Heparin Induced Thrombocytopenia; A Diagnostic Dilemma.

The diagnosis of HIT can be challenging. Short of the serotonin release assay, which can be difficult to get in a timely manner, the 4T method developed by Lo et al., in conjunction with the HIT elisa assay (EIA), is the best established criteria for the rapid diagnosis of Heparin Induced Thrombocytopenia (HIT). We sought to determine how often the 4Ts were being applied at a large academic institution. In addition, we sought to understand the outcome of patients started on direct thrombin inhibitors (DTIs) who did not meet the 4T/EIA criteria for HIT.

A retrospective case series identified patients placed on DTI for HIT treatment from 10/2006 to 5/2008. The 4T pretest probability was then applied retrospectively. HIT EIA Ab testing, optical densities, duration of DTI treatment, and bleeding incidence were recorded. Liberal and modified conservative criteria from Lo et al.'s study were used to define patients with HIT. Overdiagnosis of HIT was then recorded based on the above criteria.

76 patients were included for analysis. The average age was 61.4 years. Based on the 4T pretest probability, 30 patients were classified as low risk, 37 intermediate risk, and 9 high risk for HIT. The average HIT OD for low, intermediate, and high pretest probability was 0.49, 1.17, and 2.05.

76% and 36% of patients treated with DTIs for HIT met the liberal and modified conservative HIT definitions, respectively. 18 patients (24%) did not meet liberal HIT criteria but remained on DTI treatment for 1 to 60 days. Cost of DTI overuse in this subset was $84,960. Cost of DTI use in patients who did not meet modified conservative criteria was $462,246. All 16 patients who developed bleeding complications fulfilled a liberal diagnosis of HIT while only 6 of these patients fulfilled the modified conservative definition of HIT.

1) The 4T method for diagnosing HIT is not being applied to a large percentage of patients in a tertiary academic center. 2) There is an overdiagnosis of HIT regardless of definition criteria with 24% of patients treated for HIT not meeting the liberal definition while 64% of patients did not meet the modified conservative definition. This overdiagnosis has both a significant monetary and morbidity cost.

No relevant conflicts of interest to declare.