Thrombosis and ELISA optical density values in hospitalized patients with heparin-induced thrombocytopenia

The natural history of heparin-induced thrombocytopenia (HIT) in the absence of thrombosis was previously established using functional assays for confirmation of diagnosis (e.g. C-14 serotonin release assay). An enzyme-linked immunosorbent assay (ELISA) that detects the presence of antibodies directed against the heparin-platelet factor-4 (PF4) complex has largely replaced functional assays in many medical centers. Although the ELISA is highly sensitive for detecting HIT antibodies, its usefulness for predicting thrombotic outcomes has not been clearly established. We performed a retrospective chart review of all hospitalized patients at a University hospital who tested seropositive for HIT by a commercial ELISA during 2001 and 2002. A total of 63 inpatients were identified as HIT positive by ELISA. Forty-eight patients had no apparent HIT-associated thrombosis at the time of HIT seropositivity (i.e. isolated HIT) and only one was treated prophylactically with a direct thrombin inhibitor. The 30-day thrombosis rate for patients with isolated HIT was 17% (eight of 48). Higher ELISA optical density (OD) measurements correlated significantly with thrombosis (1.41 +/- 0.87 vs. 0.79 +/- 0.46, P < 0.001). Patients with isolated HIT and an OD measurement of greater than or equal to 1.0 demonstrated nearly a 6-fold increased risk of thrombosis compared with those with OD values between 0.4 and 0.99 (odds ratio 5.74, 95% confidence interval 1.73, 19.0; absolute rate of thrombosis, 36/,, vs. 9%, respectively, P = 0.07). We conclude that in hospitalized patients with isolated HIT, the presence of heparin-PF4 antibodies detected by ELISA was associated with a significant risk of subsequent thrombosis and higher ELISA values were observed among patients suffering thrombotic events.