Brief Report THROMBOSIS AND HEMOSTASIS Plasma exchange to remove HIT antibodies: dissociation between enzyme-immunoassay and platelet activation test reactivities

Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction caused by platelet-activating IgG antibodies that recognize multimolecular PF4/heparin complexes. Therapeutic plasma exchange (TPE) has been recommended as a way to remove HIT antibodies quickly, as might be required to permit administration of heparin for urgent cardiac surgery. However, HIT antibodies (IgG) are not as effectively removed by TPE as IgM. Using serial pre-/ post-TPE sera obtained from a patient with subacute HIT (ie, recent HITwith platelet count recovery but persistingHIT antibodies) who underwent repeated TPE pre-cardiac surgery, we compared antibody reactivity by the C-serotonin-release assay (SRA)—a functional (platelet activation) test for HIT antibodies—vs an IgG-specific antiPF4/heparin enzyme-immunoassay (EIA). We found that although a negative SRA could be achieved quickly post-TPE, corresponding EIA reactivities remained strongly positive. This observation proved to be a general feature of HIT antibody reactivity, because 15 other acute HIT sera showed rapid diminution of SRA reactivity upon serial dilutions, but with major reductions in EIA reactivity requiring much greater sample dilutions.