Patients treated with unfractionated heparin during open heart surgery are at high risk to form antibodies reactive with heparin:platelet factor 4 complexes

Transient Autoreactive PF4 and Antiphospholipid Antibodies in COVID-19 Vaccine Recipients

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare autoimmune condition associated with recombinant adenovirus (rAV)-based COVID-19 vaccines. It is thought to arise from autoantibodies targeting platelet factor 4 (aPF4), triggered by vaccine-induced inflammation and the formation of neo-antigenic complexes between PF4 and the rAV vector. To investigate the specific induction of aPF4 by rAV-based vaccines, we examined sera from rAV vaccine recipients (AZD1222, AD26.COV2.S) and messenger RNA (mRNA) based (mRNA-1273, BNT162b2) COVID-19 vaccine recipients. We compared the antibody fold change (FC) for aPF4 and for antiphospholipid antibodies (aPL) of rAV to mRNA vaccine recipients. We combined two biobanks of Dutch healthcare workers and matched rAV-vaccinated individuals to mRNA-vaccinated controls, based on age, sex and prior history of COVID-19 (AZD1222: 37, Ad26.COV2.S: 35, mRNA-1273: 47, BNT162b2: 26). We found no significant differences in aPF4 FCs after the first (0.99 vs. 1.08, mean difference (MD) = -0.11 (95% CI -0.23 to 0.057)) and second doses of AZD1222 (0.99 vs. 1.10, MD = -0.11 (95% CI -0.31 to 0.10)) and after a single dose of Ad26.COV2.S compared to mRNA-based vaccines (1.01 vs. 0.99, MD = 0.026 (95% CI -0.13 to 0.18)). The mean FCs for the aPL in rAV-based vaccine recipients were similar to those in mRNA-based vaccines. No correlation was observed between post-vaccination aPF4 levels and vaccine type (mean aPF difference -0.070 (95% CI -0.14 to 0.002) mRNA vs. rAV). In summary, our study indicates that rAV and mRNA-based COVID-19 vaccines do not substantially elevate aPF4 levels in healthy individuals.

Rivaroxaban Reduces the Dabigatran Dose Required for Anticoagulation During Simulated Cardiopulmonary Bypass

BACKGROUND

Heparin is the standard anticoagulant for cardiopulmonary bypass (CPB); however, there are problems with its use that make the development of suitable alternatives desirable. Currently, no ideal alternative exists. We have previously reported that the direct thrombin inhibitor dabigatran can prevent coagulation in simulated CPB at high concentrations. These high concentrations may cause difficulties in achieving the reversal of dabigatran with idarucizumab, given the markedly different pharmacokinetics of the 2 drugs. Herein, we test the hypothesis that the addition of the anti-Xa drug rivaroxaban would provide suitable anticoagulation at a lower concentration of dabigatran given likely synergy between the 2 classes of drugs. The primary goal of the study was to investigate whether the addition of rivaroxaban reduces the concentration of dabigatran necessary to allow 2 hours of simulated CPB.

METHODS

The study was performed in sequential steps. Blood collected from consenting healthy donors was used throughout. First, we added graded concentrations of dabigatran and rivaroxaban alone and in combination and assessed inhibition of anticoagulation using thromboelastometry. Using results from this step, combinations of dabigatran and rivaroxaban were tested in both Chandler loop and simulated CPB circuits. Dabigatran and rivaroxaban were added before recalcification, and the circuits were run for 120 minutes. In both models of CPB, 120 minutes of circulation without visible thrombus was considered successful. In the Chandler loop system, idarucizumab was added to reverse anticoagulant effects. In the CPB circuits, the arterial line filters were examined using scanning electron microscope (SEM) to qualitatively assess for fibrin deposition.

RESULTS

In vitro analysis of blood samples treated with dabigatran and rivaroxaban showed that dabigatran and rivaroxaban individually prolonged clotting time (CT) in a dose-dependent manner. However, when combined, the drugs behaved synergistically. In the Chandler loop system, dabigatran 2400 and 4800 ng/mL plus rivaroxaban (150 ng/mL) effectively prevented clot formation and reduced the dynamics of clot propagation for 120 minutes. Idarucizumab (250-1000 µg/mL) effectively reversed anticoagulation. In the CPB circuits, dabigatran (2500 ng/mL) and rivaroxaban (200 ng/mL) were successful in allowing 120 minutes of simulated CPB and prevented fibrin deposition. Biomarkers of coagulation activation did not increase during simulated CPB. Heparin controls performed similarly to dabigatran and rivaroxaban.

CONCLUSIONS

The dual administration of oral anticoagulant drugs (dabigatran and Rivaroxaban) with different pharmacologic mechanisms of action produced synergistic inhibition of coagulation in vitro and successfully prevented clotting during simulated CPB.

High-Dose Dabigatran Is an Effective Anticoagulant for Simulated Cardiopulmonary Bypass Using Human Blood

BACKGROUND

Currently no ideal alternative exists for heparin for cardiopulmonary bypass (CPB). Dabigatran is a direct thrombin inhibitor for which a reversal agent exists. The primary end point of the study was to explore whether Dabigatran was an effective anticoagulant for 120 minutes of simulated CPB.

METHODS

The study was designed in 2 sequential steps. Throughout, human blood from healthy donors was used for each experimental step. Initially, increasing concentrations of Dabigatran were added to aliquots of fresh whole blood, and the anticoagulant effect measured using kaolin/tissue factor-activated thromboelastography (rapidTEG). The dynamics of all thromboelastography (TEG) measurements were studied with repeated measures analysis of variance (ANOVA). Based on these data, aliquots of blood were treated with high-concentration Dabigatran and placed in a Chandler loop as a simple ex vivo bypass model to assess whether Dabigatran had sufficient anticoagulant effects to maintain blood fluidity for 2 hours of continuous contact with the artificial surface of the PVC tubing. Idarucizumab, humanized monoclonal antibody fragment, was used to verify the reversibility of Dabigatran effects. Finally, 3 doses of Dabigatran were tested in a simulated CPB setup using a heart-lung machine and a commercially available bypass circuit with an arteriovenous (A-V) loop. The primary outcome was the successful completion of 120 minutes of simulated CPB with dabigatran anticoagulation, defined as lack of visible thrombus. Thromboelastographic reaction (R) time was measured repeatedly in each bypass simulation, and the circuits were continuously observed for clot. Scanning Electron Microscopy (SEM) was used to visualize fibrin formation in the filters meshes during CPB.

RESULTS

In in vitro blood samples, Dabigatran prolonged R time and reduced the dynamics of clot propagation (as measured by speed of clot formation [Angle], maximum rate of thrombus generation [MRTG], and time to maximum rate of thrombus generation [TMRTG]) in a dose-dependent manner. In the Chandler Loop, high doses of Dabigatran prevented clot formation for 120 minutes, but only at doses higher than expected. Idarucizumab decreased R time and reversed anticoagulation in both in vitro and Chandler Loops settings. In the A-V loop bypass simulation, Dabigatran prevented gross thrombus generation for 120 minutes of simulated CPB.

CONCLUSIONS

Using sequential experimental approaches, we showed that direct thrombin inhibitor Dabigatran in high doses maintained anticoagulation of blood for simulated CPB. Idarucizumab reduced time for clot formation reversing the anticoagulation action of Dabigatran.

Heparin-induced thrombocytopenia (HIT): Review of incidence, diagnosis, and management

Heparin-induced thrombocytopenia (HIT) is a life and limb-threatening complication of heparin exposure. Here, we review the pathogenesis, incidence, diagnosis, and management of HIT. The first step in thwarting devastating complications from this entity is to maintain a high index of clinical suspicion, followed by an accurate clinical scoring assessment using the 4Ts. Next, appropriate stepwise laboratory testing must be undertaken in order to rule out HIT or establish the diagnosis. In the interim, all heparin must be stopped immediately, and the patient administered alternative anticoagulation. Here we review alternative anticoagulation choice, therapy alternatives in the difficult-to-manage patient with HIT, and the problem of overdiagnosis.

Improved prediction of HIT in the SICU using an improved model of the Warkentin 4-T system: 3-T

BACKGROUND

The Warkentin 4-T scoring system for determining the pretest probability of heparin-induced thrombocytopenia (HIT) has been shown to be inaccurate in the ICU and does not take into account body mass index (BMI).

METHODS

Prospectively collected data on patients in the surgical and cardiac ICU between January 2007 and February 2016 who were presumed to have HIT by clinical suspicion were reviewed. Patients were categorized into 3 BMI groups and assigned scores: Normal weight, overweight, and obese. Multivariate analyses were used to identify independent predictors of HIT.

RESULTS

A total of 523 patients met inclusion criteria. Multivariate analysis showed that only BMI, Timing, and oTher variables were independently associated with HIT. This new 3-T model was better than a five-component model consisting of the entire 4-T scoring system plus BMI (AUC = 0.791).

CONCLUSIONS

Incorporating patient 'T'hickness into a pretest probability model along with platelet 'T'iming and the exclusion of o'T'her causes of thrombocytopenia yields a simplified "3-T" scoring system that has increased predictive accuracy in the ICU.

Characterization of platelet factor 4 amino acids that bind pathogenic antibodies in heparin-induced thrombocytopenia

Essentials Many patients produce antibodies but few lead to heparin-induced thrombocytopenia (HIT). Pathogenic epitopes are difficult to identify as HIT antibodies are polyclonal and polyspecific. KKO binding to platelet factor 4 (PF4) depends on 13 amino acids, three of which are newly observed. Five amino acids in PF4 can help distinguish pathogenic from non-pathogenic antibodies. SUMMARY: Background Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction that results in thrombocytopenia and, in some patients, thrombotic complications. HIT is mediated by antibodies that bind to complexes of platelet factor 4 (PF4) and heparin. The antigenic epitopes of these anti-PF4/heparin antibodies have not yet been precisely defined, because of the polyspecific immune response that characterizes HIT. Objectives To identify PF4 amino acids essential for binding pathogenic HIT antibodies. Methods Alanine scanning mutagenesis was utilized to produce 70 single point mutations of PF4. Each PF4 mutant was used in an enzyme immunoassay (EIA) to test their capacity to bind a platelet-activating murine monoclonal anti-PF4/heparin antibody (KKO) and HIT patient sera (n = 9). Results and Conclusions We identified 13 amino acids that were essential for binding KKO because they directly affected either the binding site or the antigenic conformation of PF4. We also identified 10 amino acids that were required for the binding of HIT patient sera and five of these amino acids were required for binding both KKO and the HIT patient sera. The 10 amino acids required for binding HIT sera were further tested to differentiate pathogenic HIT antibodies (platelet activating, n = 45) and non-pathogenic antibodies (EIA-positive but not platelet activating, n = 28). We identified five mutations of PF4 that were recognized to be essential for binding pathogenic HIT antibodies. Using alanine scanning mutagenesis, we characterized possible binding sites of pathogenic HIT antibodies on PF4.

Heparin-Induced Thrombocytopenia: A Comprehensive Clinical Review

Heparin-induced thrombocytopenia is a profoundly dangerous, potentially lethal, immunologically mediated adverse drug reaction to unfractionated heparin or, less commonly, to low-molecular weight heparin. In this comprehensive review, the authors highlight heparin-induced thrombocytopenia's risk factors, clinical presentation, pathophysiology, diagnostic principles, and treatment. The authors place special emphasis on the management of patients requiring procedures using cardiopulmonary bypass or interventions in the catheterization laboratory. Clinical vigilance of this disease process is important to ensure its recognition, diagnosis, and treatment. Misdiagnosis of the syndrome, as well as misunderstanding of the disease process, continues to contribute to its morbidity and mortality.

Platelet Count Trends and Prevalence of Heparin-Induced Thrombocytopenia in a Cohort of Extracorporeal Membrane Oxygenator Patients

OBJECTIVES

To assess the prevalence of heparin-induced thrombocytopenia and to study platelet count trends potentially suggestive of heparin-induced thrombocytopenia in a population of extracorporeal membrane oxygenator patients.

DESIGN

Retrospective cohort study.

SETTING

A total of 926-bed teaching hospital.

PATIENTS

Extracorporeal membrane oxygenator patients who survived longer than 48 hours from extracorporeal membrane oxygenator initiation between January 1, 2009, and December 31, 2013.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Demographic and clinical data were collected prospectively on all extracorporeal membrane oxygenator patients. Heparin-induced thrombocytopenia testing results and platelet count variables were obtained from the electronic medical record. We used our institutional algorithm to interpret the results of heparin-induced thrombocytopenia testing. Ninety-six extracorporeal membrane oxygenator patients met the inclusion criteria. Eight patients met the algorithm criteria for heparin-induced thrombocytopenia diagnosis and seven of those had documented thromboembolic event while on extracorporeal membrane oxygenator (prevalence of heparin-induced thrombocytopenia and heparin-induced thrombocytopenia related thrombosis, 8.3 and 7.3, respectively). Heparin-induced thrombocytopenia positive patients were younger; all underwent venoarterial extracorporeal membrane oxygenator; spent more hours on extracorporeal membrane oxygenator; had significantly higher heparin-induced thrombocytopenia enzyme-linked immunosorbent assays optical density; had a higher prevalence of thromboembolic events and reached platelet count nadir later. There was no difference in mortality between heparin-induced thrombocytopenia positive and negative patients. Comparison of platelet count trends revealed that there was no statistically significant difference between the predefined study groups.

CONCLUSIONS

Prevalence of heparin-induced thrombocytopenia and heparin-induced thrombocytopenia-related thrombosis among extracorporeal membrane oxygenator patients at our institution is relatively high. Using platelet count trends to guide decision to test for heparin-induced thrombocytopenia is not an optimal strategy in extracorporeal membrane oxygenator patients. Without a validated pretest probability clinical score, serosurveillance in a defined high-risk group of extracorporeal membrane oxygenator patients may be needed.

Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an immune complication of heparin therapy caused by antibodies to complexes of platelet factor 4 (PF4) and heparin. Pathogenic antibodies to PF4/heparin bind and activate cellular FcγRIIA on platelets and monocytes to propagate a hypercoagulable state culminating in life-threatening thrombosis. It is now recognized that anti-PF4/heparin antibodies develop commonly after heparin exposure, but only a subset of sensitized patients progress to life-threatening complications of thrombocytopenia and thrombosis. Recent scientific developments have clarified mechanisms underlying PF4/heparin immunogenicity, disease susceptibility, and clinical manifestations of disease. Insights from clinical and laboratory findings have also been recently harnessed for disease prevention. This review will summarize our current understanding of HIT by reviewing pathogenesis, essential clinical and laboratory features, and management.

Heparin-induced thrombocytopenia: natural history, diagnosis, and management

Heparin-induced thrombocytopenia (HIT) is an under-recognized, limb- and life-threatening complication of pharmacologic heparin administration. Antibody formation against heparin complexed to platelet factor 4 (PF4) is central to the pathogenesis of HIT. Heparin: PF4 antibodies promote platelet activation and aggregation as well as excess thrombin generation which may lead to clinical thrombosis. HIT should be suspected in patients who develop thrombocytopenia with or without associated arterial or venous thrombosis while on heparin. HIT is a clinical diagnosis. Specialized HIT assays should be interpreted with care. The cornerstone of HIT management is the discontinuation of all forms of heparin exposure and the institution of anticoagulation with an alternative agent. The direct thrombin inhibitors lepirudin and argatroban are currently available and approved for use in patients with HIT.

Heparin-Induced Thrombocytopenia

Heparin's anticoagulation effect makes it the principal component in the immediate treatment of all thrombotic diseases. After hemorrhage, the most important complication of heparin is thrombocytopenia, the decrease in the number of blood platelets. Ten cases of "heparin-induced thrombocytopenia" are described, and guidelines for diagnosing heparin-induced thrombocytopenia in patients who present for cardiac and vascular surgery are given. Methods of treating patients with heparin-induced thrombocytopenia, including use of arachidonic acid derivatives, antiplatelet agents, other anticoagulants, heparinoids, natural and synthetic thrombin inhibitors, hirudin, and defibrinogenation with ancrod, are discussed.

Management of Patients with Heparin-Induced Thrombocytopenia During Cardiac Surgery

The role of heparin-induced thrombocytopenia and its possibly severe complications in patients undergoing cardiac surgery is increasingly appreciated. Increasing evidence indicates that in patients without the presence of antibodies, unfractionated heparins can be safely used during cardiopulmonary bypass, if their employment is restricted to this short period. This strategy is the first choice in patients with a current negative status of antibodies and in patients in whom surgery can be delayed until antibodies cannot be detected by laboratory tests. To date, however, no alternative anticoagulant to heparin during cardiopulmonary bypass has been approved. With the introduction of the class of direct thrombin inhibitors, effective anticoagulants have become available; however, they have no antidote and require specific monitoring. The availability of direct thrombin inhibitors is currently restricted. The administration of unfractionated heparin with anti-platelet agents such as prostaglandins or the short-acting platelet glycoprotein llb/Illa antagonist tirofiban is another option. Despite successful use of these strategies in fairly large numbers of patients, recent information suggests that in the case of tirofiban, renal failure may cause persistence of the agent with subsequent severe hemorrhage. If surgery is restricted to coronary artery bypass grafting, recent data show that off-pump strategies with use of the direct thrombin inhibitor bivalirudin appear to be a promising option. Therefore, if surgery cannot be postponed, the anticoagulation protocol and the surgical strategy must be adjusted to the condition of the patient and the experience of the center in order to reduce the risk of these “offlabel” strategies.

Immune pathogenesis of heparin-induced thrombocytopenia

The immune response to heparin is one of the most common drug-induced allergies, and yet, atypical for a drug hypersensitivity reaction. Whereas most drug-induced allergies are rare, idiosyncratic and life-long, the allergic response to heparin is common, predictable in certain clinical settings and transient. Advances in the last decade with regards to structural characterisation of the PF4/heparin antigenic complex, contributions of innate immunity and development of animal models have provided insights into the distinctive features of the HIT immune response. Recent descriptions of the crystal structure of the PF4/heparin complex, alongside other biophysical studies, have clarified the structural requirements for immunogenicity and heparin-dependency of antibody formation. Studies of interactions of PF4 with bacterial cell walls as well as epidemiologic associations of anti-PF4/heparin antibody formation and infection suggest a role for immune priming and explain the rapid evolution of an isotype-switched immune response in sensitised patients. Murine models have greatly facilitated investigations of cellular basis of the HIT response and identified a major role for T-cells and marginal zone B-cells, but key findings have yet to be validated in human disease. This chapter will summarise recent investigations of the HIT immune response in the context of major pathways of immune activation and identify areas of uncertainty.

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 (14)C-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 (14)C-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.

Heparin-induced thrombocytopenia

Heparin is widely used for the prevention and treatment of thrombotic and particularly cardiovascular disorders. Unfortunately, 0.5 to 3.0% of patients given heparin develop an immune reaction, commonly termed Type II heparin-induced thrombocytopenia (HIT). This is characterized by a moderate thrombocytopenia and in some patients, a venous or arterial thrombosis. This frequently leads to disastrous sequelae, such as limb amputation and death. The pathophysiological basis of this serious adverse drug reaction is the production of an immunoglobulin G antibody that reacts with an antigenic complex consisting of heparin and platelet factor 4. A significant risk factor for the development of HIT is recent surgery, and the frequency of developing an antiheparin-platelet factor 4 or HIT antibody is particularly high in cardiac surgery patients, although surprisingly, only a few of these patients actually develop the clinical syndrome of HIT. This review will discuss the frequency, pathophysiology, clinical features, diagnosis and management of HIT.

The frequency of heparin-induced thrombocytopenia in Taiwanese patients undergoing cardiopulmonary bypass surgery

BACKGROUND/PURPOSE

There are few studies on heparin-induced thrombocytopenia (HIT) reported from Taiwan and Asian countries. We conducted a prospective study to investigate the frequency of HIT in patients undergoing cardiopulmonary bypass surgeries.

METHODS

A cohort of 54 patients was enrolled from January 01, 2010 to October 31, 2011. Patients' clinical information was obtained for 4T score classification. Plasma (2-4 mL) was also collected before surgery and on Days 5 and 10 following heparin administration during the bypass procedure. This was tested for anti-heparin/PF4 antibodies and functional assay using flow cytometry (FC).

RESULTS

The mean platelet count for this cohort followed the expected pattern in the postoperative setting. Seven of the 54 (13%) patients had positive antibodies assays before bypass surgery. This increased to 32% on Day 5 and was markedly elevated to 63% on Day 10 after surgery. Only one of the 54 patients (1.8%) was found to have both positive antibody assay and platelet activation, but no clinical HIT/thrombosis developed.

CONCLUSION

Our study is the first report on the rates of HIT in the setting of cardiopulmonary bypass surgery in Taiwan and demonstrated no clinical HIT occurrence, despite the high frequency of HIT antibody in our cohort.

Diagnosis and management of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder caused by antibodies to platelet factor 4/heparin (PF4/H) complexes. It presents with declining platelet counts 5 to 14 days after heparin administration and results in a predisposition to arterial and venous thrombosis. Establishing the diagnosis of HIT can be extremely challenging. It is essential to conduct a thorough clinical evaluation in addition to laboratory testing to confirm the presence of PF4/H antibodies. Multiple clinical algorithms have been developed to aid the clinician in predicting the likelihood of HIT. Once HIT is recognized, an alternative anticoagulant should be initiated to prevent further complications.

Critical role for mouse marginal zone B cells in PF4/heparin antibody production

Heparin-induced thrombocytopenia (HIT) is an immune-mediated disorder that can cause fatal arterial or venous thrombosis/thromboembolism. Immune complexes consisting of platelet factor 4 (PF4), heparin, and PF4/heparin-reactive antibodies are central to the pathogenesis of HIT. However, the B-cell origin of HIT antibody production is not known. Here, we show that anti-PF4/heparin antibodies are readily generated in wild-type mice on challenge with PF4/heparin complexes, and that antibody production is severely impaired in B-cell-specific Notch2-deficient mice that lack marginal zone (MZ) B cells. As expected, Notch2-deficient mice responded normally to challenge with T-cell-dependent antigen nitrophenyl-chicken γ globulin but not to the T-cell-independent antigen trinitrophenyl-Ficoll. In addition, wild-type, but not Notch2-deficient, B cells plus B-cell-depleted wild-type splenocytes adoptively transferred into B-cell-deficient μMT mice responded to PF4/heparin complex challenge. PF4/heparin-specific antibodies produced by wild-type mice were IgG2b and IgG3 isotypes. An in vitro class-switching assay showed that MZ B cells were capable of producing antibodies of IgG2b and IgG3 isotypes. Lastly, MZ, but not follicular, B cells adoptively transferred into B-cell-deficient μMT mice responded to PF4/heparin complex challenge by producing PF4/heparin-specific antibodies of IgG2b and IgG3 isotypes. Taken together, these data demonstrate that MZ B cells are critical for PF4/heparin-specific antibody production.

Improving the specificity of the PF4 ELISA in diagnosing heparin-induced thrombocytopenia

Heparin induced thrombocytopenia (HIT) is a serious complication of heparin therapy. The PF4 ELISA is a serologic assay that provides laboratory support for the clinical diagnosis of HIT, but it is often positive in patients who do not have the syndrome. We examined whether the specificity of the PF4 ELISA can be improved by 1) taking antibody potency into consideration, 2) by measuring only IgG antibodies, and 3) by utilizing a high concentration heparin inhibition step. We reviewed clinical information on 116 patients whose samples were referred for HIT antibody testing and assigned each a clinical score related to the likelihood of the patient having HIT. The scores were then correlated with serologic findings. Patients with strongly positive PF4ELISA results (OD ≥ 1.0) using both versions of the assay (IgG/A/M and IgG only) had clinical scores and SRA activity that were significantly higher than those having reactive or negative results. When the IgG-only PF4 ELISA was used, only the strongly positive result group had significantly higher clinical scores and SRA release, and fewer samples were classified as weakly positive or reactive, suggesting that detection of IgG only in the PF4 ELISA improves the assay's specificity. The heparin inhibition step identified "reactive" samples that were associated with clinical scores and SRA release indistinguishable from the "negative" result groups, confirming that this step further improves specificity of the test. This study supports utilizing these 3 modifications of the PF4 ELISA to improve specificity in supporting the clinical diagnosis of HIT.

Prognostic importance of preoperative anti-PF4/heparin antibodies in patients undergoing cardiac surgery. A systematic review

It was the objective of this study to obtain best estimates of the prevalence of anti-PF4/heparin antibodies in patients not suspected to have clinical heparin-induced thrombocytopenia (HIT) prior to undergoing cardiac surgery and to determine whether preoperative antibody status and antibody class is predictive of postoperative thromboembolic outcomes, non-thromboembolic outcomes, length of stay, and mortality. PubMed and EMBASE online databases were searched up to July 2011, and we included studies involving adults undergoing cardiac surgery examining the relationship between preoperative anti-PF4/heparin antibodies (ELISA) and postoperative clinical outcomes. Five studies involving a combined total of 2,332 patients met our inclusion criteria. Preoperative anti-PF4/heparin antibodies were detected in 5-22% of patients. No study demonstrated an association between preoperative anti-PF4/heparin antibodies and postoperative thromboembolic outcomes or mortality. Three studies demonstrated a statistically significant association between preoperative anti-PF4/heparin antibodies and length of stay while two showed an association with non-thromboembolic complications. In the one study that examined outcomes by anti-PF4/heparin antibody class, IgM antibodies predicted non-thromboembolic complications and length-of-stay. None of the studies reported prior heparin exposure, and most studies did not examine the relationship of the absolute value of antibody titres (ELISA OD) and risk, nor the incidence of true/clinical HIT in preoperative positive or negative patients. In conclusion, pre-formed anti-PF4/heparin antibodies are common in patients undergoing cardiac surgery, but the available literature does not support that they predict postoperative thromboembolic complications or death. There does appear to be an association between anti-PF4/heparin antibodies and non-thromboembolic adverse events, but a causal relationship is unlikely.

Platelet factor 4/heparin antibodies in blood bank donors

Platelet factor 4 (PF4)/heparin antibody, typically associated with heparin therapy, is reported in some heparin-naive people. Seroprevalence in the general population, however, remains unclear. We prospectively evaluated PF4/heparin antibody in approximately 4,000 blood bank donors using a commercial enzyme-linked immunosorbent assay for initial and then repeated (confirmatory) testing. Antibody was detected initially in 249 (6.6%; 95% confidence interval [CI], 5.8%-7.4%) of 3,795 donors and repeatedly in 163 (4.3%; 95% CI, 3.7%-5.0%) of 3,789 evaluable donors. "Unconfirmed" positives were mostly (93%) low positives (optical density [OD] = 0.40-0.59). Of 163 repeatedly positive samples, 116 (71.2%) were low positives, and 124 (76.1%) exhibited heparin-dependent binding. Predominant isotypes of intermediate to high seropositive samples (OD >0.6) were IgG (20/39 [51%]), IgM (9/39 [23%]), and indeterminate (10/39 [26%]). The marked background seroprevalence of PF4/heparin antibody (4.3%-6.6%) with the preponderance of low (and frequently nonreproducible) positives in blood donors suggests the need for further assay calibration, categorization of antibody level, and studies evaluating clinical relevance of "naturally occurring" PF4/heparin antibodies.

Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an immune-mediated hypercoagulable disorder caused by antibodies to platelet factor 4 (PF4) and heparin. HIT develops in temporal association with heparin therapy and manifests either as an unexplained thrombocytopenia or thrombocytopenia complicated by thrombosis. The propensity for thrombosis distinguishes HIT from other common drug-induced thrombocytopenias. Diagnosing HIT in hospitalized patients is often challenging because of the frequency of heparin use, occurrence of thrombocytopenia from other causes, and development of asymptomatic PF4/heparin antibodies in patients treated with heparin. This review summarizes our current understanding of the pathogenesis, clinical features, diagnostic criteria, and management approaches in HIT.

Complications related to deep venous thrombosis prophylaxis in trauma: a systematic review of the literature

Deep venous thrombosis prophylaxis is essential to the appropriate management of multisystem trauma patients. Without thromboprophylaxis, the rate of venous thrombosis and subsequent pulmonary embolism is substantial. Three prophylactic modalities are common: pharmacologic anticoagulation, mechanical compression devices, and inferior vena cava filtration. A systematic review was completed using PRISMA guidelines to evaluate the potential complications of DVT prophylactic options. Level one evidence currently supports the use of low molecular weight heparins for thromboprophylaxis in the trauma patient. Unfortunately, multiple techniques are not infrequently required for complex multisystem trauma patients. Each modality has potential complications. The risks of heparin include bleeding and heparin induced thrombocytopenia. Mechanical compression devices can result in local soft tissue injury, bleeding and patient non-compliance. Inferior vena cava filters migrate, cause inferior vena cava occlusion, and penetrate the vessel wall. While the use of these techniques can be life saving, they must be appropriately utilized.

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

BACKGROUND

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Optimization of a murine immunization model for study of PF4/heparin antibodies

SUMMARY BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a life-threatening thrombotic illness caused by drug-dependent antibodies recognizing complexes of platelet factor 4 (PF4) and heparin. Little is known about the immune pathogenesis of HIT, in particular factors influencing PF4/heparin antibody formation. To gain insight into the biologic basis of heparin sensitization, we have recently developed an animal model using wild-type (WT) mice in which murine PF4/heparin antibodies (anti-mPF4/H) arise de novo after antigen challenge.

OBJECTIVES AND METHODS

This report describes technical refinements to the murine model and describes additional biologic features of the immune response to mPF4/heparin.

RESULTS

Our studies indicate that antibody responses to mPF4/heparin are dependent on murine strain, injection routes and doses of mPF4 and heparin. C57BL/6 mice are more immunologically responsive to mPF4/heparin antigen than BALB/c mice and robust immunization can be achieved with intravenous, but not intraperitoneal, administration of antigen. We also observe a direct relationship between initial concentrations of mPF4 and antibody levels. Additionally, we demonstrate that mPF4/H immune response in mice decays with time, is not associated with thrombocytopenia and displays characteristics of immune recall on re-exposure to antigen.

CONCLUSIONS

These studies describe and characterize a murine model for studying the immunologic basis of PF4/heparin sensitization.

Review: Recent Advances in Argatroban-Warfarin Transition in Patients With Heparin-induced Thrombocytopenia

Heparin-induced thrombocytopenia is a devastating, life-threatening, immune-mediated complication of therapy with unfractionated heparin, and less frequently, with low molecular weight heparin. Direct thrombin inhibitors are now standard therapy for the prevention of thrombosis in heparin-induced thrombocytopenia. Argatroban, a small synthetic molecule that inhibits thrombin at its active site, is increasingly used as the direct thrombin inhibitors of choice. Transition to longer term oral anticoagulation needs to be instituted after the platelet count has risen, because of the persistent risk of thrombosis. Although guidelines available in the literature outline the management of heparin-induced thrombocytopenia, they are not presented in a concise and comprehensive manner easily followed by physicians. This article reviews current recommendations, relevant studies, and clinical management trials carried out on patients with heparin-induced thrombocytopenia and provides updated, detailed guidelines for treatment of heparin-induced thrombocytopenia with emphasis on a key part of the management, the argatroban—warfarin transition.

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.

Formation of anti-platelet factor 4/heparin antibodies after cardiac surgery: influence of perioperative platelet activation, the inflammatory response, and histocompatibility leukocyte antigen status

BACKGROUND

Anticoagulation therapy with heparin induces antibodies that recognize multimolecular complexes of platelet factor 4 bound to heparin (anti-platelet factor 4/heparin antibodies). Considering that cardiac surgery induces an intense platelet activation and proinflammatory response, we examined the relationship between formation of anti-platelet factor 4/heparin antibodies and plasma levels of platelet factor 4 and interleukin 6. We also examined the relationship between anti-platelet factor 4/heparin seroconversion and the histocompatibility leukocyte antigen system.

METHODS

In 71 patients undergoing cardiac surgery, anti-platelet factor 4/heparin antibody levels were evaluated by means of enzyme-linked immunosorbent assay preoperatively and 14 days postoperatively. Platelet serotonin release assays were performed to assess the platelet-activating potential of the antibodies. Plasma levels of platelet factor 4 and interleukin 6 were assayed at prespecified time points. Histocompatibility leukocyte antigen status was assessed preoperatively in all patients and was compared with that of 6156 healthy subjects.

RESULTS

Thirty-seven (52%) patients had anti-platelet factor 4/heparin antibodies with an OD value of 0.45 or greater in 1 or more of the assays. Applying strict seroconversion criteria (>2-fold increase in Optical Density), only 16 (22.5%) patients had evidence of anti-platelet factor 4/heparin antibody seroconversion after the operation. Neither the presence of anti-platelet factor 4/heparin antibodies nor seroconversion influenced postoperative outcomes. The CW4 allele was significantly more frequent among seroconverted patients (46.9% vs 19.1%, P = .002). Platelet factor 4 levels did not influence seroconversion. Patients with anti-platelet factor 4/heparin levels of 0.45 OD units or greater 14 days after the operation had significantly higher interleukin 6 levels measured 1 hour after protamine administration.

DISCUSSION

Patients with a greater amount of perioperative inflammation could be more likely to have anti-platelet factor 4/heparin antibodies 1 to 2 weeks later. We provide additional evidence that the histocompatibility leukocyte antigen CW4 confers genetic susceptibility in an acquired inflammatory disorder that includes the anti-platelet factor 4/heparin immune response.

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.

Determinants of PF4/heparin immunogenicity

Heparin-induced thrombocytopenia (HIT) is an antibody-mediated disorder that occurs with variable frequency in patients exposed to heparin. HIT antibodies preferentially recognize large macromolecular complexes formed between PF4 and heparin over a narrow range of molar ratios, but the biophysical properties of complexes that initiate antibody production are unknown. To identify structural determinants underlying PF4/heparin immunogenicity, we characterized the in vitro interactions of murine PF4 (mPF4) and heparin with respect to light absorption, size, and surface charge (zeta potential). We show that PF4/heparin macromolecular assembly occurs through colloidal interactions, wherein heparin facilitates the growth of complexes through charge neutralization. The size of PF4/heparin macromolecules is governed by the molar ratios of the reactants. Maximal complex size occurs at molar ratios of PF4/heparin at which surface charge is neutral. When mice are immunized with complexes that differ in size and/or zeta potential, antibody formation varies inversely with heparin concentration and is most robust in animals immunized with complexes displaying a net positive zeta-potential. These studies suggest that the clinical heterogeneity in the HIT immune response may be due in part to requirements for specific biophysical parameters of the PF4/heparin complexes that occur in settings of intense platelet activation and PF4 release.

Influence of sample collection and storage on the detection of platelet factor 4-heparin antibodies

Heparin-induced thrombocytopenia is a life threatening thrombotic disorder caused by antibodies to platelet factor 4 (PF4) and heparin. Commercial immunoassays are frequently used for the detection of PF4-heparin antibodies, and several studies have reported that higher antibody titers are more frequently associated with adverse events. It is not known if conditions involving sample preparation and/or storage affect the operational characteristics of PF4-heparin immunoassays. We compared the detection of PF4-heparin antibodies from 48 patient samples collected concordantly in serum separator tubes or tubes containing EDTA or sodium citrate. We also examined the effects of extended sample storage on whole blood collected in serum separator, EDTA, or citrate tubes at 4 degrees C for up to 96 hours on antibody detection. We noted that serum or plasma anticoagulated with sodium citrate or EDTA yielded comparable results. In addition, we could not demonstrate any significant sample deterioration after storage at 4 degrees C in any medium for up to 4 days. These findings suggest that PF4-heparin antibodies are largely insensitive to the effects of sample preparation and storage.

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.

Positive Heparin-Platelet Factor 4 Antibody Complex and Cardiac Surgical Outcomes

BACKGROUND

Given the large number of patients undergoing cardiac operations annually, it is important to identify populations at high risk for adverse outcomes. This observational study was conducted to determine the incidence of preoperative heparin-platelet factor 4 (HPF4) antibodies and to assess the associated risk of postoperative adverse outcomes in a nonselected cardiac surgery patient population.

METHODS

Between March 2002 and December 2004, 1114 (92%) of 1209 patients undergoing cardiac surgery with heparin were tested in an unselected manner for HPF4 antibodies. Main outcome measures were HPF4 antibody seropositivity and fatal and nonfatal adverse clinical outcomes after cardiac surgery.

RESULTS

Of those screened, 60 (5.4%) of 1114 had positive HPF4 antibodies preoperatively. These patients had longer mean postoperative length of stay (14.0 days versus 9.8 days, p = 0.05), a higher incidence of prolonged (> or = 96 hours) mechanical ventilation (20.3% versus 9.2%, p = 0.02), acute limb ischemia (5.1% versus 0.9%, p = 0.03), renal complications including dialysis (20.3% versus 10.5%, p = 0.03), and gastrointestinal complications (15.3% versus 5.9%, p = 0.01). Stepwise logistic regression analysis showed positive HPF4 antibody status to be an independent predictor for adverse outcome and was associated with a higher risk for renal complications, including dialysis (adjusted odds ratio 2.2; 95% confidence interval, 1.1 to 4.3), than was diabetes.

CONCLUSIONS

In this large patient series, the presence of HPF4 antibodies before surgical heparin administration was an independent and clinically significant risk factor for postoperative adverse events after cardiac surgery. An optimal preoperative cardiac surgery risk profile should include HPF4 antibody status.

Prevalence of Heparin/Platelet Factor 4 Antibodies Before and After Cardiac Surgery

BACKGROUND

The clinical significance of heparin/platelet factor 4 (PF4) antibodies in subjects undergoing cardiac surgery has not been systematically studied. We prospectively investigated whether the presence of heparin/PF4 antibodies would predict clinical thrombosis in this population.

METHODS

In 299 patients scheduled for cardiac surgery between October 2003 and March 2005, the heparin/PF4 antibodies and platelet count were measured immediately prior to, and 5 days after, surgery. The patients were followed up at 30 days for thrombotic complications.

RESULTS

The prevalence of the heparin/PF4 antibodies was 4.3% (13 of 299) prior to surgery and increased more than fivefold to 22.4% (62 of 277) postoperatively (p < 0.0001). Thromboembolic events occurred in 8.8% of patients with negative antibody and in 6.3% of patients with positive antibody (p = 0.77). Of the 62 patients with positive heparin/PF4 antibodies postoperatively, 22 (35.5%) were treated with a nonheparin anticoagulant. There was a trend toward higher rates of thromboembolic events in subjects who were thrombocytopenic compared with those who were not (17.1% and 6.7%, respectively, p = 0.06), regardless of antibody status. Two out of 8 patients (25%) with both thrombocytopenia and a positive antibody (clinical heparin-induced thrombocytopenia [HIT]) suffered a thromboembolic event, compared with 17 of 222 (7.7%) without clinical HIT (p = 0.13).

CONCLUSIONS

The high prevalence of antibodies to the heparin/PF4 complex after cardiac surgery and the low rate of thromboembolic complications in this population suggest that the antibody alone does not confer an increased risk of thrombotic complications. Monitoring for thrombocytopenia is recommended.