Antifactor Xa Levels in Four Patients with Burn Injuries Who Received Enoxaparin to Prevent Venous Thromboembolism

Four patients with severe burn injuries received enoxaparin 40 mg twice/day subcutaneously for the prophylaxis of venous thromboembolism (VTE). Peak antifactor Xa levels were measured 4 hours after administration of a dose, and trough antifactor Xa levels were measured 30 minutes before the next scheduled dose. Ultrasonography was performed once/week to assess the presence of VTE. Any occurrence of major bleeding was documented in the patients' charts. All patients had trough antifactor Xa levels below 0.1 U/ml. Enoxaparin dosages were subsequently adjusted to achieve trough antifactor Xa levels of 0.1-0.2 U/ml. This required dosages higher than those typically recommended for VTE prophylaxis (40 mg every 24 hrs or 30 mg every 12 hrs). One patient needed more than 60 mg every 12 hours. No patient had a venous thromboembolic event or major bleeding. The low antifactor Xa levels that were observed suggest that a reduced dose-response relationship may exist between subcutaneously administered enoxaparin and antifactor Xa activity in patients with severe burn injuries. Prospective studies should be performed to further investigate this relationship.