This project will critically examine the pharmacokinetics of prophylactic doses of enoxaparin in surgical patients, and will evaluate how alteration of enoxaparin dose magnitude and frequency affects peak and trough aFXa levels as well as risk for re-operative hematoma. If subtherapeutic aFXa levels are observed, the study will design, implement and test a clinical protocol to optimize post-operative aFXa levels. Although not an explicit Aim, this study will also provide important preliminary data on VTE rates in surgical patients with in range and out of range aFXa levels.