Effect of Plasmapheresis on the Anti–Factor Xa Activity of Enoxaparin in an Obese Adolescent Patient

To our knowledge, the effect of plasmapheresis on the antifactor Xa activity of enoxaparin has never been reported. We describe a 13-year-old, obese (92-kg) adolescent female who was treated with enoxaparin for a pulmonary embolism while receiving plasmapheresis for suspected autoimmune encephalitis, and experienced clinically significant reductions in antifactor Xa activity following plasmapheresis. She received five courses of plasmapheresis, with the final two during treatment with enoxaparin. Her antifactor Xa concentrations were highly variable, and we hypothesized that plasmapheresis was affecting these levels. To test this hypothesis, antifactor Xa concentrations were measured prior to and immediately following the patient's last plasmapheresis treatment, and then again 2 days after plasmapheresis. The rate of antifactor Xa activity decline was 0.28 IU/mL/hour with plasmapheresis and only 0.088 IU/mL/hour on the day without plasmapheresis, representing a greater than 3-fold difference. The changes in antifactor Xa activity due to plasmapheresis altered the final enoxaparin dosage required to remain in the therapeutic range of 0.5–1 IU/mL (0.98 mg/kg/dose while receiving plasmapheresis vs 0.69 mg/kg/dose without plasmapheresis). Our patient's data suggest that plasmapheresis can significantly alter enoxaparin's anticoagulant effect as measured by antifactor Xa concentrations, which could cause a decreased anticoagulant effect during plasmapheresis and an increased risk of bleeding on plasmapheresis discontinuation. If concurrent enoxaparin-based anticoagulation and plasmapheresis are necessary, close monitoring of antifactor Xa levels is advisable. Dose escalations and reductions of enoxaparin may be necessary when initiating and discontinuing plasmapheresis, respectively. This article is protected by copyright. All rights reserved.