Infection is a known cause of impaired consciousness. Sometimes, the absence of fever delays diagnosis and treatment of infections in patients with impaired consciousness. This study aimed to identify a better index than body temperature for detecting infection in patients with impaired consciousness by using area under the receiver operating characteristic (AUROC) curves (≥ 0.7 was significant) and stratum-specific likelihood ratios (SSLRs, ＜0.2 or ＞5 was significant) according to patients’ vital signs (blood pressure, heart rate, body temperature, respiratory rate, and SpO2) or age. Of the 1,853 consecutive patients with impaired consciousness aged ≥15 years who were transported to our hospital between 2011 and 2014, 451 (24.3％) had infection. The AUROC for body temperature was 0.701 for diagnosing infection, whereas the AUROCs for other vital signs were ＜ 0.7. Because no strata of body temperature showed values ＜ 0.2 for diagnosing infection, we could not exclude infection in patients with impaired consciousness with low body temperatures. Next, we developed a novel index called HAR/S, the product of heart rate (beats/min), age (years), and respiratory rate (breaths/min) divided by systolic blood pressure (mmHg). The AUROC for HAR/S was 0.809 for diagnosing infection; this value was higher than that for body temperature; the SSLR for HAR/S ＜700 was 0.190 for diagnosing infections, which was ＜0.2. HAR/S ≥700 can be a new index for detecting infections in patients with impaired consciousness regardless of their body temperature, with sensitivity, specificity, and negative predictive values of 0.911, 0.468, and 0.941, respectively.