© 2014 National Association of EMS Physicians. Objective. To evaluate the ability of out-of-hospital physiologic measures to predict serious injury for field triage purposes among older adults and potentially reduce the undertriage of seriously injured elders to non-trauma hospitals.Methods. This was a retrospective cohort study involving injured adults 55 years and older transported by 94 emergency medical services (EMS) agencies to 122 hospitals (trauma and non-trauma) in 7 regions of the western United States from January 1, 2006 to December 31, 2008. We evaluated initial out-of-hospital Glasgow Coma Scale (GCS) score, systolic blood pressure (SBP), respiratory rate, heart rate, shock index (SBP ÷ heart rate), out-of-hospital procedures, mechanism of injury, and patient demographics. The primary outcome was "serious injury," defined as Injury Severity Score (ISS) ≥ 16, as a measure of trauma center need. We used multivariable regression models, fractional polynomials and binary recursive partitioning to evaluate appropriate physiologic cut-points and the value of different physiologic triage criteria.Results. A total of 44,890 injured older adults were evaluated and transported by EMS, of whom 2,328 (5.2%) had ISS ≥ 16. Nonlinear associations existed between all physiologic measures and ISS ≥ 16 (unadjusted and adjusted p ≤ 0.001 for all,), except for heart rate (adjusted p = 0.48). Revised physiologic triage criteria included GCS score ≤ 14; respiratory rate < 10 or > 24 breaths per minute or assisted ventilation; and SBP < 110 or > 200 mmHg. Compared to current triage practices, the revised criteria would increase triage sensitivity from 78.6 to 86.3% (difference 7.7%, 95% CI 6.1-9.6%), reduce specificity from 75.5 to 60.7% (difference 14.8%, 95% CI 14.3-15.3%), and increase the proportion of patients without serious injuries transported to major trauma centers by 60%.Conclusions. Existing out-of-hospital physiologic triage criteria could be revised to better identify seriously injured older adults at the expense of increasing overtriage to major trauma centers.