ABSTRACT: Currently available triage and monitoring tools are often late to detect life-threatening clinically significant physiological aberrations and provide limited data in prioritizing bleeding patients for treatment and evacuation. The Compensatory Reserve Index (CRI) is a novel means of assessing physiologic reserve, shown to correlate with central blood volume loss under laboratory conditions. The purpose of this study was to compare the noninvasive CRI device with currently available vital signs in detecting blood loss. Study subjects were soldiers volunteering for blood donation (n = 230), and the control group was composed of soldiers who did not donate blood (n = 34). Data collected before and after blood donation were compared, receiver operator characteristic curves were generated after either donation or the appropriate time interval, and areas under the curves (AUCs) were compared. Compared with pre–blood loss, blood donation resulted in a mean reduction of systolic blood pressure by 3% (before, 123 mmHg; after, 119 mmHg; P < 0.01). The CRI demonstrated a 16% reduction (before, 0.74; after, 0.62; P < 0.01). Heart rate, diastolic blood pressure, and oxygen saturation remained unchanged. The AUC for change in CRI was 0.81, 0.56 for change in heart rate, 0.53 for change in systolic blood pressure, 0.55 and 0.58 for pulse pressure and shock index, respectively. The AUCs for detecting mild blood loss at a single measurement were 0.73 for heart rate, 0.60 for systolic blood pressure, 0.62 for diastolic blood pressure, 0.45 for pulse oximetry, and 0.84 for CRI. The CRI was better than standard indices in detecting mild blood loss. Single measurement of CRI may enable a more accurate triage, and CRI monitoring may allow for earlier detection of casualty deterioration.