Effects of Plasma Exchange and Heparin Concentration on the Serotonin Release Assay in Heparin-Induced Thrombocytopenia.

Abstract

Heparin-induced thrombocytopenia (HIT) is a hypercoagulable state caused by a transient antibody to heparin-bound platelet factor 4 (PF4). Treatment involves discontinuing heparin and administering a nonheparin anticoagulant. Procedures requiring heparin, such as cardiopulmonary bypass, are preferably delayed until the offending antibody is no longer detectable. For patients with a high-titer anti-PF4-heparin antibody and who require exposure to heparin, therapeutic plasma exchange (TPE) has been used to remove the antibody. Recent work indicates that a functional assay for detecting platelet-activating antibodies in HIT patients, the serotonin release assay (SRA), is preferable to ELISAs for anti-PF4-heparin antibodies for following the effectiveness of plasma exchange. Two cases of acute heparin-induced thrombocytopenia managed with plasma exchange before emergent cardiac surgery were evaluated with SRAs using a range of heparin concentrations that included those used in cardiopulmonary bypass. We observed that a single round of plasma exchange led to greater reduction in platelet reactivity at heparin concentrations between 1 and 3 U/mL than at lower concentrations, consistent with the impression that heparin-PF4-antibody complexes form optimally within a limited heparin concentration range. The findings suggest there may be a range of heparin concentration in which cardiac surgery may be safely performed in HIT patients, and that a single TPE in an emergent setting may lower antibody concentration sufficiently to lower platelet reactivity in the presence of heparin.