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

Prospective evaluation of the '4Ts' score and particle gel immunoassay specific to heparin/PF4 for the diagnosis of heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a severe disease that is often difficult to diagnose. A clinical scoring system, the '4Ts' score, has been proposed to estimate its probability before laboratory testing, and a particle gel immunoassay (H/PF4 PaGIA) has also been developed for rapid detection of HIT antibodies.

AIM

To evaluate the performance of both methods when HIT is suspected clinically.

METHODS

Two hundred thirteen consecutive patients were included in four centers. The probability of HIT was evaluated using the 4Ts score blind to antibody test results. HIT was confirmed only when the serotonin release assay (SRA) was positive.

RESULTS

The risk of HIT was evaluated by the 4Ts score as low (LowR), intermediate (IR) or high (HR) in 34.7%, 60.6% and 4.7% of patients, respectively. The negative predictive value (NPV) of the 4Ts score was 100%, as the SRA was negative in all LowR patients. PaGIA was negative in 176 patients without HIT (99.4%, NPV) and the negative likelihood ratio (LR-) was 0.05. PaGIA was positive in 37 patients, including 21 with HIT (positive predictive value = 56.8%), with a positive LR of 11.4. A negative PaGIA result decreased the probability of HIT in IR patients from 10.9% before assay to 0.6%, whereas a positive result did not substantially increase the likelihood for HIT.

CONCLUSION

The use of the 4Ts score with PaGIA appears to be a reliable strategy to rule out HIT.

Heparin-Induced Thrombocytopenia

Unfractionated heparin is widely used for numerous clinical situations. A well-known adverse effect of heparin exposure is thrombocytopenia. Heparin-induced thrombocytopenia is a clinicopathologic syndrome that can be associated with severe complications and significant mortality. The pathophysiology of heparin-induced thrombocytopenia includes an immune-mediated reaction to heparin that activates platelets and results in an acquired hypercoagulability. Diagnosis of heparin-induced thrombocytopenia should incorporate clinical signs and symptoms and laboratory testing for heparin-induced thrombocytopenia antibodies. Therapy should include discontinuation of heparin, initiation of a direct thrombin inhibitor, and eventually therapy with warfarin (only after the platelet count is at least 100 x 10(9)/L).

A rare disease

Thrombocytopenia is a common finding in critically ill patients. Heparin-induced thrombocytopenia is an infrequent cause of a low platelet count. Intensivists should use the diagnostic classification system developed by the International Society on Thrombosis and Haemostasis to diagnose heparin-induced thrombocytopenia. The clinical relevance of the presence of anti-heparin/platelet factor 4 complex autoantibodies in the absence of clinical heparin-induced thrombocytopenia remains unknown.

Beyond the platelet count: heparin antibodies as independent risk predictors

A major potential side effect of heparin is immunogenicity, eliciting antibody development to a protein complex comprised of platelet factor 4 and heparin. Nevertheless, the clinical implications of heparin antibody positive patients remain broad, ranging from no apparent clinical consequences to life-threatening arterial and venous thromboemboli. The "Iceberg Model" has been proposed to depict this spectrum, with a relatively large population of antibody-positive patients forming the base of the iceberg, a smaller population of thrombocytopenic patients in the middle and a limited number of patients with thrombocytopenia and thrombosis comprising the apex. An underlying assumption of this model is that thrombosis occurs only in settings of relative or absolute thrombocytopenia. However, several recent studies suggest that antibody formation to platelet factor 4/heparin complexes, even in the absence of thrombocytopenia, may be associated with thrombotic events. In this review, we summarize these data, consider potential mechanisms for thrombosis, and suggest recommendations for testing and management of antibody-positive patients.

Heparin-induced thrombocytopenia in intensive care patients

OBJECTIVE

To summarize new information on frequency of heparin-induced thrombocytopenia (HIT) in patients treated in intensive care units (ICU), developments in the interpretation of assays for detecting anti-PF4/heparin antibodies, and treatment of HIT patients.

STUDY SELECTION

All data on the frequency of laboratory-confirmed HIT in ICU patients were included; for laboratory testing of HIT and treatment of patients, this review focuses on recent data that became available in 2005 and 2006.

DATA EXTRACTION AND SYNTHESIS

HIT is a potentially life-threatening adverse effect of heparin treatment caused by platelet-activating antibodies of immunoglobulin G class usually recognizing complexes of platelet factor 4 and heparin. HIT is more often caused by unfractionated heparin than low-molecular-weight heparin and is more common in postsurgical than in medical patients. In the ICU setting, HIT is uncommon (0.3-0.5%), whereas thrombocytopenia from other causes is very common (30-50%). For laboratory diagnosis of HIT antibodies, both antigen assays and functional (platelet activation) assays are available. Both tests are very sensitive (high negative predictive value) but specificity is problematic, especially for the antigen assays, which also detect nonpathogenic immunoglobulin M and immunoglobulin A class antibodies. Detection of immunoglobulin M or immunoglobulin A antibodies could potentially lead to adverse events such as bleeding if a false diagnosis of HIT prompts replacement of heparin by an alternative anticoagulant. For treatment of HIT, three alternative anticoagulants are approved: the direct thrombin inhibitors, lepirudin and argatroban, and the heparinoid, danaparoid (not approved in the United States). Recent data indicate that the approved dosing regimens of the direct thrombin inhibitors are too high, especially in ICU patients.

CONCLUSIONS

HIT affects <1% of ICU patients even though 30-50% develop thrombocytopenia. The choice of the optimal alternative anticoagulant depends on patient characteristics. Many ICU patients require lower doses of alternative anticoagulant than those recommended by the manufacturer.

Outcome of postoperative critically ill patients with heparin-induced thrombocytopenia: an observational retrospective case-control study

INTRODUCTION

Heparin-induced thrombocytopenia (HIT) is described as a decrease in platelet count associated with heparin administration and is an immune-mediated adverse drug reaction that can cause both arterial and venous thromboses. It can be a life-threatening complication of heparin exposure. Little data concerning incidence, predisposing factors, or outcome in critically ill surgical patients are available.

METHODS

All critically ill, postoperative patients admitted between January 1, 2000, and December 31, 2001, to a surgical intensive care unit (SICU) who tested positive by an enzyme-linked immunosorbent assay for the HIT antibody (HPIA; Diagnostica Stago, Inc., Parsippany, NJ, USA) were identified. Patient risk factors and outcomes were abstracted retrospectively from the medical record and compared with those from control patients matched for age, gender, diagnosis, severity of illness, and date of SICU admission.

RESULTS

Two hundred and ten patients out of 2,046 patients (10%) admitted to the SICU had HIT assays performed. Nineteen patients (0.9% of admissions; 9% of tested individuals) had positive tests. HIT-antibody-positive patients, compared with 19 matched controls, had an increased risk of death or major thrombotic complications (37% versus 10%; P < 0.05) and prolonged length of intensive care unit (ICU) stay (20 days versus 10 days; P < 0.05). Exposure to heparin via intravascular flushes alone was sufficient to generate HIT antibodies in 12 of 19 (63%) patients. Five patients received platelet transfusions after the diagnosis of HIT was known; four of these patients died.

CONCLUSION

Heparin flushes were the most common cause of HIT in this study. HIT-antibody-positive patients had an increased risk of death or major complications and a prolonged length of ICU stay. Platelet transfusions often were administered despite a positive HIT test result and were associated with a high mortality rate. Treatment algorithms that minimize exposure to heparin and contraindicate platelet transfusions merit further study.

Vancomycin-induced immune thrombocytopenia

BACKGROUND

Vancomycin has only rarely been implicated as a cause of thrombocytopenia, and there is only limited evidence that this complication is caused by immune mechanisms. We conducted a study to determine whether thrombocytopenia is caused by vancomycin-dependent antibodies in patients being treated with vancomycin.

METHODS

We identified and characterized vancomycin-dependent, platelet-reactive antibodies in patients who had been referred for testing during a 5-year period because of a clinical suspicion of vancomycin-induced thrombocytopenia. We obtained clinical information about the patients from their referring physicians.

RESULTS

Drug-dependent, platelet-reactive antibodies of the IgG class, the IgM class, or both were identified in 34 patients, and clinical follow-up information was obtained from 29 of these patients. The mean nadir platelet count in these patients was 13,600 per cubic millimeter, and severe bleeding occurred in 10 patients (34%). Platelet levels returned to baseline in all 26 surviving patients after vancomycin was stopped. In 15 patients, the drug was continued for 1 to 14 days while other possible causes of thrombocytopenia were investigated. Vancomycin-dependent antibodies were not found in 25 patients who had been given vancomycin and in whom thrombocytopenia did not develop.

CONCLUSIONS

Severe bleeding can occur in patients with vancomycin-induced immune thrombocytopenia. The detection of vancomycin-dependent antiplatelet antibodies in patients receiving the antibiotic in whom thrombocytopenia develops, and the absence of antibodies in patients given the drug in whom platelet counts remain stable, indicate that these antibodies are the cause of the thrombocytopenia.

[Multidisciplinary consultation for the prescription of danaparoid in suspected heparin-induced thrombocytopenia]

INTRODUCTION

Heparin-induced thrombocytopenia (HIT) is a serious immune complication of heparin therapy and presents a risk of severe thromboembolic events. Withdrawal of heparin together with administration of an alternative antithrombotic agent is always necessary in patients with suspected HIT. Diagnosis of this complication, however, is often difficult, particularly in hospitalized patients. The aim of this study was to evaluate the impact of multidisciplinary consultation on the appropriate prescription of danaparoid, widely used as an alternative antithrombotic treatment in HIT.

MATERIAL AND METHODS

Multidisciplinary consultation between clinician, hematologist, and pharmacist called for reassessment of the HIT diagnosis at day 3 and between day 3 and 10 after the beginning of danaparoid treatment. Continuation or stopping treatment depended on their joint conclusion. All danaparoid prescriptions were evaluated according to this procedure for one year.

RESULTS

HIT was suspected in 26 in-patients. The multidisciplinary approach made it possible to reassess the HIT diagnosis on day 3 and stop the alternative treatment in 42.3% of cases. Danaparoid use decreased by 52% compared with the previous year.

CONCLUSION

Multidisciplinary consultations between clinician, hematologist, and pharmacist appear useful for minimizing inappropriate prescription of this alternative treatment in cases of suspected HIT.

Heparin-induced Thrombocytopenia, a Prothrombotic Disease

Heparin-induced thrombocytopenia (HIT) is a serious, yet treatable prothrombotic disease that develops in approximately 0.5% to 5% of heparin-treated patients and dramatically increases their risk of thrombosis (odds ratio, 37). The antibodies that mediate HIT (ie, heparin-platelet factor 4 antibodies) occur more frequently than the overt disease itself, and, even in the absence of thrombocytopenia, are associated with increased thrombotic morbidity and mortality. HIT should be suspected whenever the platelet count drops more than 50% from baseline (or to <150 x 10(9)/L) beginning 5 to 14 days after starting heparin (or sooner if there was prior heparin exposure) or new thrombosis occurs during, or soon after heparin treatment, with other causes excluded. When HIT is strongly suspected, with or without complicating thrombosis, heparins should be discontinued, and a fast-acting, nonheparin alternative anticoagulant such as argatroban should be initiated immediately. With prompt recognition, diagnosis, and treatment of HIT, the clinical outcomes and health economic burdens of this prothrombotic disease are improved significantly.

Argatroban Anticoagulation for Heparin-Induced Thrombocytopenia in Elderly Patients

BACKGROUND

Argatroban, a direct thrombin inhibitor that has reduced clearance in elderly versus younger volunteers, is used for thromboprophylaxis or treatment in heparin-induced thrombocytopenia (HIT).

OBJECTIVE

To evaluate the effect of aging on argatroban therapy, including dosage, anticoagulant responses, clinical outcomes and factors influencing those responses, in elderly patients with HIT or a history of HIT.

METHODS

This was a retrospective multicentre database analysis of 118 inpatients treated with argatroban at six medical centres between August 2001 and January 2005. Sixty-two adults aged >/=65 years were administered argatroban for clinically diagnosed HIT (n = 54) or a history of HIT (n = 8). Argatroban infusion was adjusted to achieve activated partial thromboplastin times (aPTTs) 1.5-3 times baseline. All study measures and analyses were prospectively defined. Argatroban dosage patterns, aPTTs and platelet count responses, and 37-day outcomes (death, amputation, new thrombosis, major bleeding) were summarised for patients stratified by age (65-74 years [n = 31]; 75-84 years [n = 26]; >/=85 years [n = 5]) to identify possible age-related trends. Regression analyses explored relationships between dose and patient age, liver function and renal function. Cox proportional hazards models evaluated the effect of age, dose, gender, aPTT and platelet count on the risk of new thrombosis.

RESULTS

In each age group, the median argatroban dosage was initially 1.0 microg/kg/min and was generally maintained at or near that dose during therapy (median, 5-7 days). Therapeutic aPTTs occurred within 11.5 hours; the median aPTT during therapy was 54.7 seconds, without obvious trend by age. By regression analysis, the initial and mean argatroban dosages decreased 0.08-0.09 microg/kg/min with each 0.2 mg/dL increase in serum creatinine, but no association was detected between dose and patient age, serum total bilirubin, calculated creatinine clearance or blood urea nitrogen. Platelet counts recovered within 6-7 days of initiating therapy, without apparent trend by age. No patient experienced amputation or major bleeding, and no patient in the oldest group died or had new thrombosis. Overall, 13 (21%) patients died (9 in the 65-74 years group; 1 receiving argatroban) and 5 (8%) had new thrombosis (4 in the 65-74 years group; 2 receiving argatroban), comparing favourably with previously reported rates, irrespective of patient age. By univariate (but not multivariate) analysis, the risk of new thrombosis decreased with increasing argatroban dose (hazard ratio 0.020; 95% CI 0.001, 0.757; p = 0.035). No effect of age or the other covariates considered on thrombotic risk was detected.

CONCLUSION

Argatroban at a median initial dosage of 1.0 microg/kg/min, adjusted to achieve median aPTTs of 54.7 seconds during therapy, generally provided safe, adequate anticoagulation across a wide age range in elderly patients with HIT or a history of HIT. In these elderly patients, age was not a significant determinant of argatroban dosage or thrombotic risk. Prospective evaluation of this initial dose of argatroban in the elderly is warranted.

Testing for Heparin-Induced Thrombocytopenia Antibodies

Heparin-induced thrombocytopenia (HIT) has a distinct clinical profile and unique pathogenesis. It is caused by platelet-activating IgG antibodies that recognize multimolecular complexes of platelet factor 4 (PF4) bound to heparin or certain other polyanions. Although an immune response to PF4/heparin associated with heparin treatment is very common, clinical HIT occurs only among the minority of patients whose antibodies are capable of strongly activating platelets. This explains why certain platelet activation assays and anti-PF4/polyanion immunoassays have high sensitivity for HIT and why diagnostic specificity is highest for those assays that preferentially detect pathogenic antibodies, such as the washed platelet activation assays or immunoassays that detect only IgG antibodies. Negative results obtained in a solid-phase PF4/polyanion immunoassay generally exclude HIT (high negative predictive value), especially in a setting of a low pretest probability. In addition, because the magnitude of a positive test result correlates with greater likelihood of HIT, a Bayesian diagnostic approach that combines pretest probability and the magnitude of a positive test result is recommended. Recent studies suggest that presence of anti-PF4/polyanion antibodies in certain clinical settings confers an adverse prognosis, even without clinically evident HIT. Whether such antibodies impart "forme fruste" HIT or are simply a surrogate marker for a non-HIT adverse risk factor such as inflammation is unresolved.

Think of HIT

Heparin-induced thrombocytopenia, or HIT, can present in many ways, ranging from common-isolated thrombocytopenia, venous thromboembolism, acute limb ischemia-to less common but specific presentations-necrotizing skin lesions at heparin injection sites, post-bolus acute systemic reactions, and adrenal hemorrhagic necrosis (secondary to adrenal vein thrombosis). Many patients with HIT have mild or moderate thrombocytopenia: the median platelet count nadir is 60 x 10(9)/L, and ranges from 15 to 150 x 10(9)/L in 90% of patients, most of whom evince a 50% or greater fall in the platelet count. HIT that begins after stopping heparin ("delayed-onset HIT") is increasingly recognized. Factors influencing risk of HIT include type of heparin (unfractionated heparin > low-molecular-weight heparin), type of patient (surgical > medical), and gender (female > male). Since timely diagnosis and treatment of HIT may reduce the risk of adverse outcomes, this review focuses on those clinical circumstances that should prompt the clinician to "think of HIT." Coumarin anticoagulants such as warfarin are ineffective in acute HIT and can even be deleterious by predisposing to micro-thrombosis via protein C depletion (venous limb gangrene and skin necrosis syndromes). Thus, it is important to avoid or postpone coumarin while managing HIT hypercoagulability, focusing on agents that inhibit thrombin directly (lepirudin, argatroban) or that inhibit its generation (danaparoid, fondaparinux). Post-marketing experience suggests that standard dosing of lepirudin is too high; current recommendations are to avoid the initial lepirudin bolus and to begin with lower infusion rates, even in patients without overt renal dysfunction.