Risk of Subsequent Thrombosis in Patients with Isolated Heparin-Induced Thrombocytopenia
Authors: Wolfe Heather R , Shen Yu-Min 
Published: October 2019

Abstract
Background: Heparin-Induced Thrombocytopenia (HIT) is one of the most important and commonly encountered immune-mediated drug reactions which can lead to significant morbidity and mortality. In up to 25% of patients, thrombosis can be the presenting finding of HIT. Isolated HIT occurs in patients without clinically evident thrombosis at the time of diagnosis. The management of HIT focuses on reducing the risk of thrombotic complications. In addition to prompt discontinuation of heparin products, anticoagulation is recommended. The risk of thrombosis is thought to remain high for up to 4 weeks due to persistent circulating PF4-heparin antibodies. Prior studies have shown that in patients with isolated HIT, the subsequent 30-day risk of thrombosis is up to 53%.
Guidelines recommend 3 months of anticoagulation for patients with HIT-associated thrombosis. The optimal duration of anticoagulation in isolated HIT is unknown. Many experts recommend prophylactic anticoagulation for up to 4 weeks after diagnosis. However in the era of direct thrombin inhibitors, the need for prophylactic anticoagulation and risk of thrombosis beyond normalization of platelet count remains to be determined. The aim of this study was to determine the incidence of subsequent thrombosis in patients with isolated HIT.
Methods: In this retrospective review, we identified patients with a documented positive HIT (heparin-PF4 antibody) enzyme immunoassay (ELISA) at our affiliated hospitals between January 2006 and December 2016. Laboratory data from these patients were reviewed. Patients who met criteria for HIT, defined as HIT ELISA optical density ≥ 2 or positive serotonin release assay were included in the analysis. From this cohort of patients with confirmed HIT, clinical data was extracted from the electronic medical record, including anticoagulation management and documented thrombotic events within 3 months of HIT diagnosis. Patients with evidence of thrombosis at the time of diagnosis were excluded in the final analysis as our focus was on patients with isolated HIT. The incidence of subsequent thrombosis was compared in patients who received anticoagulation versus those who did not receive anticoagulation.
Results: A total of 403 charts were reviewed from patients with a documented positive HIT ELISA, defined as an optical density of > 0.4. Of the 403 patients, 49.5% (n = 107) met criteria for HIT, with an optical density > 2 or positive serotonin release assay. In patients with HIT, 48.6% (n = 52) did not have evidence of thrombosis at the time of diagnosis. Of these patients with isolated HIT, 14 patients (26.9%) received no prophylactic anticoagulation, 8 patients (15.4%) received prophylactic anticoagulation for at least 4 weeks but less than 3 months, and 30 patients (57.7%) were anticoagulated for 3 months or longer. The total incidence of subsequent thrombosis in isolated HIT was 7.7%. In the cohort of patients that did not receive prophylactic anticoagulation, two patients (14.3%) developed a thrombotic event in the 3 months following the HIT diagnosis. No documented thrombotic complications occurred in the eight patients that received prophylactic anticoagulation for at least 4 weeks but less than 3 months. Three patients (10%) who received long-term anticoagulation (≥3 months) developed a subsequent thrombotic event. There was no significant difference in the incidence of subsequent thrombosis in patients who did not receive prophylactic anticoagulation versus patients who received short-term prophylactic anticoagulation (p=0.141). In addition, there was no significant difference in the incidence of subsequent thrombosis in patients who did not receive prophylactic anticoagulation versus those who received long-term anticoagulation (p=0.701).
Conclusions: The optimal duration of anticoagulation in patients with isolated HIT is unknown. Our incidence of thrombosis following the diagnosis of HIT was lower than previously described. Patients who did not receive prophylactic anticoagulation did not have a significantly higher incidence of thrombosis compared to patients who received short-term prophylactic or long-term anticoagulation. The study was limited by the retrospective nature and small sample size. Further studies are needed to better understand the ideal length of anticoagulation to prevent thrombotic complications without leading to unnecessary increased risk of bleeding.

Disclosures
No relevant conflicts of interest to declare.