Laboratory diagnosis of heparin-induced thrombocytopenia: advantages of a functional flow cytometric test in comparison to the heparin-induced platelet-activation test

Nearly one third of patients with heparin-induced thrombocytopenia (HIT) will progress to overt thrombosis. Owing to the severity of MIT, a reliable prompt diagnosis is mandatory. In this study 248 consecutive samples from patients referred to our laboratory for HIT diagnosis and 97 specimens from normal controls were prospectively evaluated in parallel using the heparin-induced platelet aggregation (HIPA) test and a flow cytometric (FC) test. The HIPA test resulted in 214 negative, 17 indeterminate and 17 positive samples of patients. The FC method detects activated platelets induced by heparin-immune complexes using the highly sensitive recombinant probe annexin V and pooled platelets from multiple donors. The criteria for positive FC test results included an increase in platelet activiation of at, least 11% at 0.3 IU/mL heparin concentration in the tube, and a ratio of more than 1.5 between platelet activation at 0.3 and 200 IU/mL heparin. According to the cut-off level 17 patients who showed indeteminate HIPA test results had 14 negative and 3 indeterminate corresponding FC test results. Only one of these patients (HIPA test indeterminate, FC test indeterminate) had no other obvious medical cause for thrombocytopenia than HIT. Infections or inflammations did not show any association with the FC test results, whereas thromboembolic events displayed a significant patelet activation at pharmacological heparin concentration. Therefore the FC test is associated to the complications of HIT. In conclusion, the FC test, which is fast and practical, showed a good agreement with the HIPA test and may be an accurate and useful test for HIT.