Heparin‐induced thrombocytopenia (
HIT) is a clinical‐pathological disorder; thus, laboratory testing for the pathogenic platelet‐activating antiplatelet factor 4 ( PF4)/heparin antibodies is central for diagnosis. The “iceberg” model summarizes the inter‐relationship between platelet activation assays and PF4‐dependent immunoassays, with platelet‐activating antibodies comprising a subset of anti‐ PF4/heparin antibodies. The platelet serotonin‐release assay ( SRA), performed by reference laboratories, has high sensitivity and specificity for HIT(~95% each), and is especially suited for detecting highly pathogenic HITsera containing both heparin‐dependent and heparin‐independent platelet‐activating antibodies; this latter subgroup of antibodies explains “autoimmune HIT” disorders (delayed‐onset, persisting, spontaneous, heparin “flush,” fondaparinux‐associated). Recently, SRA‐negative HIThas become recognized, in which serum from some HITpatients contains subthreshold levels of platelet‐activating antibodies (by SRA) that become detectable using a PF4‐enhanced platelet activation assay. Unusual immunologic features of HITinclude early antibody detectability (at onset of platelet count fall) and antibody transience (seroreversion). Widely available PF4‐dependent enzyme immunoassays ( EIAs) have high sensitivity but poor specificity for HIT, although specificity is enhanced with IgG‐specific EIAs and strong positive results; unfortunately, EIAresults are usually not available in real time. Automated rapid immunoassays, such as the chemiluminescence immunoassay ( CLIA) and latex immunoturbidimetric assay ( LIA), facilitate real‐time laboratory diagnosis. Recently available likelihood ratio ( LR) data for positive ( LR+) and negative ( LR−) test results allow clinicians to adjust their pretest probabilities for HIT, using Bayesian analysis, into real‐time posttest probabilities that are dramatically increased (test positive) or decreased (test negative). Moreover, (semi‐)quantitative CLIA‐ and LIA‐positive results (weak, moderate, strong positive) can further refine the posttest probability of HIT.