Management of heparin-induced thrombocytopenia: a critical comparison of lepirudin and argatroban

Heparin-induced thrombocytopenia (HIT) is a transient hypercoagulability state initiated, paradoxically, by the anticoagulant, heparin. It is characterized by antibody-induced activation of platelets, leading to thrombin generation. Many patients with HIT develop thrombosis; even when heparin is stopped because of "isolated HIT" detected during routine platelet count monitoring, 25-50% of patients subsequently develop symptomatic thrombosis. Thus, an alternative anticoagulant should be substituted for heparin when HIT is strongly suspected. Two direct thrombin inhibitors (DTIs), lepirudin and argatroban, have been studied for prevention and treatment of thrombosis in HIT patients. Lepirudin is a polypeptide that binds irreversibly to the fibrin-binding and catalytic sites on thrombin (bivalent inhibitor). In contrast, argatroban is a synthetic, small-molecule DTI that binds reversibly to the catalytic site alone (univalent inhibitor). Results of historically controlled clinical trials suggest both agents are effective for preventing and treating thrombosis in HIT. However, these agents have not been compared directly, and important differences in study design limit conclusions from indirect comparison. For example, lepirudin was given for 12-14 days (mean) in treatment studies of thrombosis complicating HIT, whereas argatroban was given only for 6-7 days, a difference that could explain apparent lower thrombosis rates (and greater bleeding) with lepirudin. Recently, the transition from DTI therapy to oral anticoagulation in patients with deep venous thrombosis (DVT) complicating HIT has been identified as a risk period for coumarin-induced venous limb gangrene. Thus, the DTI should be given alone during acute HIT, with oral anticoagulants deferred until substantial resolution of the thrombocytopenia has occurred.     

Heparin-induced thrombocytopenia: diagnosis and management

Heparin-induced thrombocytopenia (HIT) is an immune reaction in response to platelet factor 4-heparin complexes, which results in increased platelet activation and thrombocytopenia beginning on the 4th-5th day after heparin exposure induced by IgG antibody production. Platelet activation can lead to arterial thrombosis, but more commonly platelet microparticle formation contributes to venous thrombosis. Accurate diagnosis of HIT is based on the presence of clinical features, including a 50% fall in platelet count, appropriate timing of thrombocytopenia, development of new thrombosis despite thrombocytopenia and heparin therapy, and the absence of a more likely cause of thrombocytopenia. Documentation of an anti-PF4-heparin antibody is necessary, but is not sufficient to make the diagnosis since antibody formation occurs in a variety of clinical settings without the development of thrombocytopenia or thrombosis. Once HIT is suspected or confirmed, all forms of heparin should be discontinued and an alternative form of anticoagulation should be administered until the platelet count recovers. Treatment options include intravenous administration of argatroban, lepirudin, and bivalirudin; subcutaneous administration of fondaparinux has also been described. Warfarin therapy, if indicated, should be avoided until platelet recovery. Re-exposure to heparin can be avoided by use of alternative anticoagulants for most circumstances. Heparin-induced thrombocytopenia (HIT) has been the focus of increasing attention over the past 15-20 years. As interventions for HIT are developed, there is a need to accurately diagnose the condition, which can be challenging especially in severely ill patients.     

Clinical evaluation of acute systemic reaction and detection of IgG antibodies against PF4/heparin complexes in hemodialysis patients

Heparin-induced thrombocytopenia (HIT) is a pathophysiological syndrome caused by platelet-activating antibodies that recognize PF4/heparin complexes. The abrupt onset of HIT following intravenous bolus heparin is known as an acute systemic reaction. Clinical features of this type of HIT may be similar to those of common complications during hemodialysis. The aim of the study was to identify whether the clinical features of the acute systemic reaction are caused by HIT or dialytic complications. Twenty-seven dialytic patients who had thrombocytopenia and clinical features of an acute systemic reaction were enrolled out of 202 HIT-suspected patients. Thirteen patients had HIT confirmed due to the presence of positive functional and immunoassays. Eight of the thirteen patients presented with acute systemic reactions due to HIT. The most common symptom of acute systemic reaction was dyspnea. The other nineteen patients, involving both HIT and non-HIT patients, had dialysis-complicated ASR. The major feature of the acute systemic reaction in hemodialysis was hypotension and its relevant symptoms. An immunoassay for the detection of IgG antibodies against PF4/heparin complexes (HIT-IgG) showed the wide-range linearity of the calibration curve by employing three concentrations of recombinant mouse monoclonal antibodies for PF4/heparin complexes. The results are expressed as micrograms of IgG in one milliliter. Significantly high levels in thirteen HIT patients were compared with levels in fourteen non-HIT patients. The highest median of 1,530 μg/ml (IQR: 3,267-813) was obtained in the presence of HIT associated with an acute systemic reaction. In HIT patients who did not show characteristics of an HIT-derived acute systemic reaction, the median was 339 μg/ml (1,178-834). Despite showing a positive ELISA, nine non-HIT patients without any platelet-activating antibodies showed a value of 97 μg/ml (166-56). The lowest median of 8.3 μg/ml (11-6) was in non-HIT patients with a negative ELISA. In conclusion, measurements of HIT-IgG -specific antibodies can facilitate an appropriate estimation in hemodialysis patients of whether the clinical features of an acute systemic reaction are caused by HIT or dialytic complications.     

CXCL4-platelet factor 4, heparin-induced thrombocytopenia and cancer

Platelet factor 4 (CXCL4-PF4) is a chemokine that binds to and neutralizes heparin and other negatively charged proteoglycans, but is also involved in angiogenesis and cancer development. In some patients exposed to heparin, antibodies are generated against the CXCL-PF4/heparin complex that may activate platelets and coagulation and lead to thrombocytopenia and arterial or venous thrombosis, a condition commonly named heparin induced thrombocytopenia (HIT). HIT has been investigated in numerous clinical settings, but there is limited data on the epidemiology and phenotype of HIT in cancer patients. The present review describes the role of CXCL4-PF4 in cancer, the immunobiology, clinical presentation and diagnosis of HIT, and the specific problems faced in cancer patients.     

Heparin-induced thrombocytopenia in neurologic disease treated with unfractionated heparin

The risk for immune-mediated heparin-induced thrombocytopenia (HIT) in neurologic patients receiving unfractionated heparin (UFH) is not known. In a prospective study of 200 patients, the authors found a 2.5% rate of HIT and a 2% rate of HIT-associated thromboses, suggesting that neurologic patients treated with UFH are at considerable risk for development of HIT and its complications. Prevalence of heparin-induced antibodies was 20.5% and was dependent on heparin dose. It was higher in cerebrovascular than in noncerebrovascular (29.4% versus 11.2%, p < 0.01) patients.     

Clinical characteristics and laboratory testing of patients with suspected HIT: A survey on current practice in 11 university hospitals in France

Summary

We undertook a survey of French university hospital hematological laboratories to ascertain the clinical characteristics of patients with suspected HIT, the laboratory tests performed, and the therapeutic strategy adopted in current practice.

Methods

A standardized medical records database for patients with suspected HIT was sent to 19 laboratories. During two months, all consecutive patients for whom a biological test was performed were included.

Results

169 patients were included, 27 (16%) patients having a final diagnosis of HIT. At the time HIT was suspected, the heparin duration and the level of thrombocytopenia were similar in HIT- positive and HIT-negative groups. The use of unfractionated heparin, a therapeutic heparin dose regimen and the presence of thrombotic complications were significantly more frequent in HIT-positive patients. When the heparin dose regimen was taken into account, only thrombotic complications under a therapeutic dose regimen were significantly increased in HIT-positive patients. Eighty-six percent of patients presented at least one alternative diagnosis of thrombocytopenia without significant difference between the two groups. Laboratory tests were performed after a mean of 0.3 days and mainly consisted of antigen assays. At the time HIT was suspected, heparin was stopped in 56 (33%) patients, being replaced mainly by danaparoid. Only three laboratories declared they usually received all the necessary clinical information to establish the likelihood of HIT.

Conclusion

In current practice in France, the clinical probability of HIT is rarely established, leading to systematic requests for laboratory HIT tests.     

Antiplatelet factor 4/heparin antibodies in patients with gram negative bacteremia

Heparin-induced thrombocytopenia (HIT) is an antibody-mediated syndrome of thrombocytopenia and prothrombotic state that follows exposure to heparin. However, spontaneous HIT has been described in the setting of infection, without evidence of previous heparin administration. Since PF4 binds to lipid A portion of lipopolysaccharide, we tested for the presence of antiPF4/heparin antibodies in patients with gram-negative bacteremia. Patients with bacteremia had higher titers of antiPF4/heparin antibodies compared to normal controls 26.3 ± SD 34 units, N = 32 versus 6.3 ± SD 2.38 units, N = 10, P = 0.001. FITC-labeled PF4 interacted with lipopolysaccharide in a concentration-dependent manner as determined by quenching of the emission spectrum following excitation at λ 488. In addition, immunoaffinity purified antiPF4/Heparin antibodies from 3 patients with HIT cross-reacted with PF4/heparin complex. These results show that PF4/LPS complex is immunogenic and can elicit cross-reacting antibodies against PF4/Heparin, providing an explanation for the presence of these antibodies in individuals, who were never been exposed to heparin before. These antibodies may also be at least partly responsible for the thrombocytopenia associated with infection.     

Heparin-induced thrombocytopenia in neurologic patients treated with low-molecular-weight heparin

The authors prospectively studied the risk for immune-mediated heparin-induced thrombocytopenia (HIT) in neurologic patients during administration of low-molecular-weight heparin (LMWH) vs unfractionated heparin (UFH). None of 111 neurologic patients receiving LMWH developed HIT, whereas HIT occurred in 2.5% of 200 patients treated with UFH (p = 0.17). The rate of heparin-induced antibodies in patients treated with LMWH was lower than in patients treated with UFH (1.8 vs 20.5%; p < 0.001).     

Heparin induced thrombocytopenia and thrombosis in a tertiary care hospital

Heparin-induced thrombocytopenia (HIT) results from development of an antibody to a complex of heparin and platelet factor 4 (PF4) resulting in thrombocytopenia and a prothrombotic state with serious clinical consequences. The diagnosis depends on a combination of both the clinical presentation and laboratory detection of an appropriate antibody.ObjectiveTo determine the frequency, clinical characteristics and laboratory correlates of HIT in a tertiary care hospital.MethodsA retrospective review of all case of HIT over a thirty month period in a tertiary care hospital was conducted.ResultsHIT was diagnosed in 136 patients including 114/28,091 (0.48%) of those receiving only unfractionated heparin, 22/6,559 (0.33%) of those that received both unfractionated and low-molecular-weight heparin (LMWH) and in 2/2498 (0.08%) of those receiving only LMWH (P = 0.02 compared to those receiving only unfractionated heparin). HIT occurred in 62/16,939 patients (0.39%) of patients receiving subcutaneous (SC) heparin or LMWH compared to 69/11,152 (0.62%) of patients receiving intravenous (IV) therapy (P = 0.003). Of all patients with exposure to heparin products, 41/34,650 (0.1%) developed symptomatic thrombosis. The optical density (OD) of the ELISA was significantly higher in patients with HIT and thrombosis (1.2 +/- 0.8) compared to those without thrombosis (0.9 +/- 0.6, P = 0.03).ConclusionHIT develops in approximately 0.4% of all patients exposed to heparin at a tertiary care hospital but is significantly less frequent in those treated with LMWH only than in those who receive unfractionated heparin. A higher antibody titer is associated with the development of thrombosis. The occurrence of HIT could be decreased by reducing exposure to unfractionated heparin, and the diagnosis could be improved by reporting the OD of the ELISA test result.     

A new approach in the study of the molecular and cellular events implicated in heparin-induced thrombocytopenia. Formation of leukocyte-platelet aggregates.

Heparin-induced thrombocytopenia (HIT), a relatively common complication of heparin therapy, results of platelet activation, via the receptor for the Fc domain of IgG (FcgammaRIIa), by heparin-dependent-antibodies, commonly directed against the heparin-platelet factor 4 (H-PF4) antigenic complex. Our strategy was to use whole blood allowing the study of leukocyte-platelet interactions. Experiments were performed with blood from healthy donors incubated with HIT patients' plasma and different concentrations of heparin. We showed that 75% of the HIT patients' plasma induced the formation of leukocyte-platelet-aggregates in a heparin-dependent-manner. The formation of leukocyte-platelet-aggregates induced by HIT plasma in the presence of heparin was (i) independent of the healthy blood donor FcgammaRIIa polymorphism, (ii) correlated with the levels of anti H-PF4 IgG antibodies contained in the patients' plasma, and to a lesser extent to anti H-PF4 IgM antibodies, and (iii) was mediated by P-selectin. This report opens new prospects in the study of the molecular and cellular events implicated in HIT.     

A flow cytometric assay of platelet activation marker P-selectin (CD62P) distinguishes heparin-induced thrombocytopenia (HIT) from HIT with thrombosis (HITT).

Heparin induced thrombocytopenia (HIT) is a well-known complication of heparin administration but usually resolves upon discontinuation without sequelae. However, a small proportion of HIT patients develop thrombosis associated with HIT, designated as HITT, which is often life-threatening and may lead to gangrene and amputations. Existing laboratory methods of confirming HIT/HITT do not distinguish between HIT and HITT. We report a flow cytometric assay of platelet activation marker CD62P to distinguish the effects of addition of HIT vs. HITT plasma to normal blood. Briefly, normal whole blood was incubated with platelet-poor plasma from 12 patients with HITT, 30 with HIT, and 65 controls, in presence and absence of heparin, and expression of CD62P was assayed by flow cytometry. When the ratios of fluorescent intensity of CD62P with heparin divided by that without heparin were compared, HITT plasma induced significantly higher ratios than HIT plasma (HITT ratios approximately 2.5 vs. HIT ratios approximately 1.2; p <0.001). Eleven of 12 HITT patients were positive by this test but only 5 of 30 HIT patients were positive (p <0.0005). In a case of HIT with silent thrombosis, this assay gave a positive results prior to clinically evident thrombosis. In conclusion, this method distinguishes HITT from HIT and may be clinically useful in the detection of HITT, allowing early intervention for preventing catastrophic thrombosis.     

Clinical features of heparin-induced thrombocytopenia including risk factors for thrombosis. A retrospective analysis of 408 patients

Immune mediated heparin induced thrombocytopenia (HIT) is a prothrombotic adverse effect of heparin. However, only a subgroup of patients with HIT develops thromboembolic complications. We aimed to identify risk factors for developing HITassociated thrombosis. We analyzed a registry of patients with clinical suspicion of HIT who tested positive using a sensitive functional assay. Patient information was obtained by a standardized questionnaire. By multivariate analysis the association of age, gender, type of patient population, and magnitude of the platelet count decline with the frequency, type (venous or arterial), and temporal pattern of thrombotic events was assessed. In 408 HIT patients we observed predominance of venous thrombosis (2.4:1), with 40% of patients developing a pulmonary embolism. However, in the subgroup of post-cardiovascular surgery patients there was predominance of arterial thrombosis (1:8.5). The type of arterial thrombosis (limb artery thrombosis > thrombotic stroke > myocardial infarction) was the converse of that observed with typical atherothrombotic clots in non-HIT populations. In 59.8% of patients HIT-related thrombosis manifested either on the same day a platelet count decrease >50% was documented (26.3%) or before the decrease in platelet counts (33.5%). The most important risk factors for thrombosis were orthopedic/trauma surgery and the magnitude of platelet count decrease. HIT-associated thrombosis occurs in a considerable proportion of patients before platelet counts decrease by more than 50%.     

Polymorphonuclear leukocyte and monocyte activation induced by plasma from patients with heparin-induced thrombocytopenia in whole blood

Heparin-induced thrombocytopenia (HIT), a severe complication of heparin therapy, results from platelet activation by heparin-dependent antibodies. Previously, we have shown that plasma from patients with HIT (HIT plasma) induces leukocyteplatelet aggregation in blood. In this report, we examined leukocyte activation by HIT plasma and the contribution of heparin and platelets to this activation, in whole blood. Degranulation of leukocytes from HIT patients was evaluated as a leukocyte activation marker. We showed that polymorphonuclear leukocytes (PMN) and monocytes were the leukocyte subpopulations involved in platelet-leukocyte aggregation induced by HIT plasma in healthy donor blood. PMN and monocyte activation, reflected by increased surface expression of the CD11b adhesion molecule, was induced by HIT plasma in a heparin-dependent manner. The CD11b increase induced by HIT plasma was observed on PMN only when they were associated with platelets. Moreover, the increased CD11b expression on monocytes and PMN correlated strongly with the degree of platelet adhesion to these cells. Degranulation of leukocytes from HIT patients and control subjects (non-HIT heparin-treated patients and healthy subjects) was evaluated in vivo by measuring the plasma myeloperoxidase concentration. HIT plasma contained higher myeloperoxidase concentrations than control plasma, suggesting leukocyte degranulation during HIT. In conclusion, this study provides the first evidence that PMN activation is induced by HIT plasma. HIT plasma induced PMN and monocyte activation in a heparin-dependent manner. In whole blood, platelet association with monocytes and PMN, and the activation of these leukocytes by HIT plasma were interrelated. Finally, leukocyte degranulation could be involved in HIT physiopathology.     

Antiplatelet Factor 4—Heparin Antibodies in Patients with Antiphospholipid Antibodies

Antibodies directed against platelet factor 4-heparin are present in patients with heparin-induced thrombocytopenia (HIT). Additionally, it has been suggested that heparin can be an antigenic target of antiphospholipid antibodies (aPL). We investigated the presence of heparin-platelet factor 4-induced antibodies (HPIA) in 33 patients with aPL. There were 30 patients with lupus anticoagulant, 25 with anticardiolipin antibodies, 21 with anti-β₂ glycoprotein I, and 18 with antiprothrombin antibodies. 20 patients had a history of thrombosis and 19 had received heparin during the last 60 months. We found 7 (21.2%) who had HPIA; 5 of them also had anti-β₂ glycoprotein I antibodies. Four patients had severe thrombocytopenia and suspicion of HIT. Among them, two presented high positive HPIA results, one of them with positive platelet aggregation test. The third patient showed grey zone HPIA and borderline aggregation test and the fourth one had negative results. Among patients without a history of HIT, 2 who had never received heparin presented high positive, one a moderate positive, and one a grey zone HPIA result; all of them with negative aggregation tests. Five positive sera samples were incubated with cardiolipin liposomes in the presence of β₂ glycoprotein I, and whereas an inhibition greater than 50% was achieved in anticardiolipin and anti-β₂ glycoprotein I activities, HPIA results did not change. We demonstrate that HPIA could be frequently found in patients with aPL. They are responsible for HIT in some cases but can also be found in patients who have not received heparin. Whether they predispose patients with aPL to HIT is not known; nevertheless, a close follow-up of heparin treatment in these patients seems to be mandatory.     

Heparin-induced thrombocytopenia in children: 12 new cases and review of the literature

Immune-mediated heparin-induced thrombocytopenia (HIT) is a rare but severe adverse effect of heparin therapy. Only few data are available on clinical presentation, diagnosis and management of HIT in children. Records of all patients sent to our laboratory between 1995 and November 2003 were reviewed. To identify literature reports a Medline search was performed, the reference lists of those publications were screened and the abstracts of meetings on thrombosis and hemostasis between 2000 and 2003 were assessed. We identified 12 new HIT patients between 13 months and 18 years of age from our laboratory and 71 reports on HIT in children in the literature. For the assessment of frequency of HIT all studies enrolling > 100 patients were analyzed. HIT is rare in children. In pediatric patients, there seem to be two risk groups: newborns and infants under 4 years of age undergoing cardiac surgery (incidence approximately 1-2%), and teenagers treated with heparin for thrombosis. For confirmation of HIT in children, antigen assays are most important. There are conflicting data on the optimal cut-off, with one randomized, double-blind trial indicating that the cut-off established in adults is appropriate. There are no systematic studies on alternative anticoagulants in children affected by HIT. Most data are available for lepirudin and danaparoid. Substitution of unfractionated heparin by low-molecular-weight heparins for regular anticoagulation may reduce the incidence of HIT.     

Laboratory testing for heparin-induced thrombocytopenia is inconsistent in North America: a survey of North American specialized coagulation laboratories

Heparin-induced thrombocytopenia (HIT) is a serious complication of heparin therapy. As HIT is considered a clinico-pathologic entity, laboratory practices have an important role in diagnosing or excluding HIT. It was the objective of this study to assess the current status of laboratory testing for HIT in North America. An online survey consisting of 67 questions related to laboratory testing for HIT was developed by the North American Specialized Coagulation Laboratory Association (NASCOLA), and distributed to its 59 members. The survey included queries about HIT test ordering practices, HIT immunoassay and activation assays performed, and reporting practices. Data was collected from the 44 NASCOLA laboratories who responded. Of these sites, 88% performed immunoassays for HIT, commonly using commercial assays. However, sites varied in practices related to use of controls, immunoglobulin class of antibody detected, and in result interpretation and reporting. Platelet activation assays for HIT were performed by 36% of sites, commonly using assays of serotonin release (50%) or heparin-induced platelet aggregation (43%). Sites varied in the use of washed platelets versus platelet-rich plasma, controls, and heparin concentrations. This survey is the first comprehensive assessment of patterns of practice in HIT testing among diagnostic coagulation laboratories in North America. We observed site-specific variability of testing methods encompassing all stages of testing, including pre-analytical handling, testing methodologies, and result interpretation and reporting. The variability in HIT platelet activation assay methods among institutions indicates a need for proficiency testing to assess assay performance, and for consensus guidelines on HIT laboratory testing.     

Disruption of PF4/H multimolecular complex formation with a minimally anticoagulant heparin (ODSH)

Recent studies have shown that ultra-large complexes (ULCs) of platelet factor 4 (PF4) and heparin (H) play an essential role in the pathogenesis of heparin-induced thrombocytopenia (HIT), an immune-mediated disorder caused by PF4/H antibodies. Because antigenic PF4/H ULCs assemble through non-specific electrostatic interactions, we reasoned that disruption of charge-based interactions can modulate the immune response to antigen. We tested a minimally anticoagulant compound (2-O, 3-O desulfated heparin, ODSH) with preserved charge to disrupt PF4/H complex formation and immunogenicity. We show that ODSH disrupts complexes when added to pre-formed PF4/H ULCs and prevents ULC formation when incubated simultaneously with PF4 and UFH. In other studies, we show that excess ODSH reduces HIT antibody (Ab) binding in immunoassays and that PF4/ODSH complexes do not cross-react with HIT Abs. When ODSH and unfractionated heparin (UFH) are mixed at equimolar concentrations, we show that there is a negligible effect on amount of protamine required for heparin neutralisation and reduced immunogenicity of PF4/UFH in the presence of ODSH. Taken together, these studies suggest that ODSH can be used concurrently with UFH to disrupt PF4/H charge interactions and provides a novel strategy to reduce antibody mediated complications in HIT.     

Low prevalence of heparin-induced thrombocytopenia after cardiac surgery in Thai patients

Introduction

Heparin induced-thrombocytopenia (HIT) has been well recognized in Western countries. However, there are no data in the Thai population. We therefore investigated the prevalence of anti-platelet factor 4 (PF4)/heparin antibodies, HIT, and its thrombotic complications in Thai patients undergoing cardiac surgery using unfractionated heparin.

Materials and methods

Seventy-three consecutive patients were prospectively enrolled in this study. Blood samples before operation and week 1, week 2, and week 3 after operation were collected from each patient for HIT antibody screening by enzyme-linked immunosorbent assay using IgG antibody specific to the PF4/heparin complex. Positive samples were further analyzed by ¹⁴C-serotonin release assay. Complete blood count was performed daily during the first week, then weekly for 3 weeks.

Results

No patient had detectable anti-PF4/heparin antibodies at baseline. Five patients sero-converted during the course of the study for anti-PF4/heparin IgG: 3 (4.1%) at week 1, 4 (5.5%) at week 2, and 5 (6.8%) at week 3 after surgery. However, none of these patients had anti-PF4/heparin antibodies that resulted in ¹⁴C-serotonin release to be considered clinically significant antibodies. Post-operative thrombocytopenia after the operation was found in 35 patients (47.9%), but was not considered to be caused by HIT. Thromboembolic events occurred in 3 patients (4.1%) during follow up; however, none of these patients had positive PF4/heparin antibody tests.

Conclusions

Our study represents the first study to examine Thai patients exposed to heparin in the context of cardiac surgery. We found a lower prevalence of positive anti-PF4/heparin antibodies and clinical HIT than previously published studies.     

Validation of whole blood impedance aggregometry as a new diagnostic tool for HIT: results of a large Australian study

Heparin-induced thrombocytopenia (HIT) remains a challenge, with diagnosis confirmed only by functional assays. The gold standard 14C-serotonin release assay (SRA) is highly sensitive but technically challenging and unsuitable for routine use. We conducted a large study to validate whole blood impedance aggregometry (WBIA) as a suitable diagnostic tool for HIT. WBIA and SRA were used to test 181 samples positive for H-PF4 antibodies by PaGIA or ELISA. Using the same high responder donor, 77 samples were positive by WBIA (aggregation with low-dose but not high-dose heparin). Using the strict definition for SRA positivity, 72 samples were true HIT. In nine samples, serotonin release with high-dose heparin dropped by > 50% but was still >20%; these were retested after a one-half dilution and 8/9 became positive. Ten other samples were discrepant between the two assays: one strongly positive (89% release) and six weakly positive samples by SRA (average release 56%) were WBIA negative. When these samples were retested using a random donor, only two remained SRA positive. Three samples were strongly WBIA positive but SRA negative; two were retested by SRA with 0.5IU/ml heparin and one became positive. Under controlled conditions, using the same selected high-responder donor, WBIA and SRA performed similarly with slightly increased sensitivity of the WBIA when using the strict definition of SRA positivity. WBIA is easy to perform with rapid turn-around time and warrants a multi-laboratory trial to complete its validation as a confirmatory assay for platelet-activating HIT antibodies.