Current insights into the laboratory diagnosis of HIT.

[heparin-induced thrombocytopenia]

Heparin-induced thrombocytopenia (HIT ) is an adverse drug reaction and prothrombotic disorder caused by immunization against platelet factor 4 (PF 4 ) after complex formation with heparin or other polyanions . After antibody binding to PF 4 /heparin complexes , HIT antibodies are capable of intravascular platelet activation by cross-linking Fc gamma receptor IIa (FcγRIIa ) on the platelet surface leading to a platelet count decrease and /or thrombosis . In contrast to most other immune-mediated disorders , the currently available laboratory tests for anti-PF 4 /heparin antibodies show a high sensitivity also for clinically irrelevant antibodies . This makes the diagnosis of HIT challenging and bears the risk to substantially overdiagnose HIT . The strength of the antigen assays for HIT is in ruling out HIT when the test is negative . Functional assays have a higher specificity for clinically relevant antibodies , but they are restricted to specialized laboratories . Currently , a Bayesian approach combining the clinical likelihood estimation for HIT with laboratory tests is the most appropriate approach to diagnose HIT . In this review , we give an overview on currently available diagnostic procedures and discuss their limitations .