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EDITORIAL

HIT Happens: Diagnosing and Evaluating the Patient with Heparin-Induced Thrombocytopenia

Andreas Greinacher, MD^*, and Jerrold H. Levy, MD

From the *Institut für Immunologie und Transfusionsmedizin, Ernst-Moritz-Arndt Universität Greifswald, Greifswald, Germany; and Emory University School of Medicine, Atlanta, Georgia.

Address correspondence to Jerrold H. Levy, MD, Emory Hospital, 1364 Clifton Road, Atlanta, GA 30322. Address e-mail to jlevy01{at}emory.edu.

Heparins are extensively administered in the perioperative setting, and are the current drug of choice for anticoagulation in extracorporeal circulation and cardiopulmonary bypass. Unfractionated heparin is a mixture of highly sulfated glycosaminoglycans with a molecular weights ranging from 3 to 40 kDa, although the average molecular weight of most commercial unfractionated heparin preparations is in the range of 12-15 kDa.1 Life-threatening anaphylactic reactions to heparin have been reported.2 Some reports of allergic reactions are associated with specific lots as noted on the Food and Drug Administration web site (http://www.fda.gov/cder/drug/infopage/heparin/default.htm), and are most likely caused by contamination. However, the most common immune response to heparin is heparin-induced thrombocytopenia (HIT). HIT is a serious, antibody-mediated complication of heparin therapy that occurs in approximately 0.5%-5% of patients treated with heparin.3 It confers significant risks of thrombosis and devastating outcomes. About 38%-76% of patients with HIT develop thrombosis, nearly 10% of whom need limb amputation, and approximately 20%-30% of whom die within 1 mo.4,5 HIT antibodies are transient and typically disappear within 3 mo. In patients with persistent antibodies, reexposure to heparin can be disastrous. The annual health care cost of HIT complications in cardiac surgery alone is estimated to be $300 million.6

The pathogenesis of HIT has been described previously. Briefly, it is mediated by immunoglobulin G (IgG) antibodies to a complex of platelet factor 4 (PF4, a heparin-binding protein stored in platelets) and heparin. Anti-PF4/heparin antibodies recognize epitopes newly exposed on PF4 when it is conformationally changed by binding to heparin, and bind heparin-PF4 complexes on the platelet surface to form immune complexes. The platelets in turn are activated by the Fc domain of the IgG in the immune complexes. Activated platelets release microparticles that promote thrombin formation and create a prothrombotic state. Reports suggest that 7%-50% of heparin-treated patients produce heparin-PF4 antibodies.7 Cardiovascular surgery patients are at risk of forming the antibodies because of multiple factors, the most important being administration of unfractionated heparin during the massive platelet activation and PF4 release that accompanies cardiopulmonary bypass. This is aggravated by multiple episodes of heparin administration including potential for occult administration.7–9 The antibodies, irrespective of thrombocytopenia, are associated with increased morbidity or mortality in various clinical settings.5,10

This month’s journal features a clinical study describing HIT in pediatric patients11 and a case report highlighting the importance of interpretation of HIT test results in the clinical context.12 In the pediatric study, Mullen et al. prospectively characterized the incidence of seroconversion in two pediatric populations.11 They evaluated 135 consecutive patients, including 60 neonates undergoing first time cardiac surgery and 75 children undergoing reoperative cardiac surgery with prior unfractionated heparin exposure. Preoperative and postoperative day (POD) 5 and 10 blood samples were used to determine the presence of PF4 IgG, IgA, and IgM antibodies with an enzyme-linked immunosorbent assay (ELISA). Initially, no antibodies were detected preoperatively in either group. On POD 5, antibodies were present in 1 of 60 (1.7%) of the neonatal patients; and in 12 of 75 (16%) of the reoperations. On POD 10, the authors identified antibodies in 1 of 60 (1.7%) of the neonatal patients, and in 39 of 75 (52%) of the reoperations. Clinical HIT occurred only in a reoperative patient (1.3%) who seroconverted and developed HIT without thrombosis or skin lesions. The authors conclude that HIT is rare in pediatric cardiac surgical patients. This study provides several important insights into HIT. Although many of the children had heart catheter interventions before surgery, none of them was anti-PF4/heparin antibody positive on the day of surgery. This further supports the idea that cardiac surgery is the main trigger for HIT antibody induction, and antibodies start to occur on day 4 reaching a maximum at day 10-12. Thus, when a clinical score is applied for assessing the pretest probability of HIT, the day of surgery should be taken as day 0. Up to now, there has been no evidence in the literature that HIT antibodies are boosted by reexposure after a typical anamnestic immune response. Indeed, the onset of HIT seems to be the same regardless whether patients had been preexposed to heparin or not.13,14 In a previous study of 111 adult cardiac surgical patients, heparin-dependent antibodies were detected by the 5th POD in 13% of patients by serotonin release assay (SRA) and 51% by ELISA.15 In the pediatric reoperations, antibody seroconversion (16% on POD 5 and 52% on POD 10) was similar to that reported in adult patients. The incidence of seroconversion (1.7% on both POD 5 and 10) in neonates was less than the older children having reoperations. As noted by the authors, although the exact cause of this is unknown, there are two potential explanations. One is that HIT antibodies develop more easily in patients being preimmunized. The other is that the immune system undergoes changes during early childhood, enabling the formation of this type of antibody. It is noteworthy that no patients in either pediatric population tested positive for anti-PF4/heparin antibodies before surgery. In adult cardiac patients, heparin-dependent antibodies have been detected preoperatively in 5% of patients by SRA and 19% of patients with ELISA.15 In adults, a positive ELISA preoperatively was also an independent predictor for postoperative death or prolonged hospital in one study10 and for clinically significant multiorgan postoperative complications in another study.16 Is this an antibody issue or rather is it that antibodies are higher in reoperations, where patients are more likely to have an increased event rate? The finding of the present study suggests that anti-PF4/heparin antibodies in adults before surgery might be an indicator for additional risk factors, e.g., perhaps vascular and endothelial dysfunction. All the complex factors that govern antibody formation may contribute to the worse outcomes.

The case report by Dellaria et al. demonstrates the importance of interpreting a HIT antibody test in the clinical context, the clinical utility of functional testing, and the need for a confirmatory step to exclude heparin-independent antibody reactivity in the ELISA tests.12 Their patient had a history of HIT 2 yr before evaluation. Although the test becomes negative in most patients within 3 mo after the acute episode of HIT, their patient continued to have a positive ELISA for anti-PF4/heparin antibody. A functional SRA was also sent to a reference laboratory to characterize the initial positive optical density (OD) value of 0.976. The sample was sent on a Monday morning and became available 3 days later, on Wednesday afternoon, standard turnaround times for this test. The patient’s serum did not release serotonin after the addition of a therapeutic amount of heparin and was reported as negative for the presence of platelet-activating HIT antibodies. Unfractionated heparin was used successfully for anticoagulation. Preoperatively, it was also noted that the anti-PF4/heparin ELISA (Genetics Testing Institute GTI PF4 Enhanced®) was not reduced when high heparin concentrations were added. The addition of high-dose heparin is a confirmatory procedure that evaluates the specificity of antibodies for PF4/heparin complexes, as high concentrations of heparin disrupt the PF4/heparin complexes.17 Thus, the ELISA is considered positive only when the OD without heparin is >0.4 and inhibition with heparin is >50%. Antibodies, which do not show inhibition, are more likely to bind either to uncomplexed PF4, or to artifacts resulting from PF4 interactions with the plastic surface of the ELISA plate (unpublished data from the Greifswald laboratory).

The use of antigenic and functional testing for HIT antibody is important for patient management.18 Antigenic tests such as ELISA19 and rapid particle gel immunoassays detect antibodies to complexes of PF4 and heparin or complexes of PF4 and other polyanions.20 These commercial tests are sensitive for PF4/heparin antibodies but are not specific for HIT. These tests have a high negative predictive value and are good for excluding HIT.21 Measurement of only IgG antibodies enhances clinical specificity,22 as do antibody titers based on the OD.4 In contrast, functional tests using washed platelets such as the ¹⁴C-SRA and heparin-induced platelet activation test detect heparin-dependent, platelet-activating antibodies.19 These assays have higher specificity for HIT and should be used to confirm the diagnosis. Unfortunately, they are not readily available for rapid evaluation in many countries,19 although it is feasible to provide a network of trained laboratories with 24 h turnaround time.23

It is increasingly obvious that we are over-diagnosing HIT in our patient population, especially in patients scheduled for cardiac surgery, if the diagnosis is based on the commercially available antigen tests only. The importance of testing for functional antibodies needs to be considered using a SRA or other confirmatory tests, as reported by Dellaria. This is particularly necessary in those patients in whom either the OD in the commercial ELISAs is <1.0, or in whom the clinical context gives a low probability for HIT. Although HIT is a prothrombotic and potentially catastrophic disease, accurate diagnosis and therapeutic modalities need to be considered. Better interpretation of laboratory tests in the context of the clinical presentation will help. Revised guidelines for diagnosing and managing HIT will be forthcoming within the coming weeks from the American College of Chest Physicians.

	Footnotes

 
Accepted for publication April 4, 2008.

Reprints will not be available from the author.

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This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Greinacher, A. Articles by Levy, J. H. Search for Related Content PubMed PubMed Citation Articles by Greinacher, A. Articles by Levy, J. H. Related Collections Cardiovascular Complications Inflammation Patient Safety Pharmacology