Standard low-molecular-weight heparin dosing may be suboptimal for venous thromboembolism prophylaxis. We aimed to identify independent predictors of subprophylactic Xa (subXa) levels in trauma patients treated under a novel early chemoprophylaxis algorithm. A retrospective analysis of trauma patients from July 2016 to June 2017 who received enoxaparin 40 mg twice daily and had peak Xa levels drawn was performed. Patients were divided into cohorts based on having a subXa (<0.2 IU/mL) or prophylactic (≥0.2 IU/mL) Xa level. In all, 124 patients were included, of which 38 (31%) had subXa levels, and 17 (14%) had Xa levels greater than 0.4 IU/mL. Of the subXa cohort, 35 (92%) had their dosage increased, and the repeat Xa testing that was done in 32 revealed that only 75% reached prophylactic levels. The median time to the initiation of chemoprophylaxis was 21.9 hours (interquartile range [IQR], 11.45-35.07 hours). Patients who were defined as having lower risk of having a complication as a result of bleeding had a shorter time to starting prophylaxis than those at higher risk (18.39 hours [IQR 5.76-26.51 hours] vs. 29.5 hours [IQR 16.23-63.07 hours], p < 0.01).There was no difference in demographics, weight, body mass index, creatinine, creatinine clearance, injury severity score, type of injury, weight-based dose, time to chemoprophylaxis, or bleeding complications between the cohorts. No independent predictors of subXa level were identified on multivariable logistic regression. A significant number of trauma patients fail to achieve prophylactic Xa levels. Intrinsic factors may prevent adequate prophylaxis even with earlier administration and higher dosing of low-molecular-weight heparin. Therapeutic, level IV.