Are We Doing Too Many HIT Tests?

Heparin-induced thrombocytopenia (type II-HIT) is a serious immune mediated clinicopathologic syndrome that may lead to significant arterial or venous thromboembolism and moderate to severe thrombocytopenia. Type II-HIT is estimated to occur with a frequency of 0.2 to 5.0 % after exposure to heparin.

To determine the incidence of type II-HIT and the necessity of its testing within a single, 450-bed, urban, teaching, community hospital.

A retrospective review of the hospital database of inpatient and laboratory medical records was performed for a one-year period from July 2009 to July 2010. Medical records were reviewed to determine how many patients were tested for HIT (i.e., solid phase immunoassay or H-PF4-ELISA) - with the test results and how many confirmatory Serotonin Release Assays (SRA) were performed and the test results. Data was categorized into subgroups depending on the type of care, diagnosis, type of heparin and the dosage of heparin. The percentage of SRA positivity for different strengths of H-PF4-ELISA positivity was also investigated with optical density (OD) at 405 nm (weakly positive with O.D 0.4–0.99, intermediately positive with O.D. 1–1.99 and strongly positive if O.D. equal or more than 2). Pretest probability was also calculated by using 4T's score for all patients with positive ELISA.

A total of 19,474 patient admissions occurred over the one year period. An estimated 213 patients had H-PF4-ELISA tests done. Overall H-PF4-ELISA testing incidence was 1% (213 out of 19,474), overall incidence of H-PF4-ELISA positivity out of all admissions during that year was 0.2% (42 out if 19,474). In this study higher frequency of HIT (H-PF4 ELISA) testing was observed in patients who were treated in ICU and cardiac care unit as well as patients treated for sepsis and venous thromboembolism (VTE). The most common type of heparin associated with higher rate of HIT testing was unfractionated heparin-UFH (prophylactic dose more than therapeutic dose) as compared to low molecular weight heparin-LMWH. The incidence of ELISA test positivity was greatest among patients in the ICU and cardiac care unit and patients with sepsis and VTE. Higher incidence of ELISA positivity was associated with UFH. Out of all performed HIT tests ELISA positivity was 19.71% (42 out of 213) and 171 ELISA tests were negative. 36 of the positive ELISA tests were weakly positive, indicating very low probability of HIT. Out of 42 positive ELISA tests 50% (total 21 of 42) had SRA and none of the SRA returned positive. For all 42 patients with positive ELISA tests, the pretest probability was calculated using the 4T score. These show that of the 42 positive ELISA tests, 52.38% (22 of 42) had a low probability, 28.57 (12 of 42) had an intermediate probability, and only 11.9% (5 of 42) had a high probability.

From this study, we conclude that in our institute we are doing too many HIT studies. To avoid unnecessary testing and improve cost effectiveness, this study emphasizes the importance of considering clinical situation and pretest probability very carefully, prior to testing.

No relevant conflicts of interest to declare.