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Heparin-induced thrombocytopenia (HIT) is a complication caused by antibodies directed to the heparin–platelet factor 4 (PF4) complex with a seemingly paradoxical high risk of thrombosis. Discontinuation of heparin and administration of an alternative anticoagulant is important in prevention of catastrophic thrombosis. Diagnosis is challenging and based on clinical probability models (Warkentin 4 Ts and Chong scale) and, to a lesser degree, laboratory testing. Enzyme-linked immunosorbent assay (ELISA) measurement of heparin-PF4 antibodies is commonly used but has low predictive values for thrombosis. We analyzed 105 cases of suspected HIT and compared optical density values and the Warkentin 4 Ts for sensitivity, specificity, positive predictive value, and negative predictive value (NPV). The predictive value of ELISA alone was inferior to the Warkentin 4 Ts score. The sensitivity and NPV of the clinical score was improved by incorporating ELISA results. The combination of a negative ELISA result with low probability 4 Ts resulted in an NPV of 94%.