Background: In the prehospital environment, the failure of medical providers to recognize latent physiologic derangement in patients with compensated shock may risk undertriage. We hypothesized that the shock index (SI; heart rate divided by systolic blood pressure [SBP]), when used in the prehospital setting, could facilitate the identification of such patients. The objective of this study was to assess the association between the prehospital SI and the risk of massive transfusion (MT) in relatively normotensive blunt trauma patients. Methods: Admissions to a Level I trauma center between January 2000 and October 2008 with blunt mechanism of injury and prehospital SBP >90 mm Hg were identified. Patients were categorized by SI, calculated for each patient from prehospital vital signs. Risk ratios (RRs) and 95% confidence intervals (CI) for requiring MT (>10 red blood cell units within 24 hours of admission) were calculated using SI >0.5 to 0.7 (normal range) as the referent for all comparisons. Results: A total of 8,111 patients were identified, of whom 276 (3.4%) received MT. Compared with patients with normal SI, there was no significant increased risk for MT for patients with a SI of ≤0.5 (RR, 1.41; 95% CI, 0.90-2.21) or >0.7 to 0.9 (RR, 1.06; 95% CI, 0.77-1.45). However, a significantly increased risk for MT was observed for patients with SI >0.9. Specifically, patients with SI >0.9 to 1.1 were observed to have a 1.5-fold increased risk for T (RR, 1.61; 95% CI, 1.13-2.31). Further increases in SI were associated with incrementally higher risks for MT, with an more than fivefold increase in patients with SI >1.1 to 1.3 (RR, 5.57; 95% CI, 3.74-8.30) and an eightfold risk in patients with SI >1.3 (RR, 8.13; 95% CI, 4.60-14.36). Conclusion: Prehospital SI > 0.9 identifies patients at risk for MT who would otherwise be considered relatively normotensive under current prehospital triage protocols. The risk for MT rises substantially with elevation of SI above this level. Further evaluation of SI in the context of trauma system triage protocols is warranted to analyze whether it triage precision might be augmented among blunt trauma patients with SBP >90 mm Hg. © 2011 by Lippincott Williams & Wilkins.