Heparin-induced thrombocytopenia (HIT) has a distinct clinical profile and unique pathogenesis. It is caused by platelet-activating IgG antibodies that recognize multimolecular complexes of platelet factor 4 (PF4) bound to heparin or certain other polyanions. Although an immune response to PF4/heparin associated with heparin treatment is very common, clinical HIT occurs only among the minority of patients whose antibodies are capable of strongly activating platelets. This explains why certain platelet activation assays and anti-PF4/polyanion immunoassays have high sensitivity for HIT and why diagnostic specificity is highest for those assays that preferentially detect pathogenic antibodies, such as the washed platelet activation assays or immunoassays that detect only IgG antibodies. Negative results obtained in a solid-phase PF4/polyanion immunoassay generally exclude HIT (high negative predictive value), especially in a setting of a low pretest probability. In addition, because the magnitude of a positive test result correlates with greater likelihood of HIT, a Bayesian diagnostic approach that combines pretest probability and the magnitude of a positive test result is recommended. Recent studies suggest that presence of anti-PF4/polyanion antibodies in certain clinical settings confers an adverse prognosis, even without clinically evident HIT. Whether such antibodies impart "forme fruste" HIT or are simply a surrogate marker for a non-HIT adverse risk factor such as inflammation is unresolved.