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 anti–factor Xa activity of enoxaparin has never been reported. We describe a 13‐year‐old, obese (92‐kg) girl who was treated with enoxaparin for a pulmonary embolism while receiving plasmapheresis for suspected autoimmune encephalitis and who experienced clinically significant reductions in anti–factor Xa activity after plasmapheresis. She received five courses of plasmapheresis, with the final two administered during treatment with enoxaparin. Her anti–factor Xa concentrations were highly variable, and we hypothesized that plasmapheresis was affecting these levels. To test this hypothesis, anti–factor Xa concentrations were measured before and immediately after the patient's last plasmapheresis treatment, and then again 2 days after plasmapheresis. The rate of anti–factor 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 anti–factor 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 anti–factor 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 anti–factor Xa levels is advisable. Dose escalations and reductions of enoxaparin may be necessary when initiating and discontinuing plasmapheresis, respectively.