BACKGROUND: Heparin-induced thrombocytopenia (HIT) is a significant complication of heparin therapy induced by antibodies to heparin/platelet factor 4 (PF4) complexes. We investigated the diagnostic performance of four commercial immunoassays that detect the anti-heparin/PF4 antibody. METHODS: Four different anti-heparin/PF4 antibody assays were performed in 39 patients with suspected HIT: HemosIL AcuStar HIT-IgG, HemosIL AcuStar HIT-total antibody (Ab) (Instrumentation Laboratory, USA), STic Expert HIT (Diagnostica Stago, France), and PF4 Enhanced (Immucor GTI Diagnostics, USA). Patients were diagnosed with HIT when the Chong score was > or =5. RESULTS: The estimated sensitivity and specificity for diagnosis of HIT were 33.3% and 80.0% for AcuStar HIT-IgG, 55.6% and 53.3% for AcuStar HIT-total Ab, 100.0% and 37.9% for STic Expert HIT, and 33.3% and 66.7% for PF4 Enhanced. All specificities significantly increased when 4Ts scores were included in the diagnosis. The areas under the curves (AUCs) for predicting thrombosis in the AcuStar HIT-IgG, AcuStar HIT-total Ab, and PF4 Enhanced assays were 0.639, 0.522, and 0.681, respectively. When the results of each assay were analysed along with 4Ts scores, the AUC increased to 0.927 in the AcuStar HIT-IgG assay and 0.944 in the AcuStar HIT-total Ab and PF4 Enhanced assays. CONCLUSIONS: The STic Expert HIT assay had high sensitivity but low specificity for diagnosis of HIT. The performances of the three other immunoassays were comparable to each other. Specificity significantly increased when assay data were combined with 4Ts scores. Differences in the diagnostic performance of the four immunoassays were not evident, and simultaneous consideration of clinical scoring systems improved performance.