Heparin-induced thrombocytopenia-associated thrombosis: from arterial to venous to venous limb gangrene

Heparin-induced thrombocytopenia (HIT) is an acquired immune-mediated hypercoagulability state that is strongly associated with thrombosis. During the 1970s and 1980s, the prevailing concept was that HIT was associated only with arterial thrombosis, through its unique pathogenesis via heparin-dependent, platelet-activating IgG antibodies. However, in 1990, when I began to encounter HIT in my clinical practice, I found that most such patients developed symptomatic venous thrombosis. This historical sketch summarizes some of the research that challenged the dogma of HIT being a mainly arterial prothrombotic disorder. Two studies - one a substudy of a randomized trial of post-orthopedic surgery thromboprophylaxis, and the second a retrospective five-hospital analysis of consecutive patients with positive test results for HIT antibodies - showed a marked predominance of venous over arterial thrombosis complicating HIT (~ 4 : 1). By the end of the 1990s, an even more dramatic manifestation of HIT-associated venous thrombosis was recognized: venous limb gangrene. Here, ischemic limb necrosis occurs despite palpable arterial pulses, as a result of both macrovascular and microvascular venous thrombosis. The surprising explanation was natural anticoagulant impairment (severe depletion of protein C, a vitamin K-dependent anticoagulant) resulting from treatment of HIT-associated deep vein thrombosis with warfarin (vitamin K antagonist). These insights from HIT research helped to elucidate the pathogenesis of ischemic limb losses in other intense non-HIT hypercoagulability states, including warfarin-associated venous limb gangrene complicating cancer-associated hypercoagulability, and symmetrical peripheral gangrene complicating disseminated intravascular coagulation of critical illness, in which proximate 'shock liver' helps to explain the profound failure of natural anticoagulant systems (protein C; antithrombin) in predisposing to peripheral limb microthrombosis in circulatory shock.

Cellular immune responses to platelet factor 4 and heparin complexes in patients with heparin-induced thrombocytopenia

Essentials The immunogenesis of Heparin-induced thrombocytopenia (HIT) is not well understood. Immunization to platelet factor 4 (PF4)-heparin occurs early in life, before any heparin exposure. PF4 and PF4-heparin complexes induce the proliferation of CD14+ cells. Reduced levels of regulatory cytokines contribute to immune dysregulation in HIT.

SUMMARY

Background Heparin-induced thrombocytopenia (HIT) is an adverse reaction to heparin characterized by thrombocytopenia and thrombotic complications. HIT is caused by pathogenic antibodies that bind to complexes of platelet factor 4 (PF4) and heparin, leading to platelet activation and inducing a hypercoagulable state. Previous studies have shown immunity to PF4-heparin complexes occurs early in life, even before heparin exposure; however, the immunogenesis of HIT is not well characterized. Objectives To investigate cellular proliferation in response to PF4-heparin complexes in patients with HIT. Patients/Methods Peripheral blood mononuclear cells (PBMCs) from healthy controls (n = 30), postoperative cardiac surgery patients who had undergone cardiopulmonary bypass (CPB) (n = 17) and patients with confirmed HIT (n = 41) were cultured with PF4 and PF4-heparin complexes. Cellular proliferation was assessed by [3 H]thymidine uptake and 5-ethynyl-2'-deoxyuridine detection. Results and Conclusions PBMCs proliferated in the presence of PF4, and this was enhanced by the addition of heparin in all study groups. CPB and HIT patients showed significantly greater proliferative responses than healthy controls. PBMC proliferation was antigen-specific, depended on the presence of platelets, and only CD14+ cells were identified as proliferating cells. Culture supernatants were tested for the levels of regulatory cytokines, and both CPB and HIT patients produced significantly lower levels of interleukin-10 and transforming growth factor-β1 than healthy controls. These findings further demonstrate cellular immune sensitization to PF4-heparin complexes occurs before heparin exposure, and suggests immune dysregulation can contribute to HIT.

Acute haemolysis, DIC and renal failure after transfusion of uncross-matched blood during trauma resuscitation: illustrative case and literature review

AIMS/OBJECTIVES

The aims of this study were to report a patient with acute haemolytic transfusion reaction (HTR) after transfusing uncross-matched red blood cell (RBC) units and to identify the frequency of this complication.

BACKGROUND

Uncross-matched RBC units are commonly transfused in emergencies, but the frequency of acute HTR is unknown.

METHODS

We describe a male stabbing victim who received three units of uncross-matched RBC units complicated by acute intravascular HTR, disseminated intravascular coagulation (DIC) and renal failure. We identified 14 studies evaluating the frequency of acute HTR post-emergency transfusion of uncross-matched RBC units.

RESULTS

Acute HTR was shown by haemoglobinuria, free-plasma haemoglobin and methemalbumin, with anti-K and anti-Fy^a eluted from recipient red cells; acute DIC featured severe hypofibrinogenemia, thrombocytopenia, elevated fibrin D-dimer and multiple bilateral renal infarcts. Two of the three transfused units reacted with pre-existing RBC alloantibodies [anti-K (titre, 128), anti-Fy^a (titre, 512)], explained by transfusion 25 years earlier. Our literature review found the frequency of acute HTR following emergency transfusion of uncross-matched RBC units to be 2/3998 [0·06% (95% CI, 0·01-0·21%)].

CONCLUSIONS

Although emergency transfusion of uncross-matched blood is commonly practiced at trauma centres worldwide, with low risk of acute HTR (<1/1000), our well-documented patient case demonstrates the potential for acute HTR with severe complications.

Autoimmune heparin-induced thrombocytopenia

Autoimmune heparin-induced thrombocytopenia (aHIT) indicates the presence in patients of anti-platelet factor 4 (PF4)-polyanion antibodies that are able to activate platelets strongly even in the absence of heparin (heparin-independent platelet activation). Nevertheless, as seen with serum obtained from patients with otherwise typical heparin-induced thrombocytopenia (HIT), serum-induced platelet activation is inhibited at high heparin concentrations (10-100 IU mL-1 heparin). Furthermore, upon serial dilution, aHIT serum will usually show heparin-dependent platelet activation. Clinical syndromes associated with aHIT include: delayed-onset HIT, persisting HIT, spontaneous HIT syndrome, fondaparinux-associated HIT, heparin 'flush'-induced HIT, and severe HIT (platelet count of < 20 × 109  L-1 ) with associated disseminated intravascular coagulation (DIC). Recent studies have implicated anti-PF4 antibodies that are able to bridge two PF4 tetramers even in the absence of heparin, probably facilitated by non-heparin platelet-associated polyanions (chondroitin sulfate and polyphosphates); nascent PF4-aHIT-IgG complexes recruit additional heparin-dependent HIT antibodies, leading to the formation of large multimolecular immune complexes and marked platelet activation. aHIT can persist for several weeks, and serial fibrin, D-dimer, and fibrinogen levels, rather than the platelet count, may be helpful for monitoring treatment response. Although standard anticoagulant therapy for HIT ought to be effective, published experience indicates frequent failure of activated partial thromboplastin time (APTT)-adjusted anticoagulants (argatroban, bivalirudin), probably because of underdosing in the setting of HIT-associated DIC, known as 'APTT confounding'. Thus, non-APTT-adjusted therapies with drugs such as danaparoid and fondaparinux, or even direct oral anticoagulants, such as rivaroxaban or apixaban, are suggested therapies, especially for long-term management of persisting HIT. In addition, emerging data indicate that high-dose intravenous immunoglobulin can interrupt HIT antibody-induced platelet activation, leading to rapid platelet count recovery.

Clinical effectiveness of a Bayesian algorithm for the diagnosis and management of heparin-induced thrombocytopenia

Essentials We previously published a diagnostic algorithm for heparin-induced thrombocytopenia (HIT). In this study, we validated the algorithm in an independent large healthcare system. The accuracy was 98%, sensitivity 82% and specificity 99%. The algorithm has potential to improve accuracy and efficiency in the diagnosis of HIT.

SUMMARY

Background Heparin-induced thrombocytopenia (HIT) is a life-threatening drug reaction caused by antiplatelet factor 4/heparin (anti-PF4/H) antibodies. Commercial tests to detect these antibodies have suboptimal operating characteristics. We previously developed a diagnostic algorithm for HIT that incorporated 'four Ts' (4Ts) scoring and a stratified interpretation of an anti-PF4/H enzyme-linked immunosorbent assay (ELISA) and yielded a discriminant accuracy of 0.97 (95% confidence interval [CI], 0.93-1.00). Objectives The purpose of this study was to validate the algorithm in an independent patient population and quantitate effects that algorithm adherence could have on clinical care. Methods A retrospective cohort comprised patients who had undergone anti-PF4/H ELISA and serotonin release assay (SRA) testing in our healthcare system from 2010 to 2014. We determined the algorithm recommendation for each patient, compared recommendations with the clinical care received, and enumerated consequences of discrepancies. Operating characteristics were calculated for algorithm recommendations using SRA as the reference standard. Results Analysis was performed on 181 patients, 10 of whom were ruled in for HIT. The algorithm accurately stratified 98% of patients (95% CI, 95-99%), ruling out HIT in 158, ruling in HIT in 10 and recommending an SRA in 13 patients. Algorithm adherence would have obviated 165 SRAs and prevented 30 courses of unnecessary antithrombotic therapy for HIT. Diagnostic sensitivity was 0.82 (95% CI, 0.48-0.98), specificity 0.99 (95% CI, 0.97-1.00), PPV 0.90 (95% CI, 0.56-0.99) and NPV 0.99 (95% CI, 0.96-1.00). Conclusions An algorithm incorporating 4Ts scoring and a stratified interpretation of the anti-PF4/H ELISA has good operating characteristics and the potential to improve management of suspected HIT patients.

Diagnostic accuracy of IgG-specific versus polyspecific enzyme-linked immunoassays in heparin-induced thrombocytopenia: a systematic review and meta-analysis

Essentials Immunoassay specificity varies in heparin-induced thrombocytopenia (HIT) testing. This meta-analysis examined 9 studies that tested samples by both IgG and polyspecific methods. IgG-specific assays confer superior diagnostic accuracy compared with polyspecific assays. These results further support recommendations in favor of IgG-specific testing.

SUMMARY

Background There are conflicting data on whether the IgG-specific or polyspecific antiplatelet factor 4/heparin (PF4/H) enzyme-linked immunosorbent assay (ELISA) is preferred for the laboratory diagnosis of heparin-induced thrombocytopenia (HIT). Objectives To directly compare diagnostic accuracy of IgG-specific versus polyspecific ELISA in HIT. Patients/Methods A systematic search yielded nine studies comprising 1948 patients with suspected HIT tested by both IgG-specific and polyspecific ELISAs and a reference standard against which the diagnostic accuracy of the ELISAs could be measured. Study quality was assessed by QUADAS-2 criteria. Results There was identical sensitivity for IgG-specific and polyspecific ELISAs (0.97; 95% confidence interval (CI), 0.95-0.99) and superior specificity of IgG-specific compared with polyspecific ELISA (0.87 [0.85-0.88] vs. 0.82 [0.80-0.84], respectively). Performance was similar in subgroups using the serotonin release assay and a single commercial ELISA manufacturer. The negative predictive values of IgG-specific and polyspecific ELISA were similarly high (0.99, [0.99-1.00], but the positive predictive value was superior with IgG-specific compared with polyspecific ELISA (0.56 [0.52-0.61] vs. 0.32 [0.28-0.35], respectively). The positive likelihood ratio (LR) was higher in IgG-specific than polyspecific ELISA, although negative LRs were similar. There was high risk of quality concerns in domains of index test and reference standard. Conclusions The superior diagnostic accuracy of IgG-specific ELISA reinforces the ISTH-SSC recommendation for standardization of laboratory testing for HIT. Likelihood ratios of individual assays may be used in combination with clinical scoring systems as part of an integrated diagnostic algorithm for HIT.

Protamine (heparin)-induced thrombocytopenia: a review of the serological and clinical features associated with anti-protamine/heparin antibodies

Protamine is widely used in medicine as a rapidly-acting antidote to heparin, particularly for reversing heparin anticoagulation after cardiac surgery. Protamine is also used as a stabilizing additive to certain preparations of insulin. Recent reports demonstrate that protamine and heparin form multimolecular complexes that result in high rates of immunization in post-cardiac surgery patients, particularly of immunoglobulin G (IgG) class antibodies; a subset of these anti-protamine/heparin IgG antibodies activates platelets through their FcγIIA (IgG) receptors. Although the clinical consequences of anti-protamine/heparin antibodies that are newly generated after cardiac surgery are unknown, there is evidence that platelet-activating anti-protamine/heparin antibodies already present at the time of cardiac surgery might occasionally explain more severe thrombocytopenia with delayed platelet count recovery, as well as thromboembolic complications, in the post-cardiac surgery setting. Triggers for such antibodies remain poorly-defined, but could include preoperative administration of heparin to diabetic patients receiving protamine-insulin as well as recent previous cardiac surgery. Anti-protamine/heparin antibodies have several features in common with anti-platelet factor 4 (PF4) PF4/heparin antibodies implicated in heparin-induced thrombocytopenia (HIT), including immunization by heparin-containing multimolecular complexes, predominant IgG class, pathological platelet-activating properties, relatively rapid IgG formation without IgM precedence, and antibody transience. Despite these similarities, the risk of anti-protamine/heparin antibody-mediated complications seems to affect the early post-cardiac surgery period, whereas HIT usually occurs at least 5 days following cardiac surgery. Clinicians need to become aware of this recently recognized immunohematological disorder, and research is needed to identify triggers of immunization, improve detection of pathological antibodies and identify patients at risk of this complication.

Rivaroxaban for treatment of suspected or confirmed heparin-induced thrombocytopenia study

Essentials Heparin-induced thrombocytopenia (HIT) is a thrombogenic condition that is difficult to treat. We evaluated rivaroxaban as a treatment option in patients with suspected or confirmed HIT. One patient had recurrent thrombosis and 9/10 patients with thrombocytopenia had platelet recovery. Rivaroxaban may be an effective and safe treatment option for HIT.

SUMMARY

Background Rivaroxaban is a direct oral anti-Xa inhibitor that has the potential to greatly simplify treatment of heparin-induced thrombocytopenia (HIT). Objectives To evaluate the efficacy and safety of rivaroxaban in this patient population, we conducted a multicenter, single-arm, prospective cohort study of patients with suspected or confirmed HIT. Patients/Methods Twenty-two consecutive adults with suspected or confirmed HIT received rivaroxaban 15 mg bid until a local HIT assay result was available. Participants with a positive local assay result continued rivaroxaban 15 mg bid until platelet recovery (or until day 21 if they had acute thrombosis at study entry), then stepped down to rivaroxaban 20 mg daily until day 30. Results and Conclusions The primary outcome measure, incidence of new symptomatic, objectively-confirmed venous and arterial thromboembolism at 30 days, occurred in one HIT-positive participant (4.5%; 95% confidence interval [CI], 0-23.5%) and one HIT-positive participant required limb amputation despite platelet recovery. Platelet recovery was achieved in nine out of 10 HIT-positive patients with thrombocytopenia. Rivaroxaban appears to be effective for treating patients with confirmed HIT, although the small number of patients enrolled limits precision.

Shock, acute disseminated intravascular coagulation, and microvascular thrombosis: is 'shock liver' the unrecognized provocateur of ischemic limb necrosis?

For unknown reasons, a small minority of critically ill patients with septic or cardiogenic shock, multiorgan failure, and disseminated intravascular coagulation develop symmetrical acral (distal extremity) limb loss due to microvascular thrombosis ('limb gangrene with pulses'). Case reports have described preceding 'shock liver' in some critically ill patients who developed such a picture of ischemic limb necrosis. This suggests that profoundly disturbed procoagulant-anticoagulant balance featuring uncontrolled generation of thrombin-resulting from failure of the protein C and antithrombin natural anticoagulant systems due to insufficient hepatic synthesis of these crucial proteins-could explain the microvascular thrombosis and associated limb loss. We hypothesize that shock liver is the key predisposing risk factor underlying ischemic limb necrosis in the majority of patients who develop this complication in the setting of acute disseminated intravascular coagulation complicating septic or cardiogenic shock. As shock liver precedes onset of limb ischemia by several days, therapeutic intervention may be possible.

Distinguishing between anti-platelet factor 4/heparin antibodies that can and cannot cause heparin-induced thrombocytopenia

BACKGROUND

Many patients exposed to heparin develop antibodies against platelet factor 4 (PF4) and heparin, yet only those antibodies that activate platelets cause heparin-induced thrombocytopenia (HIT). Patients who produce anti-PF4/heparin antibodies without developing HIT either have antibodies that do not cause platelet activation or produce pathogenic antibodies at levels that are insufficient to cause HIT. Understanding the differences between anti-PF4/heparin antibodies with and without HIT will improve test methods and reduce overdiagnosis.

AIMS

To investigate the presence of low levels of platelet-activating antibodies in patients investigated for HIT who had anti-PF4/heparin antibodies but failed to cause platelet activation in the (14) C-serotonin release assay (SRA).

MATERIALS/METHODS

We developed a platelet activation assay similar to the SRA using exogenous PF4 without added heparin (PF4-SRA). This assay was able to detect low levels of platelet-activating antibodies. We used this PF4-SRA to test for platelet-activating antibodies in patients investigated for HIT.

RESULTS

The PF4-SRA detected platelet-activating antibodies in seven (100%) of seven SRA-positive sera even after the samples were diluted until they were no longer positive in the standard SRA. Platelet-activating antibodies were detected in 14 (36%) of 39 patients who had anti-PF4/heparin antibodies but tested negative in the SRA and did not have clinical HIT. The clinical diagnosis of HIT was confirmed by chart review and concordant with the SRA results.

CONCLUSIONS

A subset of heparin-treated patients produce subthreshold levels of platelet-activating anti-PF4/heparin antibodies that do not cause HIT. An increase in the titer of these pathogenic antibodies, along with permissive clinical conditions, could lead to HIT.

FcγRIIa proteolysis as a diagnostic biomarker for heparin-induced thrombocytopenia

BACKGROUND

A significant challenge in the management of heparin-induced thrombocytopenia (HIT) patients is making a timely and accurate diagnosis. The readily available enzyme immunoassays (EIAs) have low specificities. In contrast, platelet activation assays have higher specificities, but they are technically demanding and not widely available. In addition, ~ 10% of samples referred for HIT testing are initially classified as indeterminate by the serotonin release assay (SRA), which further delays accurate diagnosis. HIT is characterized by platelet activation, which leads to FcγRIIa proteolysis. This raises the possibility that identification of the proteolytic fragment of FcγRIIa could serve as a surrogate marker for HIT.

OBJECTIVES

To determine the specificity of platelet FcγRIIa proteolysis induced by sera from patients with HIT, and to correlate the results with those of the SRA.

METHODS/PATIENTS

Sera from HIT patients and control patients with other thrombocytopenic/prothrombotic disorders were tested for their ability to proteolyse FcγRIIa. The results were correlated with anti-platelet factor 4 (PF4)/heparin antibodies (EIA), and heparin-dependent platelet activation (SRA).

RESULTS

Only HIT patient samples (20/20) caused heparin-dependent FcγRIIa proteolysis, similar to what was shown by the SRA. None of the samples from the other patient groups or hospital controls caused FcγRIIa proteolysis. Among nine additional samples that tested indeterminate in the SRA, FcγRIIa proteolysis resolved five samples that had a positive anti-PF4/heparin EIA result; three had no FcγRIIa proteolysis, and two were shown to have heparin-dependent FcγRIIa proteolysis

CONCLUSIONS

This study suggests that heparin-dependent FcγRIIa proteolysis is at least as specific as the SRA for the diagnosis of HIT.

Anti-PF4/heparin antibodies and venous graft occlusion in postcoronary artery bypass surgery patients randomized to postoperative unfractionated heparin or fondaparinux thromboprophylaxis

BACKGROUND

Anti-PF4/heparin antibodies are frequently generated after coronary artery bypass grafting (CABG) surgery, with platelet-activating IgG implicated in heparin-induced thrombocytopenia (HIT). It is controversial whether non-platelet-activating antibodies are associated with thrombosis.

OBJECTIVES

To determine in post-CABG patients whether thromboprophylaxis using fondaparinux vs. unfractionated heparin (UFH) reduces the frequency of anti-PF4/heparin antibodies, and whether anti-PF4/heparin antibodies are associated with early graft occlusion.

METHODS/PATIENTS

In a pre-planned secondary analysis of a randomized control trial (RCT) comparing fondaparinux vs. UFH thromboprophylaxis post-CABG, we determined the frequency of anti-PF4/heparin antibody formation by solid-phase enzyme-immunoassay (EIA) and of platelet-activating antibodies by serotonin-release assay (SRA); the SRA and fluid-phase EIA were used to assess fondaparinux cross-reactivity. We also examined whether anti-PF4/heparin antibodies were associated with early arterial or venous graft occlusion (6-week CT angiography).

RESULTS

We found no significant difference in the frequency of antibody formation between patients who received fondaparinux vs. UFH (65.3% vs. 46.0%; P = 0.069), and no significant fondaparinux cross-reactivity. Venous graft occlusion(s) occurred in 6/26 patients who formed 'strong' IgG antibodies (≥ 1.0 optical density [OD] units and ≥ 2× baseline) vs. 3/66 who did not (P = 0.0139). In both unadjusted and adjusted analyses, strong postoperative (but not pre-operative) anti-PF4/heparin IgG responses were associated with a markedly increased risk of early venous (but not arterial) graft occlusion (adjusted OR, 9.25 [95% CI, 1.73, 49.43]; P = 0.0093); notably, none of the three SRA-positive patients developed a venous graft occlusion.

CONCLUSIONS

Fondaparinux vs. UFH thromboprophylaxis postCABG does not reduce anti-PF4/heparin antibody formation. Non-platelet-activating anti-PF4/heparin IgG antibodies generated post operatively are associated with early venous graft occlusion.

A systematic evaluation of laboratory testing for drug-induced immune thrombocytopenia

BACKGROUND

Drug-induced immune thrombocytopenia (DITP) can be confirmed by the demonstration of drug-dependent platelet antibodies in vitro; however, laboratory testing is not readily accessible and test methods are not standardized.

OBJECTIVE

To identify drugs with the strongest evidence for causing DITP based on clinical and laboratory criteria.

PATIENTS/METHODS

We developed a grading system to evaluate the quality of DITP laboratory testing. The 'DITP criteria' were: (i) Drug (or metabolite) was required for the reaction in vitro; (ii) Immunoglobulin binding was demonstrated; (iii) Two or more laboratories obtained positive results; and (iv) Platelets were the target of immunoglobulin binding. Laboratory diagnosis of DITP was considered definite when all criteria were met and probable when positive results were reported by only one laboratory. Two authors applied the DITP criteria to published reports of each drug identified by systematic review. Discrepancies were independently adjudicated.

RESULTS

Of 153 drugs that were clinically implicated in thrombocytopenic reactions, 72 (47%) were associated with positive laboratory testing. Of those, 16 drugs met criteria for a definite laboratory diagnosis of DITP and thus had the highest probability of causing DITP. Definite drugs were: quinine, quinidine, trimethoprim/sulfamethoxazole, vancomycin, penicillin, rifampin, carbamazepine, ceftriaxone, ibuprofen, mirtazapine, oxaliplatin and suramin; the glycoprotein IIbIIIa inhibitors abciximab, tirofiban and eptifibatide; and heparin.

CONCLUSIONS

We identified drugs with the strongest evidence for an association with immune thrombocytopenia. This list may be helpful for ranking potential causes of thrombocytopenia in a given patient.

Fondaparinux treatment of acute heparin-induced thrombocytopenia confirmed by the serotonin-release assay: a 30-month, 16-patient case series

BACKGROUND

Fondaparinux is theoretically an attractive agent for the treatment of immune heparin-induced thrombocytopenia (HIT), a prothrombotic disorder caused by platelet-activating anti-platelet factor 4/heparin antibodies. Although reports of the use of fondaparinux for this indication have thus far been favorable, the diagnosis of HIT in most cases was not based on definitive laboratory confirmation of heparin-dependent, platelet-activating antibodies.

OBJECTIVES

To report thrombotic and major bleeding outcomes with fondaparinux in patients with a high likelihood of having acute HIT based on clinical features and a positive result in the confirmatory platelet serotonin-release assay (SRA), a sensitive and specific test for platelet-activating HIT antibodies.

METHODS/PATIENTS

We reviewed consecutive eligible patients with SRA-positive HIT (mean peak serotonin release, 91% [normal, < 20%]; mean IgG-specific PF4/heparin enzyme immunoassay result, 2.53 optical density units [normal, < 0.45 units]) in one medical center over a 30-month period who received fondaparinux for anticoagulation during acute HIT (platelet count, < 150 × 10(9) L(-1)). Where available, plasma samples were used to measure thrombin-antithrombin (TAT) complex levels.

RESULTS

Sixteen patients with SRA-positive HIT received fondaparinux: 14 surgical (11 after cardiac surgery; three after vascular surgery) and two medical (acute stroke). Fifty-six per cent of patients had HIT-associated thrombosis at the time of diagnosis. No patient developed new, recurrent or progressive thrombosis; one patient developed a major bleed (calf hematoma). One patient judged to have irreversible tissue necrosis before receiving fondaparinux therapy ultimately required limb amputation. TAT complex levels were reduced within 24 h of starting fondaparinux, and 13 of 13 patients were successfully switched to warfarin.

CONCLUSION

Fondaparinux shows promise for the treatment of patients with SRA-positive acute HIT.

HIT paradigms and paradoxes

The current major problem with HIT is its overdiagnosis. This concept follows from the HIT central paradigm: HIT is caused by a subset of antibodies against platelet factor 4 (PF4)/heparin complexes that have strong platelet-activating properties. Prospective studies show that only a minority of sera containing such antibodies exhibit platelet-activating properties. Ironically, the earliest tests for HIT--platelet activation assays--remain today the most diagnostically useful, particularly the washed platelet assays. But the wider application of PF4-dependent immunoassays, and their much greater sensitivity for the larger subset of non-platelet-activating (and non-HIT-inducing) antibodies, has resulted in HIT overdiagnosis in many centres. Studies of anti-PF4/heparin immunization in diverse clinical situations have provided insights into the factors that influence the HIT immune response. Besides the conundrum of anticoagulant-induced thrombosis (including its potentiation of coumarin-induced microthrombosis), HIT evinces numerous other paradoxes: (i) it is a platelet-activating disorder with venous thrombosis as its predominant clinical manifestation; (ii) 'delayed-onset' (or 'autoimmune') HIT can lead to dramatic worsening of HIT-associated thrombosis despite cessation of heparin; (iii) partial thromboplastin time (PTT) monitoring of direct thrombin inhibitor treatment - and confounding of PTT monitoring by HIT-associated consumptive coagulopathy - infers that the worst subset of HIT patients may fail this therapeutic approach; (iv) the highly sulfated pentasaccharide anticoagulant, fondaparinux, can (rarely) cause HIT yet appears to be an effective treatment for this disorder; and (v) the transience of the HIT immune response means that many patients with previous HIT can safely receive future heparin.

Heparin-induced thrombocytopenia: towards standardization of platelet factor 4/heparin antigen tests

BACKGROUND

Laboratory confirmation of heparin-induced thrombocytopenia (HIT) is based on detection of heparin-dependent platelet-activating antibodies. Platelet factor 4 (PF4)/heparin enzyme-immunoassays (EIA) are a widely available surrogate for platelet-activating antibodies.

OBJECTIVE

Defining the optical density (OD) reactivity profiles of a PF4/heparin EIA in reference subject and patient populations and the correlation of the EIA results (expressed in OD units) with the prevalence of platelet-activating antibodies.

PATIENTS/METHODS

Using quantile regression we determined the 97.5th percentile of PF4/heparin-immunoglobulin G (IgG) EIA reactivities in non-heparin-treated individuals [blood donors (n = 935)] and patients before heparin therapy (n = 1207). In patients with suspected HIT, we compared the correlation of EIA-IgG reactivities (Greifswald laboratory; n = 2821) and the heparin-induced platelet activation assay (HIPA) with the correlation of reactivities of another EIA-IgG (McMaster laboratory; n = 1956) with the serotonin-release assay (SRA).

RESULTS

PF4/heparin-IgG EIA OD reactivities had a lower OD 97.5th percentile in blood donors compared with patient groups before heparin treatment (P < 0.001). The percentage of sera testing positive in the functional assays strongly correlated with PF4/heparin-IgG EIA OD reactivities in both laboratories with very similar results (correlation coefficient > 0.9) when normalized OD ranges (maximum OD divided by 10) were used instead of absolute OD values.

CONCLUSIONS

Results of PF4/heparin-IgG EIA should not be reported as only positive or negative as there is no single acceptable cut-off value. Instead, reporting PF4/heparin-IgG EIA OD results in ranges allows for risk-stratified prediction for presence of platelet-activating antibodies. Use of normalized OD ranges permits a standardized approach for inter-laboratory comparisons.

Anti-PF4/heparin antibody formation postorthopedic surgery thromboprophylaxis: the role of non-drug risk factors and evidence for a stoichiometry-based model of immunization

BACKGROUND

Heparin-induced thrombocytopenia is an antibody-mediated disorder exhibiting variable frequency in different clinical settings. Antibodies recognize PF4/heparin complexes formed at optimal stoichiometric molar ratios.

OBJECTIVE

To identify clinical factors influencing risk of anti-PF4/heparin immunization.

PATIENTS/METHODS

We performed observational studies and exploratory analyses of the frequency of anti-PF4/heparin antibody formation in 6324 patients who received enoxaparin or fondaparinux in four randomized controlled trials of postorthopedic surgery thromboprophylaxis. Variables included surgery type (knee vs. hip), timing of first anticoagulant dose (pre- vs. postsurgery), circumstances of surgery (elective vs. hip fracture), anticoagulant (enoxaparin vs. fondaparinux) and body-mass index (BMI). We applied a stoichiometry-based model that predicts immunization risk based on expected differences in PF4/anticoagulant ratios in different settings, and specifically used this model to predict the effect of increasing BMI quartiles upon relative risk (RR) of immunization for fondaparinux vs. enoxaparin.

RESULTS

Anti-PF4/heparin immunization was more frequent after knee vs. hip surgery (particularly for enoxaparin), and when enoxaparin was given post- rather than pre-elective surgery; however, the opposite occurred with hip fracture surgery, that is, antibody formation was more frequent when enoxaparin or fondaparinux was given presurgery. The RR of immunization for fondaparinux vs. enoxaparin decreased significantly for increasing BMI quartiles, an effect predominantly because of increasing immunization with enoxaparin at increasing BMI quartiles.

CONCLUSIONS

Several non-drug factors--including type and circumstances of surgery, timing of first anticoagulant dose and BMI--influence risk of anti-PF4/heparin antibody formation, consistent with a stoichiometry-based immunization model of PF4 and anticoagulant ratios occurring during the early peri-operative period.

Early-onset and persisting thrombocytopenia in post-cardiac surgery patients is rarely due to heparin-induced thrombocytopenia, even when antibody tests are positive

BACKGROUND

The high frequency of thrombocytopenia in post-cardiac surgery patients makes it challenging to diagnose heparin-induced thrombocytopenia (HIT). Two platelet count profiles are reported as indicating possible HIT in these patients: profile 1 describes a platelet count fall that begins between postoperative days 5 and 10, whereas profile 2 denotes early-onset thrombocytopenia that persists beyond day 5.

OBJECTIVES

To examine how these platelet count profiles correlate with antibody status and HIT post-cardiac surgery.

METHODS

We prospectively screened 581 cardiac surgery patients for heparin-dependent antibodies by platelet factor 4 (PF4)-heparin immunoassay and platelet-activation test, and performed daily platelet counts (until day 10) with 30-day follow-up.

RESULTS

All three patients with platelet count profile 1 tested positive for platelet-activating anti-PF4-heparin IgG antibodies [odds ratio (OR) 521.7, 95% confidence interval (CI) 3.9-34,000, P = 0.002], and were judged to have HIT. In contrast, none of 25 patients with early-onset and persisting thrombocytopenia (profile 2) was judged to have HIT, including five patients testing positive for platelet-activating anti-PF4-heparin IgG antibodies. In these patients, the frequency of heparin-dependent antibodies did not differ from that in non-thrombocytopenic controls, either for anti-PF4-heparin IgG (OR 1.7, 95% CI 0.7-4.1, P = 0.31) or for platelet-activating antibodies (OR 1.9, 95% CI 0.6-5.7, P = 0.20). Multivariate analysis revealed that type of cardiac surgery, but not HIT antibody status, predicted early-onset and persisting thrombocytopenia. Together, these findings show that HIT was uncommon in this study population [overall frequency, 3/581 (0.5%), 95% CI 0.1-1.5%].

CONCLUSIONS

Thrombocytopenia that begins between 5 and 10 days post-cardiac surgery is highly predictive for HIT. In contrast, early-onset and persisting thrombocytopenia is usually caused by non-HIT factors with coinciding heparin-dependent antibody seroconversion.

Managing bleeding in anticoagulated patients with a focus on novel therapeutic agents

Heparin, low molecular weight heparin (LMWH) and coumarins are familiar to most clinicians, inexpensive, highly effective when correctly used and widely available. However, coumarin has a delayed onset of action, interacts with many medications, has a narrow therapeutic window, and can cause thrombosis in some settings (e.g. hereditary protein C deficiency, heparin induced thrombocytopenia, warfarin loading). Additionally, warfarin and heparin require monitoring of their therapeutic effect. These real and perceived limitations have led to the development of 'novel' anticoagulants. However, these new agents have one general limitation--a lack of a widely available antidote. We focus on the management of bleeding in anticoagulated patients, with particular regard to novel anticoagulants.

Heparin-induced thrombocytopenia--therapeutic concentrations of danaparoid, unlike fondaparinux and direct thrombin inhibitors, inhibit formation of platelet factor 4-heparin complexes

BACKGROUND

Treatment of heparin-induced thrombocytopenia (HIT), a disorder in which anti-platelet factor 4 (PF4)-heparin antibodies cause platelet activation and hypercoagulability, requires alternative (non-heparin) anticoagulation. Treatment options include direct thrombin inhibitors [lepirudin and argatroban (approved), and bivalirudin], danaparoid (approved) (mixture of anticoagulant glycosaminoglycans), or fondaparinux (synthetic heparin-mimicking pentasaccharide). PF4-heparin complexes form at optimal stoichiometric ratios.

OBJECTIVES

To compare the effects of these various non-heparin anticoagulants in disrupting the formation of PF4-heparin complexes, and PF4-containing immune complexes.

PATIENTS/METHODS

Sera were obtained from patients with serologically confirmed HIT. The effects of the alternative anticoagulants on PF4 and PF4-heparin complex interactions with platelets, as well as HIT antibody binding and platelet activation, were investigated.

RESULTS

Danaparoid at very low concentrations increased PF4 binding to platelets. In therapeutic concentrations, however, it decreased PF4 binding to platelets (P = 0.0004), displaced PF4-heparin complexes from platelets (P = 0.0033) and PF4 from the surface of a PF4-transfected HEK-293 EBNA cell line expressing the PF4 receptor CXCR3-B (P = 0.0408), reduced PF4-heparin complex size (P = 0.025), inhibited HIT antibody binding to PF4-heparin complexes (P = 0.001), and prevented platelet activation by HIT antibodies (P = 0.046). Although fondaparinux also interfered with PF4 binding to platelets, HIT antibody binding to PF4-heparin complexes, and activation of platelets by HIT antibodies, these effects occurred only at supratherapeutic concentrations. The direct thrombin inhibitors had no effect at any concentrations.

CONCLUSIONS

Danaparoid uniquely interferes with the pathogenesis of HIT by disrupting PF4-containing immune complexes at therapeutic dose concentrations. It is possible that these effects contribute to its therapeutic efficacy.

Quantitative interpretation of optical density measurements using PF4-dependent enzyme-immunoassays

BACKGROUND

Many laboratories test for heparin-induced thrombocytopenia (HIT) using a PF4-dependent enzyme-immunoassay (EIA). An advantage of the EIA is its simplicity; a disadvantage is that it only indirectly detects heparin-dependent, platelet-activating antibodies ('HIT antibodies').

OBJECTIVES

To determine whether the magnitude of a positive EIA result, expressed in optical density (OD) units, predicts risk of HIT antibodies, defined as a strong-positive platelet serotonin-release assay (SRA) result (>or=50% serotonin release).

PATIENTS/METHODS

We determined the risk of a strong-positive SRA result for five categories of OD reactivity (<0.40, 0.40-<1.00, 1.00-<1.40, 1.40-<2.00, and >or=2.00 OD units) using two EIAs (commercial anti-PF4/polyanion IgG/A/M and in-house anti-PF4/heparin-IgG).

RESULTS

For patient sera investigated for HIT antibodies, a weak-positive result (0.40-<1.00 OD units) in either EIA indicated a low probability (or= 2.00 units. Quantifying the EIA-SRA relationship for 1553 referred patient sera, we found that for every increase of 0.50 OD units in the EIA-IgG, the risk of a strong-positive SRA result increased by OR = 6.39 [95% confidence interval (CI), 5.13, 7.95; P < 0.0001]. For every increase of 1.00 OD units in the EIA-IgG, the risk increased by OR = 40.81 (95% CI, 26.35, 63.20; P < 0.0001).

CONCLUSIONS

The probability of HIT antibodies (strong-positive SRA result) inferred by a positive PF4-dependent EIA varies considerably in relation to the magnitude of the EIA result, expressed as OD values. In our laboratory, the probability of HIT antibodies being present reached >or=50% only when the OD level was >or=1.40 units.

Heparin-induced thrombocytopenia in patients requiring prolonged intensive care unit treatment after cardiopulmonary bypass

BACKGROUND

The diagnosis of heparin-induced thrombocytopenia (HIT) is problematic in postcardiac surgery (CS) intensive care unit (ICU) patients, as there are multiple potential explanations for thrombocytopenia and the presence of anti-platelet factor 4/heparin antibodies is not highly specific for HIT. Two platelet count profiles for HIT - a 40% or greater fall in platelet count beginning on or after day 5 (pattern 1) and persisting thrombocytopenia (< 100 x 10(9) L(-1)) beyond day 7 (pattern 2) - have been described in post-CS patients.

METHODS AND RESULTS

We examined the platelet count profiles of 329 consecutive post-CS patients who required ICU treatment beyond 7 days. Although 70 patients (21.3%) developed thrombocytopenia (57.1% pattern 1, 42.9% pattern 2), the overall incidence of HIT was only 1.8% [6/329; 95% confidence interval (95% CI) 0.7-3.9%] in these ICU patients, with more HIT patients showing a pattern 2 than a pattern 1 platelet count decrease (four vs. two patients). Notably, pattern 2 patients with HIT also showed a new proportional fall of > 30% in platelet count between postoperative days 5 and 10. Among the remaining 2242 post-CS patients without a prolonged ICU stay, only three (0.1%; 95% CI 0.03-0.4%) developed symptomatic HIT (OR 0.07; 95% CI 0.01-0.3; P = 0.0002 vs. ICU patients), all presenting with pattern 1.

CONCLUSIONS

Among post-CS ICU patients, a postoperative platelet count fall between days 5 and 10 increases diagnostic specificity for HIT, irrespective of whether this platelet count fall occurs after postoperative platelet count recovery (pattern 1) or is superimposed upon persisting postoperative thrombocytopenia (pattern 2). A prospective study is required in order to validate the findings of this retrospective analysis.

Heparin-induced thrombocytopenia: a stoichiometry-based model to explain the differing immunogenicities of unfractionated heparin, low-molecular-weight heparin, and fondaparinux in different clinical settings

INTRODUCTION

Heparin-induced thrombocytopenia (HIT) is caused by platelet-activating antibodies that recognize platelet factor 4 (PF4)/heparin complexes. The frequency of HIT is highly variable in different clinical settings, and is more frequent with unfractionated heparin (UFH) than with low-molecular-weight heparin (LMWH), despite the in vitro observation that HIT antibodies activate platelets similarly well with LMWH as with UFH. An important difference between UFH, LMWH, and fondaparinux is their widely differing plasma concentrations. We aimed to provide a model that included anticoagulant concentrations and PF4 availability as risk factors influencing the anti-PF4/heparin immune response.

MATERIALS AND METHODS

By photon correlation spectroscopy we determined the concentrations at which UFH, LMWH, and fondaparinux form complexes optimally with PF4. Plasma concentrations of UFH and LMWH were calculated based on ex vivo pharmacokinetic data, with information on fondaparinux and PF4 concentrations taken from the literature.

RESULTS AND CONCLUSIONS

The main features of our model are: optimal complex formation occurs at prophylactic-dose UFH and high PF4 levels, whereas therapeutic-dose LMWH concentrations are too high for optimal complex formation; in contrast, concentrations of fondaparinux are usually below the optimal stoichiometric range. Thus, immunization should occur more often in situations with major rather than minor platelet activation, and--for a given degree of platelet activation (PF4 availability)--as: prophylactic-dose UFH>therapeutic-dose UFH>prophylactic-dose LMWH, fondaparinux>therapeutic-dose LMWH. Our model provides a framework for explaining empirical observations that LMWH induces less anti-PF4/heparin antibodies than does UFH, and that anti-PF4/heparin antibodies are more often found in patients undergoing major surgery than in medical patients.

Heparin-induced thrombocytopenia: a prospective study on the incidence, platelet-activating capacity and clinical significance of antiplatelet factor 4/heparin antibodies of the IgG, IgM, and IgA classes

INTRODUCTION

Platelet-activating antiplatelet factor 4/heparin (anti-PF4/heparin) antibodies are the major cause of heparin-induced thrombocytopenia (HIT). However, the relative utility of functional (platelet activation) vs. antigen [enzyme-immunoassay (EIA)] assays, and the significance of assay discrepancies remain unresolved.

METHODS

Consecutive patient sera (n = 1650) referred for diagnostic HIT testing were screened prospectively by both the heparin-induced platelet activation (HIPA) test and anti-PF4/heparin EIA - including individual classes (IgG, IgA, IgM) - with clinical correlations studied. Platelet microparticle and annexin-V-binding properties of the sera were also investigated.

RESULTS

Only 205 (12.4%) sera tested positive in either the HIPA and/or EIA: 95 (46.3%) were positive in both, 109 (53.1%) were only EIA-positive, and, notably, only one serum was HIPA-positive/EIA-negative. Of 185 EIA-positive sera, only 17.6% had detectable IgM and/or IgA without detectable IgG. Among sera positive for EIA IgG, optical density values were higher when the sera were HIPA-positive (1.117 vs. 0.768; P < 0.0001), with widely overlapping values. Two HIPA-positive but EIA-IgG-negative sera became HIPA-negative following IgG depletion, suggesting platelet-activating antibodies against non-PF4-dependent antigens. Clinical correlations showed that HIPA-negative/EIA-positive patients did not develop thrombosis and had reasons other than HIT to explain thrombocytopenia. IgM/A antibodies did not increase microparticle penetration, but increased annexin-V binding.

CONCLUSIONS

The anti-PF4/heparin EIA has high ( approximately 99%) sensitivity for HIT. However, only about half of EIA-positive patients are likely to have HIT. Anti-PF4/heparin antibodies of IgM/A class and non-PF4-dependent antigens have only a minor role in HIT.

Generation of platelet-derived microparticles and procoagulant activity by heparin-induced thrombocytopenia IgG/serum and other IgG platelet agonists: a comparison with standard platelet agonists

Heparin-induced thrombocytopenia (HIT) is a relatively common, immunoglobulin-mediated adverse drug reaction associated with in vivo thrombin generation and both venous and arterial thrombosis. Serum and purified IgG from patients with HIT induce normal platelets to generate procoagulant platelet-derived microparticles, but the magnitude of this response in comparison with other IgG and standard platelet agonists is unknown. We describe a comparison of IgG platelet agonists, including HIT-IgG/serum, heat-aggregated IgG, and platelet-activating murine monoclonal antibodies, with standard 'strong' and 'weak' platelet agonists, and have determined their relative ability to generate platelet procoagulant activity. Using washed normal platelets as targets, we observed that HIT sera as well as other IgG agonists produced similar or even greater numbers of microparticles and procoagulant activity than the standard strong platelet agonists (thrombin, collagen, and thrombin receptor agonist peptide). The only exception was the non-physiological platelet agonist, calcium ionophore, which consistently produced a platelet procoagulant response even greater than the IgG agonists. We conclude that the IgG class of platelet agonists (including pathogenic HIT antibodies) is an effective trigger of the platelet procoagulant response comparable at least to strong physiological platelet agonists. These results help to explain the association between HIT, in vivo thrombin generation, and thrombosis.

Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a prothrombotic adverse drug reaction caused by heparin. As thrombocytopenia is common in hospitalized patients receiving heparin, it would be useful to have a clinical scoring system that could differentiate patients with HIT from those with other reasons for thrombocytopenia.

AIM

To compare prospectively the diagnostic utility of a clinical score for HIT in two different clinical settings.

METHODS

The pretest clinical scoring system, the '4 T's', was used to classify 100 consecutive patients referred for possible HIT in one hospital (Hamilton General Hospital, HGH) into high, intermediate, and low probability groups. This system was also used to classify likewise 236 patients by clinicians in Germany referring blood for diagnostic testing for HIT in Greifswald (GW). The clinical scores were correlated with the results of laboratory testing for HIT antibodies using the serologic criteria for HIT with high diagnostic specificity.

RESULTS

In both centers, patients with low scores were unlikely to test positive for HIT antibodies [HGH: 1/64 (1.6%), GW: 0/55 (0%)]. Patients with intermediate [HGH: 8/28 (28.6%), GW: 11/139 (7.9%)] or high scores [HGH: 8/8 (100%), GW: 9/42 (21.4%)] were more likely to test positive for clinically significant HIT antibodies. The positive predictive value of an intermediate or high clinical score for clinically significant HIT antibodies was higher at one center (HGH).

CONCLUSIONS

A low pretest clinical score for HIT seems to be suitable for ruling out HIT in most situations (high-negative predictive value). The implications of an intermediate or high score vary in different clinical settings.

Heparin-induced thrombocytopenia and vascular surgery

Heparin-induced thrombocytopenia (HIT) is of special interest to vascular surgeons as heparin is the predominant anticoagulant used before, during, and after vascular surgery. Further, the prothrombotic nature of this antibody-mediated disorder leads to a high frequency of limb ischemia due to large arterial occlusion by platelet-rich ("white") clots or because of extensive venous thrombosis involving large veins and small venules. This latter syndrome has been associated with coumarin anticoagulation of HIT-associated deep-vein thrombosis (coumarin-induced venous limb gangrene). Non-heparin anticoagulants, such as the direct thrombin inhibitors (lepirudin, argatroban), may be needed for intraoperative management of a patient with suspected acute HIT who requires vascular surgery. The transience of HIT antibodies provides a rationale for intraoperative use of heparin in a patient who has recovered from HIT and in whom HIT antibodies are no longer detectable.

Off-pump coronary artery bypass grafting for acute heparin-induced thrombocytopenia

Surgical revascularization of coronary arteries is problematic for patients with heparin-induced thrombocytopenia because the available nonheparin anticoagulants cannot be reversed pharmacologically. We used three-vessel off-pump coronary artery bypass grafting in a patient with heparin-induced thrombocytopenia, as it allowed us to use substantially lower doses of nonheparin anticoagulant (danaparoid sodium), compared with procedures requiring cardiopulmonary bypass.

Venous limb gangrene during warfarin treatment of cancer-associated deep venous thrombosis

BACKGROUND

The cause of cancer-associated venous limb gangrene is unknown but could paradoxically be due to warfarin.

OBJECTIVE

To determine the pathogenesis of venous gangrene in a patient with cancer.

DESIGN

Case report.

SETTING

University hospital in Ontario, Canada.

PATIENT

66-year-old woman with metastatic lung cancer and deep venous thrombosis.

MEASUREMENTS

Levels of vitamin K-dependent factors, additional coagulation factors, and thrombin-antithrombin complexes (marker of thrombin generation).

RESULTS

During warfarin use, venous limb gangrene developed when the international normalized ratio (INR) reached 6.0 (therapeutic range, 2.0 to 3.0); at this time, the level of protein C (a vitamin K-dependent natural anticoagulant) was severely reduced, but thrombin-antithrombin complexes remained markedly elevated. The supratherapeutic INR was explained by the greatly reduced levels of factor VII, which correlated closely with protein C levels; therefore, the high INR was a surrogate marker for severely reduced protein C activity.

CONCLUSION

Warfarin may contribute to the pathogenesis of cancer-associated venous limb gangrene by leading to severe depletion of protein C while at the same time failing to reduce thrombin generation.