Laboratory diagnosis of immune heparin-induced thrombocytopenia.

Immune heparin-induced thrombocytopenia (HIT) is a distinct immunohematologic syndrome in which laboratory detection of the pathogenic HIT antibodies is diagnostically useful. Assays can be broadly classified as platelet activation assays (which detect HIT antibodies based on their characteristic platelet-activating properties) and antigen assays (which measure antibodies reactive against platelet factor 4 complexed with heparin or other polyanions). Available tests vary considerably in their sensitivity-specificity profiles for detecting the antibodies and in their predictive values. The high sensitivity of certain assays means that HIT can be readily ruled out (high negative predictive value). However, because heparin-treated patients often generate nonpathogenic antibodies, a positive test does not necessarily indicate clinical HIT. Laboratory methods for detecting HIT antibodies have undergone an evolution similar to that of red blood cell serology and which parallels the Goldilocks tale, that is, progression from too insensitive (too hard) to too sensitive (too soft) to "just right." However, optimal diagnostic information requires that laboratory test results be interpreted in the appropriate clinical context.