Serial SRA and IgG-specific anti-PF4/heparin EIA test results in relation to 4 therapeutic plasma exchange (TPE) sessions performed on 4 consecutive days (last TPE performed 2 days before cardiac surgery using heparin). Heparin rechallenge during cardiac surgery did not result in increased levels of anti-PF4/heparin antibodies during the postoperative period (testing until postoperative day 7) (not shown). Although at the time of initial HIT diagnosis, the patient also had detectable anti-PF4/heparin antibodies of IgA class (EIA-IgA = 1.20 OD U; normal <0.45 U) and IgM class (EIA-IgM = 0.46 U; normal <0.45 U), both the IgA and IgM EIAs were negative 3 months later pre-TPE (0.30 and 0.34 OD U, respectively). EIA-IgG, enzyme-immunoassay (IgG-specific); OD, optical density; SRA, serotonin-release assay.