Heparin-induced thrombocytopenia (HIT) is caused by heparin-dependent antibodies that usually recognize multimolecular complexes of platelet factor 4-heparin (PF4-H). HIT can be viewed as a clinicopathologic syndrome (Warkentin et al., 1998). Thus, a diagnosis of HIT should be based on two criteria: (1) clinically evident abnormalities, most commonly thrombocytopenia with or without thrombosis (see Chapter 2), and (2) detection of HIT antibodies. In some ways, HIT resembles another clinicopathologic disorder, the antiphospholipid (lupus anticoagulant) syndrome (Table 1).

Two major classes of assays—platelet activation (functional) and PF4-dependent antigen—have been developed to detect HIT antibodies (Warkentin and Sheppard, 2006a) (Table 2).

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