Background: Standard hemodynamic evaluation of patients in shock may underestimate severity of hemorrhage given physiologic compensation. Blood lactate (BL) is an important adjunct in characterizing shock, and point-of-care devices are currently available for use in the prehospital (PH) setting. The objective of this study was to determine if BL levels have better predictive value when compared with systolic blood pressure (SBP) for identifying patients with an elevated risk of significant transfusion and mortality in a hemodynamically indeterminant cohort. Study Design: We selected trauma patients admitted to a level I trauma center over a 9-year period with SBP between 90 and 110 mmHg. The predictive capability of initial emergency department (ED) BL for needing ≥6 units packed RBCs within 24 hours postinjury and mortality was compared with PH-SBP and ED-SBP by comparing estimated area under the receiver operator curve (AUC). Results: We identified 2,413 patients with ED-SBP and 787 patients with PH-SBP and ED-BL. ED-BL was statistically better than PH-SBP (p = 0.0025) and ED-SBP (p < 0.0001) in predicting patients who will need ≥ 6 U packed RBCs within 24 hours postinjury (AUC: ED-BL, 0.72 vs PH-SBP, 0.61; ED-BL, 0.76 vs ED-SBP, 0.60). ED-BL was also a better predictor than both PH-SBP (p = 0.0235) and ED-SBP (p < 0.0001) for mortality (AUC: ED-BL, 0.74 vs PH-SBP, 0.60; ED-BL, 0.76 vs ED-SBP, 0.61). Conclusions: ED-BL is a better predictor than SBP in identifying patients requiring significant transfusion and mortality in this cohort with indeterminant SBP. These findings suggest that point-of-care BL measurements could improve trauma triage and better identify patients for enrollment in interventional trials. Further studies using BL measurement in the PH environment are warranted. © 2010 American College of Surgeons.