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Heparin-induced thrombocytopenia can be a serious and difficult-to-diagnose complication of heparin therapy. Serum from patients with heparin-induced thrombocytopenia can cause heparin-dependent platelet aggregation, but the low sensitivity and specificity of this test limit its clinical usefulness. In this report we describe an assay for heparin-induced thrombocytopenia that is both sensitive and specific. The improvement in the assay was accomplished by measuring platelet release instead of aggregation and by measuring platelet release at two heparin concentrations. The rationale for the use of two heparin concentrations was that sera from patients with heparin-induced thrombocytopenia caused release at therapeutic but not at high concentrations of heparin. Twenty-eight sera samples from patients suspected of having heparin-induced thrombocytopenia and 573 controls were coded and tested in the assay. The patients with possible heparin-induced thrombocytopenia were ranked according to the likelihood of having this disorder by using prospectively defined criteria. The test had a high specificity (99%); only one of 573 controls showed a positive result. The test was also very sensitive, and the likelihood of a positive test result was directly correlated with the clinical likelihood of the patient having heparin-induced thrombocytopenia. Six of six patients with definitive heparin-induced thrombocytopenia had positive test results, whereas zero of four patients in whom the diagnosis was unlikely had positive test results. The two-point test for heparin-induced thrombocytopenia represents a sensitive and specific test for this disorder. This test may be useful not only in confirming the diagnosis of this disorder but also may provide information about its pathogenesis.