Serotonin‐release assay‐positive but platelet factor 4‐dependent enzyme‐immunoassay negative: HIT or not HIT?

IgG‐specific and polyspecific PF4‐dependent enzyme‐immunoassays (EIAs) have exceptionally high sensitivity (≥99%) for diagnosis of heparin‐induced thrombocytopenia (HIT), a drug reaction caused by platelet‐activating antibodies detectable by serotonin‐release assay (SRA). The IgG‐specific EIAs are recommended for screening, as their high sensitivity is accompanied by relatively high specificity vis‐à‐vis polyspecific EIAs. We investigated the frequency of SRA‐positive/EIA‐negative (SRA+/EIA−) HIT, prompted by referral to our reference HIT laboratory of serial blood samples from a patient (“index case”) with false‐negative IgG‐specific EIAs. Despite initial clinical suspicion for HIT, repeat negative IgG‐specific EIAs prompted heparin resumption, which triggered recurrent thrombocytopenia and near‐fatal cardiac arrest, indicating likely post‐heparin HIT‐associated anaphylactoid reaction. Further investigations revealed a strong‐positive SRA, whether performed with heparin alone, PF4 alone, or PF4/heparin, with inhibition by Fc receptor‐blocking monoclonal antibody (indicating IgG‐mediated platelet activation); however, five different IgG‐specific immunoassays yielded primarily negative (or weak‐positive) results. To investigate the frequency of SRA+/EIA− HIT, we reviewed the laboratory and clinical features of patients with this serological profile during a 6‐year period in which our reference laboratory investigated for HIT using both SRA and IgG‐specific EIA. Although ~0.2% of 8546 patients had an SRA+/EIA− profile, further review of 15 such cases indicated clerical/laboratory misclassification or false‐positive SRA in all, with no SRA+/EIA− HIT case identified. We conclude that while SRA+/EIA− HIT is possible—as shown by our index case—this clinical picture is exceptionally uncommon. Moreover, the requirement for a positive EIA is a useful quality control maneuver that reduces risk of reporting a false‐positive SRA result.