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

Evaluation of a clinical scoring scale to direct early appropriate therapy in heparin-induced thrombocytopenia

Background. Heparin-induced thrombocytopenia (HIT) is a serious adverse effect associated with heparin therapy. Current laboratory confirmation for immune mediated HIT often results in false positives and unnecessary treatment, exposing individuals to possible complications. As a result, clinical evaluation has been recommended in conjunction with laboratory testing. We hypothesize that utilization of a clinical scoring scale, the 4T’s, will result in the initial appropriate therapy for suspected HIT.

Methods. This is a retrospective chart review of 108 patients who underwent ELISA testing for HIT at a university hospital. The 4T’s scale was applied, stratifying individuals into low, intermediate, and high-risk categories. Each risk score was compared to the ELISA results to determine if the 4T’s can predict the diagnosis of HIT and result in appropriate management. ELISA optical density scores as well as incidence of adverse events were also compared among risk categories.

Study Results. Individuals with low risk correlate with a negative ELISA compared to intermediate and high-risk individuals (p = 0.01 and p<0.01) and also were significantly more likely to predict institution of appropriate therapy (p<0.01). Median optical density scores were 0.184 (0.046—2.116), 0.226 (0.067—1.887), and 0.476 (0.096—1.309) for low, intermediate, and high 4T scores. Major adverse events include thrombosis and bleeding.

Conclusions. Individuals with low risk were more likely to receive initial, appropriate therapy and were also significantly more likely to have a negative ELISA test result. Individuals with low risk determined by the 4T score therefore may have therapy and serologic testing for HIT withheld.

Prospective evaluation of PF4/heparin immunoassays for the diagnosis of heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is an adverse complication of heparin caused by HIT antibodies that recognize platelet factor 4-heparin (PF4/hep) complexes leading to platelet activation. Several methods are available for the identification of HIT antibodies.

OBJECTIVES

To evaluate the clinical usefulness of different antigen-binding assays for detection of antibodies against PF4/hep complexes in a prospective study.

PATIENTS/METHODS

A prospective cohort of 500 surgical and medical patients suspected of having HIT was evaluated. The laboratory assessment included particle gel immunoassay (PaGIA), polyspecific ELISA recognizing IgG, IgM and IgA antibodies (Poly-ELISA), IgG-specific ELISA (IgG-ELISA) and the HIPA test. The pretest probability of HIT was determined using the 4T's model. Positive and negative predictive values (PPV, NPV) of each immunoassay were determined depending upon the heparin-induced platelet activation (HIPA) results and the clinical scoring. The operating characteristics of each immunoassay were determined using the receiver-operation characteristic (ROC) curve.

RESULTS

Platelet-activating antibodies were identified in 35/500 patients, all of whom had intermediate to high clinical probability. PF4/hep antibodies were detected in 124, 86 and 90 sera using Poly-ELISA (PPV = 28), IgG-ELISA (PPV = 40.6) and PaGIA (PPV = 36.6). NPV was 100% for Poly- and IgG-ELISA, but only 99.5% for PaGIA. ROC analysis revealed that PaGIA is less informative than ELISA. The IgG-ELISA provides better diagnostic information than the other assays. In addition, there is a clear correlation between optical density (OD) value and the probability of having HIT.

CONCLUSIONS

Our observation indicates that an IgG-ELISA provides the best diagnostic information of all antigen-binding assays.

Heparin-induced thrombocytopenia: a renal perspective

Heparin-induced thrombocytopenia (HIT) is a clinicopathologic syndrome in which one or more clinical events, usually thrombocytopenia or thrombosis, are temporally related to heparin administration and caused by HIT antibodies. Rapid and accurate diagnosis is essential given the high incidence of thrombosis at around the time of initial disease recognition. Discontinuation of heparin and initiation of alternative anticoagulants reduces HIT-associated morbidity and mortality. The clinical consequences of HIT in hemodialysis patients remain unclear, with several studies reporting no clinical sequelae and others describing complications such as thrombocytopenia or clotting of the extracorporeal circuit. Frequent clotting of the extracorporeal circuit has also been reported in HIT-antibody-positive patients on continuous veno-venous hemofiltration. Several recent findings are of particular interest to nephrologists. An acute systemic reaction has been described as a presentation of HIT in hemodialysis patients shortly after administration of an unfractionated heparin bolus. This syndrome is important to recognize as it might mimic a dialyzer reaction. More recently, the presence of a positive HIT-antibody test or increasing titers of HIT antibody were associated with increased mortality in hemodialysis patients, raising the question of whether these antibodies have a role in the increased cardiovascular mortality seen in these patients. HIT-antibody production is often transient and small numbers of hemodialysis patients with undetectable antibody levels have been rechallenged with heparin without adverse clinical consequences.

Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT), typically occurring in the second week of heparin therapy, is an antibody-mediated adverse drug reaction associated with increased thrombotic risk. The most important antigens are located on platelet factor 4 (PF4)/heparin complexes. All HIT is caused by platelet-activating antibodies, but not all PF4/heparin-reactive antibodies cause HIT. Thus, tests have high negative, but only moderate, positive predictive value. Cessation of heparin requires substitution with an alternative anticoagulant, but as these drugs have increased bleeding risk, they should be used in therapeutic doses only if HIT is considered very likely. Avoiding/postponing coumarin is crucial in minimizing microthrombotic complications. Recent studies of HIT immunobiology suggest that HIT mimics immunity against repetitive antigens, as they are relevant in microbial defense. Thus, understanding HIT may help unravel why host defenses can trigger autoimmunity.

Studies of the immune response in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is caused by platelet-activating antibodies that recognize PF4/heparin complexes. Uncertainties remain regarding HIT immunobiology, including the temporal relation of antibody formation to onset of thrombocytopenia, and whether immunoglobulin class switching occurs. Using serial plasma samples from 2 heparin thromboprophylaxis trials, we determined the time of onset, antibody levels, and immunoglobulin class distributions (IgG, IgA, IgM) for 12 patients with HIT and 36 patients who formed anti-PF4/heparin antibodies, but did not develop HIT (“seropositive non-HIT controls”). In patients with HIT, anti-PF4/heparin antibodies became detectable 4 days (median) after starting heparin; antibody detection preceded the platelet count decline by 2 days (median). Patients with HIT produced higher levels of IgG antibodies, but similar IgA and IgM levels, compared with seropositive non-HIT controls. Among all 48 seroconverting patients, the first day of a positive antibody test (median, day 6) did not differ among the immunoglobulin classes. Thus, the HIT immune response does not exhibit the classic paradigm of IgM class precedence/immunoglobulin class switching; rather, relatively rapid formation of IgG antibodies is observed, sometimes with concomitant IgA and IgM formation. Compared with seropositive non-HIT controls, HIT patients develop significantly higher anti-PF4/heparin IgG levels that are detectable before the onset of thrombocytopenia.

The costs of heparin-induced thrombocytopenia: a patient-based cost of illness analysis

SUMMARY BACKGROUND AND OBJECTIVES: Due to the complexity of heparin-induced thrombocytopenia (HIT), currently available cost analyses are rough estimates. The objectives of this study were quantification of costs involved in HIT and identification of main cost drivers based on a patient-oriented approach.

METHODS

Patients diagnosed with HIT (1995-2004, University-hospital Greifswald, Germany) based on a positive functional assay (HIPA test) were retrieved from the laboratory records and scored (4T-score) by two medical experts using the patient file. For cost of illness analysis, predefined HIT-relevant cost parameters (medication costs, prolonged in-hospital stay, diagnostic and therapeutic interventions, laboratory tests, blood transfusions) were retrieved from the patient files. The data were analysed by linear regression estimates with the log of costs and a gamma regression model. Mean length of stay data of non-HIT patients were obtained from the German Federal Statistical Office, adjusted for patient characteristics, comorbidities and year of treatment. Hospital costs were provided by the controlling department.

RESULTS AND CONCLUSIONS

One hundred and thirty HIT cases with a 4T-score >or=4 and a positive HIPA test were analyzed. Mean additional costs of a HIT case were 9008 euro. The main cost drivers were prolonged in-hospital stay (70.3%) and costs of alternative anticoagulants (19.7%). HIT was more costly in surgical patients compared with medical patients and in patients with thrombosis. Early start of alternative anticoagulation did not increase HIT costs despite the high medication costs indicating prevention of costly complications. An HIT cost calculator is provided, allowing online calculation of HIT costs based on local cost structures and different currencies.

Low-molecular-weight heparin-induced skin necrosis: about 2 cases

Heparin-induced skin necrosis is an infrequent condition, complicating unfractionated heparin and more rarely low-molecular-weight heparin administration. Like heparin-induced thrombocytopenia, it is associated with the presence of anti-PF4 antibodies. After reporting 2 observations, we briefly review the clinical features and management of this condition. Early recognition is of the utmost importance since heparin-induced skin necrosis can be associated with HIT, a potentially lethal disorder.

Evaluation and management of thrombocytopenia and suspected heparin-induced thrombocytopenia in hospitalized patients: The Complications After Thrombocytopenia Caused by Heparin (CATCH) registry

BACKGROUND

Thrombocytopenia and heparin-induced thrombocytopenia (HIT) are potentially devastating paradoxical side effects of heparin therapy. We explored the evaluation, management, and clinical consequences of thrombocytopenia occurring during heparin therapy in diverse clinical settings.

METHODS

CATCH was a prospective observational study that enrolled 3,536 patients in 48 US hospitals. Data were collected on 3 strata: patients receiving any form of heparin for > or =96 hours (n = 2,420); cardiac care unit (CCU) patients treated with heparin who developed thrombocytopenia (n = 1,090); patients who had an HIT assay performed (n = 449).

RESULTS

Thrombocytopenia occurred in 36.4% of patients in the prolonged heparin stratum and was associated with an increased risk of death or thromboembolic complication (OR 1.5, 95% CI 1.2-1.9). Among a subset of patients whose clinical presentation suggested they were at high risk for HIT, suspicion for HIT was uncommon (prolonged heparin stratum 19.8%, CCU stratum 37.6%) and often did not arise until > or =1 day after patients developed thrombocytopenia. Often patients were not evaluated for HIT until after they had had a thromboembolic complication (prolonged heparin stratum 43.8%, CCU stratum 61%). Even after HIT was suspected, patients often continued to receive heparin. Direct thrombin inhibitor use was infrequent (prolonged heparin stratum 29.4%, CCU stratum 35.6%). Among the few patients who underwent evaluation, HIT was confirmed in 46.7% of the prolonged heparin stratum and 31.4% of the CCU stratum.

CONCLUSIONS

Thrombocytopenia is common among patients receiving heparin, and it is associated with substantial risk for catastrophic complications. Despite the high risk for HIT in this population, recognition, evaluation, and appropriate treatment are infrequent and delayed.

Heparin-induced thrombocytopenia successfully treated with fondaparinux

We report the successful treatment of a patient with HIT-associated venous thrombosis by fondaparinux, a synthetic pentasaccharide. Although not yet approved for this indication, this new anticoagulant may be a useful alternative in the setting of HIT.

Argatroban anticoagulation in intensive care patients: effects of heart failure and multiple organ system failure

We retrospectively evaluated argatroban dosing patterns, clinical outcomes, and the effects of heart failure and multiple organ system failure on dosing requirements in 65 adult, intensive care patients administered argatroban anticoagulation for clinically suspected heparin-induced thrombocytopenia (n=56) or history of heparin-induced thrombocytopenia (n=9). Argatroban was initiated then titrated to achieve target activated partial thromboplastin times 1.5 to 3 times normal control (ie, 42-84 seconds). Overall, argatroban was initiated at 1.14+/-0.62 microg/kg/min (mean+/-SD) and administered for 11.4+/-9.5 days, with comparable dosing patterns between patients with suspected, versus previous, heparin-induced thrombocytopenia. Sixty-four (98.5%) patients achieved target activated partial thromboplastin times, typically following no or one dose adjustment. Therapeutic doses were lower in patients with, versus without, heart failure (0.58+/-0.28 vs 0.97+/-0.6 microg/kg/min, P= .042) and decreased as the number of failed organ systems increased from 1 to 2 to =3 (1.10+/-0.67 vs 0.87+/-0.47 vs 0.58+/-0.47 microg/kg/min, P= .008). From argatroban initiation until patient discharge or death, 11 (16.9%) patients (3 off argatroban) developed thromboembolic complications; 14 (21.5%) died (11 off argatroban, 7 from multiple organ system failure); and 1 (1.5%) required amputation. Nine patients (13.8%) experienced bleeding, none fatal. This experience suggests that argatroban administered at approximately 1 micro/kg/min provides adequate levels of anticoagulation in many intensive care unit patients with suspected or previous heparin-induced thrombocytopenia. Reduced doses are needed when heart failure or multiple organ system failure is present.

Timely diagnosis and management of heparin-induced thrombocytopenia in a frequent request, low incidence single centre using clinical 4T's score and particle gel immunoassay

Clinicians must promptly decide which patients suspected of having heparin-induced thrombocytopenia (HIT) warrant a change in anticoagulation. This single-centre series of 246 HIT testing referrals assessed the combination of clinical score (thrombocytopenia, timing, thrombosis, other causes of thrombocytopenia not evident; 4T's), Diamed ID-Heparin-PF4 immunoassay (PaGIA) and 14C Serotonin Release Assay (SRA) to develop a practical and safe diagnostic strategy for HIT. A total of 142/256 (58%) referrals were in patients with a low 4T's score, with 12/246 (5%) in the high scoring group. PaGIA was positive in 24/246 (9.7%) patients, whilst SRA was positive in 9/246 (3.6%). The overall positive predictive value of a positive PaGIA test alone was 37.5%, however this reached 80% for the high scoring group. Both negative PaGIA and low clinical score independently had negative predictive values of 100%. We subsequently developed an algorithm that, when applied to this cohort, would have resulted in 18/246 patients (7%) definitely requiring alternative anticoagulation, whilst a further 7/246 (2.8%) patients would have been considered on an individual basis. Ultimately (based on SRA) this would have resulted in 16/246 (6.5%) patients unnecessarily having a change in their anticoagulation, with 9/246 (3.6%) patients being 'correctly treated'. The combination of 4T's scoring and PaGIA permitted a practical and safe approach to rapid HIT diagnosis and management.

Cerebral infarction associated with heparin-induced thrombocytopenia in a patient with encephalitis

We report a patient who had cerebral infarction associated with heparin-induced thrombocytopenia (HIT) during treatment of aseptic encephalitis. In patients with intracranial inflammation, such as ours, the possibility of HIT has to be considered when heparin is used, since inflammatory cerebral lesions often cause vascular damage, which is an aggravating factor for HIT-associated thrombosis.

Heparin-induced thrombocytopenia: when a low platelet count is a mandate for anticoagulation

Heparin-induced thrombocytopenia (HIT) is an immune-mediated disorder caused by the development of antibodies to platelet factor 4 (PF4) and heparin. The thrombocytopenia is typically moderate, with a median platelet count nadir of ~50 to 60 × 109 platelets/L. Severe thrombocytopenia has been described in patients with HIT, and in these patients antibody levels are high and severe clinical outcomes have been reported (eg, disseminated intravascular coagulation with microvascular thrombosis). The timing of the thrombocytopenia in relation to the initiation of heparin therapy is critically important, with the platelet count beginning to drop within 5 to 10 days of starting heparin. A more rapid drop in the platelet count can occur in patients who have been recently exposed to heparin (within the preceding 3 months), due to preformed anti-heparin/PF4 antibodies. A delayed form of HIT has also been described that develops within days or weeks after the heparin has been discontinued. In contrast to other drug-induced thrombocytopenias, HIT is characterized by an increased risk for thromboembolic complications, primarily venous thromboembolism. Heparin and all heparin-containing products should be discontinued and an alternative, non-heparin anticoagulant initiated. Alternative agents that have been used effectively in patients with HIT include lepirudin, argatroban, bivalirudin, and danaparoid, although the last agent is not available in North America. Fondaparinux has been used in a small number of patients with HIT and generally appears to be safe. Warfarin therapy should not be initiated until the platelet count has recovered and the patient is systemically anticoagulated, and vitamin K should be administered to patients receiving warfarin at the time of diagnosis of HIT.

Heparin-induced thrombocytopenia: some working hypotheses on pathogenesis, diagnostic strategies and treatment

PURPOSE OF REVIEW

The present contribution will illustrate some evolving concepts on the pathogenesis and clinical management of heparin-induced thrombocytopenia (HIT) and describe how we approach patients with suspected HIT at our institution.

RECENT FINDINGS

HIT is caused by an autoimmune reaction leading to the formation of antibodies directed against platelet factor 4. Conditions favoring the development of anti-platelet factor 4/heparin antibodies differ from those required for the formation of macromolecular ternary complexes (HIT antibody/platelet factor 4/heparin), which are able to activate platelets and induce clinical HIT. HIT can be diagnosed by combining its pretest probability with the quantitative result of rapid HIT-antibody assays. Treatment of acute HIT requires inhibition of in-vivo thrombin generation by means of alternative nonheparin anticoagulant drugs, whose effective dosage appears to be significantly lower than the official recommendations. As HIT antibodies are transient, HIT patients can be re-exposed to heparin, provided that previous heparin treatment is remote and that anti-platelet factor 4/heparin antibodies are undetectable.

SUMMARY

In recent years, there has been a continuing elucidation of pathogenic and clinically relevant issues, which are intellectually rewarding to follow and should enable us to offer a steadily improving treatment to the HIT patients we are in charge of.

Management of anticoagulation in patients with subacute heparin-induced thrombocytopenia scheduled for heart transplantation

Anticoagulation management of patients with recent heparin-induced thrombocytopenia (HIT) requiring cardiopulmonary bypass (CPB) surgery is a serious challenge, and especially difficult in patients requiring urgent heart transplantation. As nonheparin anticoagulants during CPB bear a high risk of major bleeding, these patients are at risk of being taken off the transplant list. Short-term use of unfractionated heparin (UFH) for CPB, with restriction of UFH to the surgery itself, is safe and effective in patients with a history of HIT who test negative for antiplatelet factor 4 (PF4)/heparin antibodies. We present evidence that it is safe to expand the concept of UFH reexposure to patients with subacute HIT (ie, those patients with recent HIT in whom the platelet count has recovered but in whom anti-PF4/heparin IgG antibodies remain detectable) requiring heart transplantation, if they test negative by a sensitive functional assay using washed platelets. This can be lifesaving in patients with end-stage heart failure.

Treatment and prevention of heparin-induced thrombocytopenia: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition)

This chapter about the recognition, treatment, and prevention of heparin-induced thrombocytopenia (HIT) is part of the Antithrombotic and Thrombolytic Therapy: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). Grade 1 recommendations are strong and indicate that the benefits do, or do not, outweigh risks, burden, and costs. Grade 2 suggests that individual patient values may lead to different choices. Among the key recommendations in this chapter are the following: For patients receiving heparin in whom the clinician considers the risk of HIT to be > 1.0%, we recommend platelet count monitoring over no platelet count monitoring (Grade 1C). For patients who are receiving heparin or have received heparin within the previous 2 weeks, we recommend investigating for a diagnosis of HIT if the platelet count falls by >/= 50%, and/or a thrombotic event occurs, between days 5 and 14 (inclusive) following initiation of heparin, even if the patient is no longer receiving heparin therapy when thrombosis or thrombocytopenia has occurred (Grade 1C). For patients with strongly suspected (or confirmed) HIT, whether or not complicated by thrombosis, we recommend use of an alternative, nonheparin anticoagulant (danaparoid [Grade 1B], lepirudin [Grade 1C], argatroban [Grade 1C], fondaparinux [Grade 2C], or bivalirudin [Grade 2C]) over the further use of unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) therapy or initiation/continuation of vitamin K antagonists (VKAs) [Grade 1B]. The guidelines include specific recommendations for nonheparin anticoagulant dosing that differ from the package inserts. For patients with strongly suspected or confirmed HIT, we recommend against the use of vitamin K antagonist (VKA) [coumarin] therapy until after the platelet count has substantially recovered (usually, to at least 150 x 10(9)/L) over starting VKA therapy at a lower platelet count (Grade 1B); that VKA therapy be started only with low maintenance doses (maximum, 5 mg of warfarin or 6 mg of phenprocoumon) over higher initial doses (Grade 1B); and that the nonheparin anticoagulant (eg, lepirudin, argatroban, danaparoid) be continued until the platelet count has reached a stable plateau, the international normalized ratio (INR) has reached the intended target range, and after a minimum overlap of at least 5 days between nonheparin anticoagulation and VKA therapy rather than a shorter overlap (Grade 1B). For patients receiving VKAs at the time of diagnosis of HIT, we recommend use of vitamin K (10 mg po or 5 to 10 mg IV) [Grade 1C].

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.

Platelet transfusions in heparin-induced thrombocytopenia: a report of four cases and review of the literature

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a complication of heparin therapy associated with thrombocytopenia and thrombosis. The diagnosis of HIT is based on clinical criteria and laboratory tests, including the serotonin release assay (SRA). Because HIT patients are thrombocytopenic, platelet (PLT) transfusions may be contemplated; however, many published reviews have concluded that PLT transfusions are contraindicated in HIT because they may precipitate thrombotic events. This study reports four patients with clinically suspected HIT who received PLT transfusions without complications, and the literature regarding this subject has been reviewed.

STUDY DESIGN AND METHODS

Patients with a SRA ordered for suspected HIT were retrospectively identified. Charts of patients with positive SRAs who received a PLT transfusion when HIT was clinically suspected were reviewed for evidence of PLT transfusion safety and efficacy. A comprehensive search of the published literature regarding PLT transfusions in patients with HIT was conducted.

RESULTS

A SRA was performed on 189 patients with suspected HIT. Thirteen patients tested positive and 4 of these received a PLT transfusion. No patient developed a thrombotic complication. All 4 patients had adequate posttransfusion PLT increments. Two of the 3 patients with active bleeding had cessation of bleeding after transfusion. Review of the literature revealed no case of a complication clearly attributable to PLT transfusion.

CONCLUSION

Four patients with clinically suspected HIT and a positive SRA were transfused PLTs both efficaciously and safely. These outcomes, combined with the results of the literature review, suggest that PLT transfusions should not be withheld when clinically indicated in patients with HIT.

A clinical-laboratory approach contributing to a rapid and reliable diagnosis of heparin-induced thrombocytopenia

INTRODUCTION

Heparin-induced thrombocytopenia (HIT) remains a very challenging diagnosis. The first objective of this study was to compare the performance of the ID-H/PF4 PaGIA with the Asserachrom HPIA ELISA. The main purpose was to evaluate the diagnostic utility of the combination of the H/PF4 PaGIA with the clinical "4T's" score as a screening strategy.

MATERIALS AND METHODS

102 patients with clinical suspicion of HIT were classified into risk groups using the 4T's score. The presence of HIT antibodies was assessed by two immunoassays and confirmed by a functional flow cytometric assay.

RESULTS

Comparison of the ID-H/PF4 PaGIA with the Asserachrom HPIA ELISA demonstrated a comparable technical performance, being an excellent screening test to rule out HIT (negative predictive value or NPV=100%). According to the 4T's score, HIT was excluded in all low risk patients (NPV=100%). ELISA optical density levels were significantly different between all risk groups (P-values<0.01). In contrast, due to the low positive predictive value (22%) and weak positive likelihood ratio (2.6), a positive ID-H/PF4 PaGIA result did not considerably increase the probability of HIT.

CONCLUSION

Our study confirms the combination of the 4T's score with the ID-H/PF4 PaGIA as a reliable strategy to rule out HIT. Yet, confirming positive ID-H/PF4 PaGIA results by flow cytometry within 1-2 h after blood sampling remains necessary. This novel clinical-laboratory approach can contribute in a rapid and reliable way to the definite diagnosis of HIT.

Pharmacokinetics and pharmacodynamics of danaparoid during continuous venovenous hemofiltration: a pilot study

Background

In patients with suspected heparin-induced thrombocytopenia (HIT) who need renal replacement therapy, a nonheparin anticoagulant has to be chosen to prevent thrombosis in the extracorporeal circuit. Danaparoid, a low-molecular-weight heparinoid consisting of heparan sulphate, dermatan sulphate, and chondroitin sulphate, is recommended for systemic anticoagulation in patients with HIT. However, there are few data on the use of danaparoid in patients with acute renal failure, especially in patients dependent on renal replacement therapy such as continuous venovenous hemofiltration (CVVH). In the present study, we analyzed the pharmacokinetics and pharmacodynamics of danaparoid during CVVH in patients with suspected HIT.

Methods

Based on a mathematical model, a dosing scheme for danaparoid was designed, aiming at anti-Xa levels of 0.5 to 0.7 U/mL, with a maximum of 1.0 U/mL. This dosing scheme was prospectively tested in the first CVVH run of a cohort of five patients with suspected HIT. CVVH with a blood flow rate of 150 mL/minute and a substitution rate of 2,000 mL/hour was performed with a cellulose triacetate membrane. Danaparoid was administered as a continuous infusion of 100 anti-Xa-U/hour after a loading dose of 3,500 anti-Xa-U. Serial measurements of anti-Xa activity and prothrombin fragment F1+2 were performed at baseline, at t = 5, 15, and 30 minutes, and at t = 1, 2, 4, 8, 16, and 24 hours after the danaparoid loading dose.

Results

The median anti-Xa activity reached a maximum of 1.02 (0.66 to 1.31) anti-Xa-U/mL after 15 minutes and gradually declined to 0.40 (0.15 to 0.58) anti-Xa-U/mL over the span of 24 hours. Target anti-Xa levels were reached from 2 to 12 hours after the loading dose. Median prothrombin fragment F1+2 gradually decreased from 432 (200 to 768) to 262 (248 to 317) pmol/L after 24 hours. No bleeding or thromboembolic events occurred throughout the described treatment period.

Conclusion

Danaparoid administered by a continuous infusion of 100 anti-Xa-U/hour after a loading dose of 3,500 anti-Xa-U elicited target anti-Xa levels from 2 to 12 hours after the loading dose, without bleeding or thromboembolic events during the described CVVH treatment in patients with suspected HIT.

The clinical diagnosis of heparin-induced thrombocytopenia in patients receiving continuous renal replacement therapy

BACKGROUND

Thrombocytopenia is common in critically ill patients who receive continuous renal replacement therapy. Often, these patients receive heparin therapy and the diagnosis of heparin induced thrombocytopenia (HIT) is considered as a potential etiology. No data regarding the clinical diagnosis of HIT is available for patients receiving continuous renal replacement therapy.

PATIENTS AND METHODS

We performed a retrospective study of 29 consecutive patients who received CRRT in a medical-surgical intensive care unit (ICU) and determined trends in platelet counts following CRRT and the frequency of meeting platelet based clinical criteria for consideration of a HIT diagnosis.

RESULTS

For patient exposures to CRRT concurrent with heparin, 54% met at least one clinical threshold for consideration of the diagnosis of HIT. In 31% of exposures, both a platelet count <100,000/mm3 and a >50% decrease from baseline were seen. In contrast, the majority (73-85%) of patients receiving CRRT had a low pre-test probability of HIT using the "4T's" scoring system. Mean platelet counts while on CRRT concurrent with heparin were significantly lower than when patients received heparin alone (P < 0.02).

CONCLUSIONS

The clinical diagnosis of HIT in ICU patients initiating CRRT is challenging given the decrease in platelet counts seen following CRRT initiation in the majority of patients. A prospective study in this population is needed to optimize patient outcomes.

Utility of consecutive repeat HIT ELISA testing for heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a serious complication of heparin therapy. Limited data are available regarding repeat HIT antibody testing after an initial negative test. We conducted a retrospective study to determine the utility of repeat testing. Heparin antibodies were detected using the GTI-PF4 enzyme-linked immunoabsorbent assay, ELISA (GTI Diagnostics, Waukesha, WI). Patients (n = 137) were assigned to one of three groups based upon the initial negative test optical density (OD) range of low = 0-0.132, medium = 0.133-0.267, and high = 0.268-0.399. A pretest clinical score was retrospectively determined using the "4T's" (Thrombocytopenia, Timing of platelet fall, Thrombosis, and the absence of oTher causes of thrombocytopenia). A subsequent positive ELISA was found in 16% (22/137) of patients who underwent repeat testing. Most of these patients had a low pretest clinical score (62%). Four patients had an interval change in the pretest score between the initial negative and subsequent positive tests. Only these four patients developed HIT with thrombosis (HITT). Eighty percent of patients with a high initial negative test OD value had a positive ELISA on repeat testing; however, the initial negative test OD value could not predict whether a patient developed HITT. In contrast, an increase in the pretest clinical probability between initial and repeat testing better predicted HITT. Consecutive repeat ELISA testing for heparin antibodies may be warranted in patients with an increase in their pretest clinical score after an initial negative test as an adjunct to confirm the diagnosis of HIT.

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.

Reduced Argatroban Doses After Coronary Artery Bypass Graft Surgery

Background:

The Food and Drug Administration–approved argatroban dose for heparin-induced thrombocytopenia (HIT) is 2 μg/kg/min (0.5 μg/kg/min in hepatic impairment), adjusted to achieve activated partial thromboplastin time (aPTT) 1.5–3 times baseline. Recent data suggest that reduced doses are required after cardiovascular surgery.

Objective:

To characterize dosing requirements, aPTTs, factors affecting dosage, and clinical outcomes in patients administered argatroban after coronary artery bypass graft (CABG) surgery.

Methods:

Charts of 39 patients who underwent CABG surgery and were administered argatroban postoperatively for laboratory-confirmed HIT (n = 25), antibody-negative suspected HIT (n = 10), or previous HIT requiring anticoagulation (n = 4) were retrospectively reviewed. Patient characteristics, argatroban dosing information, aPTTs (target range 45–90 sec), and outcomes were summarized. Regression analyses explored potential effectors of dosage.

Results:

Patient features, argatroban dosing patterns, and aPTTs were similar among groups. Many patients had laboratory evidence of some hepatic and/or renal dysfunction (median [range] bilirubin 1.0 [0.3–8.0] mg/dL, creatinine clearance 47 [18–287] mL/min). Overall, median argatroban doses were 0.5 μg/kg/min initially and 0.6 μg/kg/min during therapy (median duration 5.3 days). After argatroban initiation, aPTTs were greater than 90 seconds at first assessment in 4 patients (3 with abnormal hepatic function test results) initially administered 0.5, 1, 2, and 2 μg/kg/min, respectively. Within approximately 16 hours of therapy, 33 (85%) patients achieved consecutive therapeutic aPTTs. No association was detected between mean dose during therapy and preoperative ejection fraction, routine hepatic or renal function test results (other than blood urea nitrogen [BUN]), or surgery type. A clinically insignificant association existed between dose and BUN: there was an approximately 0.15 μg/kg/min dose decrease for each 10 mg/dL BUN increase. One patient developed thrombosis, 1 underwent finger amputation, 7 died (5 after argatroban cessation), and 4 had significant bleeding.

Conclusions:

These findings suggest that reduced initial argatroban doses (eg, 0.5 μg/kg/min), adjusted to achieve therapeutic aPTTs, provide rapid, adequate anticoagulation in postoperative CABG patients with presumed or previous HIT. Prospective study of reduced initial dosing in this setting is warranted.

HITT and stent thrombosis: a "clinical" diagnosis not to be missed

Heparin induced thrombocytopenia and thrombosis (HITT) syndrome is a rare but potentially life threatening disorder, which is increasingly recognized with the widespread use of heparin. We report a rare case of early stent thrombosis secondary to HITT in a patient initially presenting with cardiogenic shock, which occurred 9 days after primary bare metal stent implantation due to subcutaneous heparin exposure. The most important factor in detection of HITT remains a high index of clinical suspicion and close monitoring of the temporal trend of thrombocytopenia, particularly when trying to distinguish HITT from other sources of thrombocytopenia seen in critically ill patients.

Thrombocytopenia associated with antithrombotic therapy in patients with cardiovascular diseases: diagnosis and treatment

Agents with antiplatelet and anticoagulant activity have been proved to be effective in reducing the incidence of complications following acute coronary syndrome, percutaneous coronary intervention, and cardiopulmonary bypass. However, these agents, including heparin, glycoprotein IIb/IIIa receptor inhibitors, and thienopyridines, are associated with increased risk of bleeding and thrombocytopenia and have been administered together with increasing frequency in a variety of cardiovascular settings. Therefore, clinicians must be familiar with the safety and rational use of these potent antithrombotic agents. Clinical features of thrombocytopenia range from bleeding to thrombosis, even death, and therapy is very different depending on the underlying cause. Additionally, patients may sometimes need urgent intervention or surgery. Thus, it is essential to quickly discriminate the etiology and start appropriate therapy. This review highlights the pathogenesis, clinical and laboratory manifestation, differential diagnosis, and treatment of antithrombotic drug-induced thrombocytopenia in cardiovascular diseases.

What is the potential for overdiagnosis of heparin-induced thrombocytopenia?

Heparin-induced thrombocytopenia (HIT) is caused by platelet-activating antibodies that recognize platelet factor 4//heparin (PF4/H) complexes. According to the "iceberg model," only a subset of anti-PF4/heparin antibodies of IgG class evincing strong platelet-activating properties cause clinical HIT. Since many centers rely predominantly on an anti-PF4/polyanion enzyme-immunoassay (EIA) to diagnose HIT, we estimated the potential for overdiagnosis when only this single test is available. We examined a database of 100 patients in whom the probability of HIT had been estimated using a clinical scoring system (4Ts), and where patients underwent systematic testing for HIT antibodies using three assays: the platelet serotonin release assay (SRA), an "in-house" EIA that detects IgG anti-PF4/heparin antibodies (EIA-IgG), and a commercial EIA that detects anti-PF4/polyanion antibodies of all three immunoglobulin classes (EIA-GTI). Whereas 16 of 100 patients fulfilled a "classic" definition of HIT (intermediate/high probability plus strong platelet-activating anti-PF4/heparin IgG antibodies), an additional 16 patients fulfilled a "liberal" definition in which any investigated patient (irrespective of the pretest probability) who had a positive EIA-GTI was considered to have HIT. The clinical features of these 16 additional patients--including generally weak antibodies and low risk for thrombosis--suggest underlying non-HIT explanations for thrombocytopenia. Patients with a positive SRA generally corresponded to those with intermediate or high pretest probability of HIT who also had strong EIA-GTI reactivity (>1.20 OD units). We conclude there is the potential to overdiagnose HIT by approximately 100% if any positive EIA is considered to "confirm" the diagnosis of HIT irrespective of the clinical scenario.

[Heparin-induced thrombocytopenia]

Hemorrhage is the most common and best-recognized complication of heparin treatment. However, a potentially more dangerous complication is the development of heparin-induced thrombocytopenia (HIT). All patients exposed to heparin, irrespective of the dose and route of administration, are at risk of developing HIT. It is due to the formation of antibodies against the heparin-platelet factor 4 complex, which cause secondary activation of platelets, coagulation and, finally, increased thrombin production. The main symptom is the sudden onset of thrombocytopenia involving a drop in the platelet count to less than 50% of the basal level, with or without the appearance of thrombotic complications some 5 to 14 days after the start of heparin therapy. Heparin-induced thrombocytopenia can be detected early in patients receiving heparin by monitoring the platelet count. Demonstration of heparin-dependent platelet activation using an antigen or functional assay confirms the clinical diagnosis. Once the diagnosis of HIT has been confirmed serologically or there is a high level of suspicion of HIT, heparin must be suspended and treatment with an alternative anticoagulant should be considered. This review contains a discussion of the diagnosis and treatment of this syndrome.

Reducing thrombotic complications in the perioperative setting: an update on heparin-induced thrombocytopenia

Heparins are widely used in the perioperative setting. Immune heparin-induced thrombocytopenia (HIT) is a serious, antibody-mediated complication of heparin therapy that occurs in approximately 0.5%-5% of patients treated with heparin for at least 5 days. An extremely prothrombotic disorder, HIT confers significant risks of thrombosis and devastating consequences on affected patients: approximately 38%-76% develop thrombosis, approximately 10% with thrombosis require limb amputation, and approximately 20%-30% die within a month. HIT antibodies are transient and typically disappear within 3 mo. In patients with lingering antibodies, however, re-exposure to heparin can be catastrophic. In the perioperative setting, heightened awareness is important for the prompt recognition, diagnosis, and treatment of HIT. HIT should be considered if the platelet count decreases 50% and/or thrombosis occurs 5-14 days after starting heparin, with other diagnoses excluded. On strong clinical suspicion of HIT, heparin should be discontinued and a parenteral alternative anticoagulant initiated, even before laboratory confirmation of HIT is obtained. Subsequent laboratory test results may help with the decision to continue with nonheparin therapy or switch back to heparin. Heparin avoidance in patients with current or previous HIT is feasible in most clinical situations, except perhaps in cardiovascular surgery. If the surgery cannot be delayed until HIT antibodies have disappeared, intraoperative alternative anticoagulation is recommended.

Clinical challenge: heparin-induced thrombocytopenia type II (HIT II) or pseudo-HIT in a patient with antiphospholipid syndrome

Treatment of patients with heparin-induced thrombocytopenia type II (HIT II) and thrombosis in some cases that represents a clinical challenge, which, if unrecognized, may lead to treatment delay or disease progression with potentially lethal outcome. We present a case of a 19-year-old patient with antiphospholipid syndrome, factor V (FV) Leiden mutation in heterozygous state, and venous thromboembolism. The patient was subjected to intravenous infusions of unfractionated heparin (UFH), and 16 days after the beginning of the treatment, his condition worsened, with thrombocytopenia and extension of thrombosis. Whereas the patient had a high clinical score for HIT II, functional and antigenic assays for the presence of HIT antibodies were negative. After repeated negative functional and antigenic assays, pseudo-HIT was suspected and nadroparin was introduced, which resulted in further worsening of the clinical presentation. Disease remission, along with complete normalization of platelet count, was finally accomplished with the introduction of lepirudin. The presence of multiple comorbid states, such as antiphospholipid syndrome, can potentially make laboratory confirmation of disease more difficult in patients with HIT II. In our opinion, it is of great importance that HIT II diagnosis be primarily clinical and that laboratory test results are carefully interpreted, especially when HIT is indicated by high clinical score values.

PF4 ENHANCED assay for the diagnosis of heparin-induced thrombocytopenia in complex medical and surgical patients

OBJECTIVE

To evaluate the sensitivity and specificity of the PF4 ENHANCED (GTI Diagnostics, Waukesha, WI) enzyme-linked immunosorbent assay for heparin-induced thrombocytopenia using the carbon-14 serotonin-release assay as the reference method.

DESIGN

A total of 34 patients were prospectively enrolled with a variety of diagnoses and suspected heparin-induced thrombocytopenia. They were clinically scored and underwent testing with the (14)C-serotonin-release assay. Enzyme-linked immunosorbent assay and (14)C-serotonin-release assay results were also available from 21 medical and surgical patients who had previously been tested.

MAIN RESULTS

With the (14)C-serotonin-release assay as the reference method, the sensitivity and specificity of the enzyme-linked immunosorbent assay were 93% and 65%, respectively. There was only one false-negative enzyme-linked immunosorbent assay. The clinical scores were frequently misleading, largely because of difficulty excluding other causes of thrombocytopenia.

CONCLUSION

Because of its high sensitivity, we believe the PF4 ENHANCED enzyme-linked immunosorbent assay should be used to identify heparin-induced thrombocytopenia in patients with multiple potential causes of thrombocytopenia, although false-positive results will not be uncommon.

Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an immune-mediated adverse drug effect that is characterized by platelet activation, hypercoagulability, and a resulting increased risk for thrombosis, both venous and arterial. This disorder is autoimmune-like, because the target antigen is a multimolecular complex of the "self" protein, platelet factor 4, and heparin. HIT usually begins 5 to 10 days after starting heparin, especially when administered intra- or perioperatively, although a rapid onset of thrombocytopenia can occur if heparin is given to a patient with circulating HIT antibodies that resulted from a recent heparin exposure. The clinical diagnosis is supported if heparin-dependent, platelet-activating antibodies are detectable. Treatment includes cessation of heparin and use of an alternative non-heparin anticoagulant, such as danaparoid, lepirudin, or argatroban. Warfarin must be avoided or postponed, as the acute phase of HIT poses a high risk for coumarin necrosis, particularly limb loss due to venous limb gangrene.

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.