Incidence of isolated heparin-induced thrombocytopenia and risk of thrombosis by IgG-specific anti-PF4/heparin ELISA

Heparin-induced thrombocytopenia (HIT) antibodies are screened by an enzyme-linked immunosorbent assay (ELISA). Polyspecific ELISA detects anti-PF4/heparin IgG, IgA, and IgM. Recently, anti-PF4/heparin IgG ELISA has been shown to be more specific. However, the impact of using the IgG-ELISA on the incidence of isolated HIT (thrombocytopenia alone without clinically evident thrombosis) and the risk of developing subsequent thrombosis are still unknown. Methods: A total of 492 consecutive patients with clinically suspected HIT at The University of Texas Southwestern Medical Center and affiliated hospitals were retrospectively reviewed from December 2008 to May 2010. RESULTS: 29 patients (6%) were diagnosed with HIT based on clinical findings and positive ELISA. 19 of the 29 patients (65%) had thrombosis at the time of diagnosis; whereas 10 of the 29 (35%) had only isolated HIT. The ten patients with isolated HIT had serial follow up for at least 3 months. 3 of 10 were treated with direct thrombin inhibitors and 5 of 10 were treated with Warfarin for at least 1 month upon discharge. None of them developed symptoms or signs of thrombosis during 3 months of follow up. Conclusion: The incidence of isolated HIT in this study was 35%, which is significantly lower than previously reported in the literature. It is possible that some patients previously thought to have HIT by the poly-specific ELISA assay had false positive results. The improved specificity of the IgG- ELISA appears to reduce the incidence of isolated HIT which may have lower risk of subsequent thrombosis.