disorders related to angiogenesis. Since its first description almost 100 years ago, the improvement of laboratory tests and the description of novel DCVs along the ADAMTS13 gene have contributed to a better and faster diagnosis of patients under critical conditions. The ability of ADAMTS13 to dissolve platelet aggregates in vitro and its antithrombotic properties makes recombinant human ADAMTS13 treatment a potential therapeutic approach targeting not only patients with cTTP but also other medical conditions.

Evaluation of thrombotic thrombocytopenic purpura and other thrombotic microangiopathies: Lessons learned from a 14-year retrospective study

INTRODUCTION

Thrombotic thrombocytopenic purpura (TTP) is a clinical thrombotic microangiopathy (TMA) syndrome defined by the pentad of symptoms. Therapeutic plasma exchange with plasma replacement is an ASFA Category I modality that can reduce morbidity and mortality if initiated early. We describe a 14-year review of patients referred for plasma exchange with a suspected diagnosis of TTP.

METHODS

For 70 patients referred for urgent plasma exchange, clinical, therapeutic, and laboratory data were retrospectively analyzed, and the diagnosis was determined.

RESULTS

Fifteen of the patients were diagnosed with TTP based upon ADAMTS-13 activity with the other 51 patients having other non-TTP TMA diagnoses. The mortality rate was significant for both TTP and non-TTP TMAs. PLASMIC scores were also calculated retrospectively and were noted to have limited value. TMA is a diagnostic challenge and encompasses different syndromes with similar presentations.

CONCLUSION

Determining an accurate diagnosis, including prompt ADAMTS-13 testing, makes it possible to initiate appropriate therapy for the multiple different TMAs that can be seen in clinical practice.

Harmonization of Hemostasis Testing Across a Large Laboratory Network: An Example from Australia

Harmonization and standardization of laboratory tests and procedures carry a variety of benefits. For example, within a laboratory network, harmonization/standardization provides a common platform for test procedures and documentation across different laboratories. This enables staff to be deployed across several laboratories, if required, without additional training, since test procedures and documentation are the "same" in the different laboratories. Streamlined accreditation of laboratories is also facilitated, as accreditation in one laboratory using a particular procedure/documentation should simplify the accreditation of another laboratory in that network to the same accreditation standard. In the current chapter, we detail our experience regarding the harmonization and standardization of laboratory tests and procedures related to hemostasis testing in our laboratory network, NSW Health Pathology, representing the largest public pathology provider in Australia, with over 60 separate laboratories.

Impact of Concomitant Thrombotic Thrombocytopenic Purpura on COVID-19 Mortality and Morbidity: A Nationwide Inpatient Sample Analysis

Utilizing the comprehensive Nationwide Inpatient Sample (NIS) database, we examined the impact of thrombotic thrombocytopenic purpura (TTP) on the outcomes of patients with coronavirus disease-19 (COVID-19), emphasizing the potential role of the ADAMTS13 enzyme in disease pathogenesis and evolution. We analyzed extensive data from the NIS database using STATA v.14.2 and accounted for potential confounders using multivariate regression analysis to uphold the validity and reliability of the study. Among 1 050 045 adult patients hospitalized with COVID-19, only 300 (0.03%) developed TTP. These patients were younger (mean age 57.47 vs 64.74, P < .01) and exhibited a higher prevalence of preexisting conditions, such as congestive heart failure (13.33% vs 16.82%, P value not provided) and end-stage renal disease (3.33% vs 3.69%, P value not provided). On multivariate regression analysis, COVID-19 patients with concomitant TTP demonstrated a significant increase in mortality (adjusted odds ratio [AOR] 3.99, P < .01), venous thromboembolism (AOR 3.33, P < .01), acute kidney injury (AOR 7.36, P < .01), gastrointestinal bleeding (AOR 10.75, P < .01), intensive care unit admission (AOR 14.42, P < .01), length of hospital stay (17.42 days, P < .01), and total hospitalization charges ($298 476, P < .01). Thrombotic thrombocytopenic purpura in COVID-19 patients elevates the risk of mortality and complications, likely driven by the thrombotic nature of TTP. Our data underline the potential significance of ADAMTS13 in COVID-19 and TTP pathophysiology, suggesting its possible role as a therapeutic target.

ADAMTS13 Activity Measurement by ELISA and Fluorescence Resonance Energy Transfer Assay

Accurate estimation of ADAMTS13 (a disintegrin-like and metalloprotease with thrombospondin type 1 motif, member 13) activity level is crucial in the diagnostic setting of differentiation between thrombotic thrombocytopenic purpura (TTP) and other thrombotic microangiopathies. The original assays were too cumbersome and time-consuming for use in the acute situation, and treatment was often based on clinical findings alone, with confirmatory laboratory assays following days or weeks later. Rapid assays are now available that can generate results fast enough to impact on immediate diagnosis and management. Assays based on fluorescence resonance energy transfer (FRET) or chemiluminescence principles can generate results in less than an hour, although they require specific analytical platforms. Enzyme-linked immunosorbent assays (ELISA) can generate results in about 4 h, but do not require specialized equipment beyond ELISA plate readers that are in regular use in many laboratories. The present chapter describes principles, performance, and practical aspects of an ELISA and a FRET assay, for quantitative measurement of ADAMTS13 activity in plasma.

Automated and Rapid ADAMTS13 Testing Using Chemiluminescence: Utility for Identification or Exclusion of TTP and Beyond

Thrombotic thrombocytopenic purpura (TTP) is a prothrombotic condition caused by a significant deficiency of the enzyme, ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). In the absence of adequate levels of ADAMTS13 (i.e., in TTP), plasma VWF accumulates, in particular as "ultra-large" VWF multimers, and this leads to pathological platelet aggregation and thrombosis. In addition to TTP, ADAMTS13 may be mildly to moderately reduced in a range of other conditions, including secondary thrombotic microangiopathies (TMA) such as those caused by infections (e.g., hemolytic uremic syndrome (HUS)), liver disease, disseminated intravascular coagulation (DIC), and sepsis, during acute/chronic inflammatory conditions, and sometimes also in COVID-19 (coronavirus disease 2019)). ADAMTS13 can be detected by a variety of techniques, including ELISA (enzyme-linked immunosorbent assay), FRET (fluorescence resonance energy transfer) and by chemiluminescence immunoassay (CLIA). The current report describes a protocol for assessment of ADAMTS13 by CLIA. This protocol reflects a rapid test able to be performed within 35 min on the AcuStar instrument (Werfen/Instrumentation Laboratory), although certain regional approvals may also permit this testing to be performed on a BioFlash instrument from the same manufacturer.

An Overview of Laboratory Testing for ADAMTS13

ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) is also called von Willebrand factor (VWF) cleaving protease (VWFCP). ADAMTS13 acts to cleave VWF multimers and thus reduce plasma VWF activity. In the absence of ADAMTS13 (i.e., in thrombotic thrombocytopenia purpura, TTP), plasma VWF can accumulate, in particular as "ultra-large" VWF multimers, and this can lead to thrombosis. Relative deficiencies in ADAMTS13 can also occur in a variety of other conditions, including secondary thrombotic microangiopathies (TMA). Of contemporary interest, COVID-19 (coronavirus disease 2019) may also be associated with relative reduction of ADAMTS13 and also pathological accumulation of VWF, with this likely contributing to the thrombosis risk seen in affected patients. Laboratory testing for ADAMTS13 can assist in the diagnosis of these disorders (i.e., TTP, TMA), as well as in their management, and can be achieved using a variety of assays. This chapter therefore provides an overview of laboratory testing for ADAMTS13 and the value of such testing to assist the diagnosis and management of associated disorders.

Auto-validation of Routine Coagulation/Hemostasis Assays with Reflex Testing of Abnormal Test Results

Hemostasis laboratories play a crucial role in the diagnosis and treatment of individuals with bleeding or thrombotic disorders. Routine coagulation assays, including the prothrombin time (PT)/international normalized ratio (INR), and activated partial thromboplastin time (APTT), are used for various purposes. These include as a screen of hemostasis function/dysfunction (e.g., possible factor deficiency) and for monitoring of anticoagulant therapy, such as vitamin K antagonists (PT/INR) and unfractionated heparin (APTT). Clinical laboratories are also under increasing pressure to improve services, especially response (test turnaround) times. There is also a need for laboratories to try to reduce error rates and for laboratory networks to standardize/harmonize processes and policies. Accordingly, we describe our experience with the development and implementation of automated processes for reflex testing and validation of routine coagulation test results. This has been implemented in a large pathology network compromising 27 laboratories and is under consideration for expansion to our larger network (of 60 laboratories). These rules have been custom-built within our laboratory information system (LIS), perform reflex testing of abnormal results, and fully automate the process of routine test validation for appropriate results. These rules also permit adherence to standardized pre-analytical (sample integrity) checks, automate reflex decisions, automate verification, and provide an overall alignment of network practices in a large network of 27 laboratories. In addition, the rules enable clinically significant results to be quickly referred to hematopathologists for review. We also documented an improvement in test turnaround times, with savings in operator time and thus operating costs. Finally, the process was generally well received and determined to be beneficial for most laboratories in our network, in part identified by improved test turnaround times.

Identification of ADAMTS13 Inhibitors in Acquired TTP

Thrombotic thrombocytopenic purpura (TTP) is a prothrombotic condition caused by a deficiency of ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). In turn, ADAMTS13 (also called von Willebrand factor (VWF) cleaving protease (VWFCP)) acts to cleave VWF multimers and thus reduce plasma VWF activity. In the absence of ADAMTS13 (i.e., in TTP), plasma VWF accumulates, in particular as "ultra-large" VWF multimers, and this leads to thrombosis. In most patients with confirmed TTP, ADAMTS13 deficiency is an acquired disorder due to the development of antibodies against ADAMTS13, which either promote clearance of ADAMTS13 from circulation or cause inhibition of ADAMTS13 activity. The current report describes a protocol for assessment of ADAMTS13 inhibitors, being antibodies that inhibit ADAMTS13 activity. The protocol reflects the technical steps that help identify inhibitors to ADAMTS13, whereby mixtures of patient plasma and normal plasma are then tested for residual ADAMTS13 activity in a Bethesda-like assay. The residual ADAMTS13 activity can be assessed by a variety of assays, with a rapid test able to be performed within 35 minutes on the AcuStar instrument (Werfen/Instrumentation Laboratory) used as an example in this protocol.

ADAMTS13 Activity: Screening Test Protocol

Accurate estimation of ADAMTS13 (a disintegrin-like and metalloprotease with thrombospondin type 1 motif, member 13) activity level is necessary for diagnosis and management of thrombotic microangiopathies (TMA). In particular, it permits distinction between thrombotic thrombocytopenic purpura (TTP) and other TMAs, prompting disorder appropriate treatment. Manual and automated quantitative assays of ADAMTS13 activity are commercially available, some providing results within less than an hour, but they require specialist equipment and personnel and tend to only be available in specialized diagnostic facilities. Technoscreen ADAMTS13 Activity is a rapid, commercially available, semiquantitative screening test employing flow-through technology and an ELISA activity assay principle. It is a simple to perform screening tool, not requiring specialist equipment or personnel. The colored end point is compared to a reference color chart containing four color intensity indicators corresponding to ADAMTS13 activity levels of 0, 0.1, 0.4, or 0.8 IU/mL. Reduced levels detected in the screening test should be confirmed by quantitative assay. The assay lends itself to use in nonspecialized laboratories, remote, and point-of-care settings.

Autoimmune Diseases Affecting Hemostasis: A Narrative Review

Hemostasis reflects a homeostatic mechanism that aims to balance out pro-coagulant and anti-coagulant forces to maintain blood flow within the circulation. Simplistically, a relative excess of procoagulant forces can lead to thrombosis, and a relative excess of anticoagulant forces can lead to bleeding. There are a wide variety of congenital disorders associated with bleeding or thrombosis. In addition, there exist a vast array of autoimmune diseases that can also lead to either bleeding or thrombosis. For example, autoantibodies generated against clotting factors can lead to bleeding, of which acquired hemophilia A is the most common. As another example, autoimmune-mediated antibodies against phospholipids can generate a prothrombotic milieu in a condition known as antiphospholipid (antibody) syndrome (APS). Moreover, there exist various autoimmunity promoting environments that can lead to a variety of antibodies that affect hemostasis. Coronavirus disease 2019 (COVID-19) represents perhaps the contemporary example of such a state, with potential development of a kaleidoscope of such antibodies that primarily drive thrombosis, but may also lead to bleeding on rarer occasions. We provide here a narrative review to discuss the interaction between various autoimmune diseases and hemostasis.

Diagnosis of Thrombotic Thrombocytopenic Purpura by ADAMTS13 Activity Quantification

BACKGROUND

Rapid quantification of ADAMTS13 activity in plasma is essential for establishing a diagnosis of thrombotic thrombocytopenic purpura (TTP); a rare, but potentially lethal disorder. The current methods for quantitating ADAMTS13 activity are manual and only available at specialized laboratories, which often results in initiation of specific treatments long before a diagnosis of TTP is established.

METHODS

We compared the performance of the HemosIL, a novel and rapid automated method, and the current standard FRET (fluorescence resonance energy transfer) method in quantitating ADAMTS13 activity using 706 consecutive plasma samples collected over a period of 14 years. The clinical accuracy of both methods was further examined using 212 diagnostic samples.

RESULTS

The correlation between the FRET and HemosIL methods in all 706 samples and in the 212 diagnostic samples was excellent (Pearson's r of 0.919 and 0.912, respectively). Both methods displayed a high degree of clinical accuracy using the current cutoff of ADAMTS13 activity <0.10 kIU/L (<10%) as diagnostic for TTP: the area under the curve (AUC) was 97.7% for the FRET method and 99.5% for the HemosIL method. When applying a lower cutoff (ADAMTS13 activity <0.05 kIU/L or <5%), the diagnostic accuracy of the HemosIL method increased further (AUC = 99.7%).

CONCLUSIONS

A novel, rapid method for ADAMTS13 quantification is comparable to the more laborious FRET method in patients with possible TTP. A rapid analysis available in the acute setting assessing patients with possible TTP allows for improved care and optimized treatment of a life-threatening condition.

The Intriguing Relationships of von Willebrand Factor, ADAMTS13 and Cardiac Disease

von Willebrand factor (VWF) is an adhesive protein involved in primary hemostasis and facilitates platelet adhesion to sites of vascular injury, thereby promoting thrombus formation. VWF exists in plasma as multimers of increasing size, with the largest (high molecular weight; HMW) expressing the greatest functional activity. A deficiency of VWF is associated with a bleeding disorder called von Willebrand disease (VWD), whereas an excess of VWF, in particular the HMW forms, is associated with thrombosis. ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motif-13), also known as VWF-cleaving protease, functions to moderate the activity of VWF by cleaving multimers of VWF and limiting the expression of the largest multimers of VWF. A deficiency of ADAMTS13 is therefore associated with an excess of (HMW forms of) VWF, and thus thrombosis. Indeed, any disturbance of the VWF/ADAMTS13 ratio or ‘axis’ may be associated with pathophysiological processes, including prothrombotic tendency. However, both thrombosis or bleeding may be associated with such disturbances, depending on the presenting events. This review evaluates the relationship of VWF and ADAMTS13 with cardiac disease, including cardiac failure, and associated pathophysiology.

ADAMTS13 testing update: Focus on laboratory aspects of difficult thrombotic thrombocytopenic purpura diagnoses and effects of new therapies

TTP is a life-threatening disorder diagnosed using a combination of clinical information and laboratory results. ADAMTS13 activity and antibody testing represent a major advance in the field, but results can sometimes be difficult to interpret due to technical aspects of the tests and characteristics of the causative antibodies in acquired TTP. Genetic testing for ADAMTS13 mutations is also now available to assist with the diagnosis of inherited TTP. This review will focus on ADAMTS13 testing and will highlight patient and laboratory aspects that can lead to diagnostic difficulty. The effects of TTP therapies on test results will also be discussed.

Laboratory testing for ADAMTS13: Utility for TTP diagnosis/exclusion and beyond

The metalloproteinase ADAMTS13 (a disintegrin with a thrombospondin type 1 motif, member 13), also known as VWF (von Willebrand factor) protease, may be assessed in a vast array of clinical conditions. Notably, a severe deficiency of ADAMTS13 characterizes TTP (thrombotic thrombocytopenic purpura), a rare but potentially fatal disorder associated with thrombosis due to accumulation of prothrombotic ultra-large VWF multimers. Although prompt identification/exclusion of TTP can be facilitated by rapid ADAMTS13 testing, the most commonly utilized assays are based on ELISA (enzyme linked immunosorbent assay) and require long turnaround time and have relatively limited throughput. Nevertheless, several rapid ADAMTS13 assays are now available, at least in select geographies. The current mini-review discusses these issues, as well as the potential utility of ADAMTS13 testing in a range of other conditions, including coronavirus disease 2019 (COVID-19).

Increased VWF and Decreased ADAMTS-13 in COVID-19: Creating a Milieu for (Micro)Thrombosis

von Willebrand factor (VWF) is a large adhesive multimeric protein involved in hemostasis. The larger the size (or number of VWF multimers), the greater the functionality of the protein. A deficiency or defect of VWF can lead to von Willebrand disease (VWD) and cause bleeding. Conversely, an increase in VWF may create an environment that promotes thrombosis. ADAMS-13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), sometimes called VWF-cleaving protease, is primarily responsible for controlling the size of VWF. The most severe deficiency (<10% of normal levels) of ADAMTS-13 arises in thrombotic thrombocytopenic purpura, a condition characterized by the presence of ultralarge VWF and clinically resulting in enhanced risk of thrombosis. However, ADAMTS-13 deficiency may result from other pathological processes. Of relevance is the recent finding that COVID-19 (coronavirus disease 2019) is associated with both increased levels and activity of VWF as well as generally decreased (or occasionally normal) activity levels of ADAMTS-13. Thus, in COVID-19 there is an alteration in the VWF/ADAMTS-13 axis, most often described by increased VWF/ADAMTS-13 ratio (or reduced ADAMTS-13/VWF ratio). COVID-19 is also associated with high prothrombotic risk. Thus, the imbalance of VWF and ADAMTS-13 in COVID-19 may be providing a milieu that promotes (micro)thrombosis, in a clinical picture resembling a secondary thrombotic microangiopathy in some patients. This review therefore assesses the literature on VWF, ADAMTS-13, and COVID-19. Whenever reported in COVID-19, VWF has always been identified as raised (compared with normal reference ranges or control populations). Reports have included VWF level (i.e., VWF antigen) and in some cases one or more VWF "activity" (e.g., collagen binding; platelet glycoprotein Ib [GPIb] binding, using ristocetin cofactor or more modern versions including VWF:GPIbR [recombinant] and VWF:GPIbM [mutant]). Whenever reported, ADAMTS-13 has been reported as "normal" or reduced; however, it should be recognized that "normal" levels may still identify a relative reduction in individual cases. Some reports also discuss the raised VWF/ADAMTS-13 (or reduced ADAMTS-13/VWF) ratio, but very few provide actual numerical data.