A comparative study of platelet factor 4‐enhanced platelet activation assays for the diagnosis of heparin‐induced thrombocytopenia

Background Functional platelet activation assays, such as the serotonin release assay (SRA), are the gold standard for the diagnosis of heparin‐induced thrombocytopenia (HIT). Recently, platelet activation assays using added platelet factor 4 (PF4) have been described and suggest improved sensitivity. Direct comparisons of these assays have not been performed. Objective We compare the performance characteristics of three PF4‐enhanced platelet activation assays, the PF4/heparin‐SRA (PF4/hep‐SRA), the PF4‐SRA, and the P‐selectin expression assay (PEA), at a single reference laboratory. Methods Serum samples from two cohorts of patients were used. The referral cohort (n = 84) included samples that had previously undergone routine diagnostic testing for HIT and tested positive or negative using the SRA. The clinical cohort (n = 101) consisted of samples from patients with clinically confirmed HIT whose serum contained platelet‐activating antibodies. We simultaneously tested all samples in PF4‐enhanced SRA‐based assays (PF4/hep‐SRA, PF4‐SRA) and the flow cytometry‐based PEA. Results In the referral cohort, the three PF4‐enhanced assays identified all samples that were previously determined to be positive in the SRA. However, specificity of the PF4/hep‐SRA was 96.6%, the PF4‐SRA was 84.7%, and the PEA was 67.8%. In the clinical cohort of samples, all SRA‐based assays displayed high performance characteristics (>92.1% sensitivity, >98.4% specificity). Sensitivity and specificity of the PEA was the lowest, 65.8% and 63.5%, respectively; but improved to 92.1% and 96.8% using preselected platelet donors. Conclusions All PF4‐enhanced assays demonstrated good performance characteristics when platelet donors were preselected. Further comparisons across multiple laboratories should be conducted for consensus on optimal HIT diagnostic testing.