Clinical and laboratory diagnosis of heparin-induced thrombocytopenia: an integrated approach

Heparin-induced thrombocytopenia (HIT) is a clinicopathologic disorder that predisposes to thrombosis. Diagnosis rests on a compatible clinical picture and laboratory evidence of antiplatelet factor 4 (PF4)/heparin antibodies that activate platelets in a heparin-dependent manner. Rapid and accurate diagnosis is paramount to avoid the perils of misdiagnosis. Clinical evaluation may be guided by scoring systems such as the 4Ts and HIT Expert Probability (HEP) score. Laboratory tests include immunoassays, such as the PF4/heparin enzyme-linked immunosorbent assay (ELISA) and functional tests such as the 14C-serotonin release assay and heparin-induced platelet activation assay. Clinical scoring systems and commercially available immunoassays have high sensitivity but modest specificity. Functional assays are more specific, but they are technically demanding. Novel laboratory assays with faster turnaround times, greater specificity, and lesser technical complexity are in development. A Bayesian approach that combines the 4T score and the PF4/heparin ELISA result may be used to estimate the probability of HIT and guide clinical decision making.