Beneficial effect of exogenous platelet factor 4 for detecting pathogenic heparin‐induced thrombocytopenia antibodies

Abstract

Summary The laboratory diagnosis of heparin‐induced thrombocytopenia ( HIT ) is based on an enzyme immunoassay combined with a functional test, and serotonin release assay ( SRA ) is the gold standard for detecting activating HIT antibodies. However, a recent atypical history of HIT prompted us to evaluate whether addition of platelet factor 4 ( PF 4) during SRA could improve its ability to detect pathogenic HIT antibodies. Using 5B9, a monoclonal antibody to PF 4/H with a human Fc fragment, we first defined the optimal PF 4 concentration for detecting low amounts of platelet‐activating IgG with SRA . Plasma samples from 50 patients with suspected HIT were then studied, and SRA was positive in 17 cases (Group SRA pos ), with relatively high levels of PF 4‐specific IgG (median optical density = 2·66). SRA was also systematically performed after adding 10 μg/ml of PF 4 in the reaction mixture, and significant serotonin release was measured with samples from 9 additional patients (Group PF 4‐ SRA pos ). Importantly, levels of PF 4‐specific IgG were similar in these samples and those from the 24 persistently SRA negative patients. Moreover, the pre‐test probability of HIT was intermediate/high in all ‘ SRA pos ’ or ‘ SRA ‐ PF 4 pos ’ patients. In conclusion, addition of exogenous PF 4 might improve the detection of pathogenic HIT antibodies by SRA .