Underdosing of Prophylactic Enoxaparin Is Common in Orthopaedic Trauma and Predicts 90-Day Venous Thromboembolism.

Objectives: To determine the feasibility and impact of real-time anti-factor Xa (aFXa) level monitoring and enoxaparin dose adjustment in orthopaedic trauma. To examine the adequacy of standard fixed-dose enoxaparin chemoprophylaxis and to examine whether patient-specific factors influence enoxaparin metabolism. Design: Prospective cohort. Setting: Academic Level-I trauma center. Patients: Postoperative adult orthopaedic trauma patients undergoing acute fracture or nonunion surgery of the pelvis, acetabulum, or lower extremity placed on 30 mg of enoxaparin twice daily. Intervention: Peak steady-state aFXa levels were drawn with a goal range of 0.2-0.4 IU/mL. Patients with out-of-range levels underwent a 10-mg dose adjustment followed by repeat aFXa draws. Main Outcome Measures: Peak and trough aFXa levels, 90-day venous thromboembolism, and bleed events. Results: Of 109 enrolled patients, 43% had inadequate initial peak aFXa levels (aFXa, 0.2 IU/mL) with standard dosing. Higher gross weight, acetabular surgery, and operation length predicted low aFXa levels (P < 0.001, 0.006, 0.004, respectively). Dose adjustment increased the proportion of patients with in-range aFXa levels from 53.2% to 87.8% (P < 0.001). Patients with low aFXa levels during hospitalization or at discharge had significantly higher 90-day deep vein thrombosis and pulmonary embolism rates compared to those with adequate aFXa levels (deep vein thrombosis 12% vs. 1.36%; P = 0.023, pulmonary embolism 8% vs. 0%; P = 0.027). There were no major bleed events. Conclusions: Patients receiving inadequate enoxaparin chemoprophylaxis were at significantly increased risk of 90-day venous thromboembolism. Standard fixed-dose enoxaparin provided inadequate chemoprophylaxis in 43% of postoperative orthopaedic trauma patients, which significantly improved with dose adjustment. Weight, acetabular surgery, and operation length predicted inadequate enoxaparin prophylaxis.