Higher optical density of an antigen assay predicts thrombosis in patients with heparin-induced thrombocytopenia

Objectives: To correlate optical density and percent inhibition of a two-step heparin-induced thrombocytopenia (HIT) antigen assay with thrombosis; the assay utilizes reaction inhibition characteristics of a high heparin concentration. Patients and Methods: Patients with more than 50% decrease in platelet count or thrombocytopenia (<150 × 10⁹/L) after exposure to heparin, who had a positive two-step antigen assay [optical density (OD) >0.4 and >50 inhibition with high concentration of heparin] were included in the study. Results: Forty of 94 HIT patients had thrombosis at diagnosis; 54/94 had isolated-HIT without thrombosis. Eight of the isolated-HIT patients developed thrombosis within the next 30 d; thus, a total of 48 patients had thrombosis at day 30. At diagnosis there was no significant difference in OD between HIT patients with thrombosis and those with isolated-HIT. However, OD was significantly higher in all patients with thrombosis (n = 48, 1.34 ± 0.89), including isolated-HIT patients who later developed thrombosis within 30 d (n = 8, 1.84 ± 0.64) as compared to isolated-HIT patients who did not develop thrombosis (0.96 ± 0.75; P = 0.011 and P = 0.008). The Receiver Operative Characteristic Curve showed that OD >1.27 in the isolated-HIT group had a significantly higher chance of developing thrombosis by day 30. None of these groups showed significant difference in percent inhibition. Multivariate analysis showed a 2.8-fold increased risk of thrombosis in females. Similarly, thrombotic risk increased with age and OD values. Conclusion: Higher OD is associated with significant risk of subsequent thrombosis in patients with isolated-HIT; percent inhibition, however, was not predictive.