Rapid diagnosis of heparin-induced thrombocytopenia (HIT) by whole blood impedance aggregometry: Results of the Australian multi centre study.

Abstract

HIT is a serious complication and remains a challenge for diagnostic laboratories and clinicians. Only a subset of IgG antibodies to complexes of platelet factor 4 (PF4) and heparin (H) trigger the clinical manifestations of HIT by activating platelets, and these can only be identified with platelet activation/aggregation (functional) assays. The 14C-serotonin release assay (SRA), the gold standard in HIT diagnosis, is highly sensitive but only performed in one Australian laboratory and hence unsuitable for routine use. We have developed a whole blood impedance aggregometry (WBIA-Multiplate) and in a small cohort of HIT-antibody positive patients have shown it to be superior to light transmission aggregometry and as sensitive as the SRA. We obtained ethics approval to conduct a multi-centre study to validate the WBIA as a suitable diagnostic tool in HIT. We have tested 158 samples positive for H-PF4 antibodies by PaGIA or ELISA. Using a selected donor (high responder), 65 samples were positive by WBIA (aggregation with low dose 0.5 IU/mL H but not with high dose). SRA was performed using the same donor platelets, serotonin was released from 80 samples (> 20% release with 0.1 IU/ mL H) with only 70 true HIT samples (release < 20% with 100 IU/ mL H). Eleven samples exhibited discrepant results: one strongly positive (89% release) and seven weakly positive by SRA (average release 40%, range 22-67%) were WBIA negative. These eight samples were retested using a random donor, only three remained SRA positive. Three samples were SRA negative but strongly WBIA positive. These discrepant cases will be analysed for clinical outcomes and potential interfering substances. In this multi-centre study, with a selected donor, WBIA had a sensitivity of 88.6% which could rise to 95% using random donors for SRA, a specificity of 96.6% and a PPV of 95.4%. WBIA is easy to perform with rapid turn-around time and warrants further investigation as confirmatory assay for platelet-activating HIT antibodies.