New approaches to the diagnosis of heparin-induced thrombocytopenia.

Heparin-induced thrombocytopenia (HIT) is a clinicopathologic syndrome that is most reliably diagnosed when a patient with a clinical scenario that is consistent with heparin-induced immunization is shown to have antiplatelet factor 4/heparin, platelet-activating IgG antibodies. A Bayesian diagnostic approach is discussed, wherein the physician estimates the pretest probability of HIT (eg, the timing and severity of thrombocytopenia in relation to heparin treatment and associated thrombosis) and determines the posttest probability using the results of HIT antibody testing. By this approach, the magnitude of a positive test result determines its likelihood ratio in influencing the posttest probability of HIT.