Age-Adjusted Oral Temperature Thresholds for Rectal Fever Diagnosis in Hospitalized Older Adults

Because older adults may present with atypical symptoms of infection, accurate thresholds for fever are critical for diagnosis and treatment. Accuracy of oral thresholds for rectal fever detection in older adults and those with frailty has not been well described in the hospital setting. The study's aim is to evaluate the diagnostic accuracy of oral fever threshold for predicting rectal fever and to propose age- and frailty-adjusted oral thresholds.

Retrospective observational cohort study.

Adults aged 45 years and older hospitalized in a multicenter tertiary hospital center with rectal and oral temperatures measured.

We identified rectal and oral temperatures within a 60-minute interval. Fever was defined as rectal temperature ≥38.0 °C and oral ≥37.5 °C. A 34-deficit frailty index was constructed. Area under the curve (AUC), sensitivity, specificity, and confusion matrices for rectal fever prediction were calculated by age and frailty groups. Receiver operating characteristic curve analysis identified age- and frailty-adjusted oral thresholds using sensitivity of 45-54 and nonfrail groups as reference, respectively.

Among 6814 participants with a mean age of 69 years (44% female), 8548 oral and rectal paired temperatures were included with 2100 (25%) paired discordances. The ≥85 years group had the lowest performance, with an AUC of 0.71 (95% CI: 0.67-0.74) and sensitivity of 0.60 (95% CI 0.55-0.65). Holding sensitivity equal across age groups, oral thresholds were 37.4 °C (95% CI 37.3-37.5 °C) for 45-54 years, 37.4 °C (95% CI 37.3-37.4 °C) for 55-64 years, 37.3 °C (95% CI 37.2-37.4 °C) for 65-74 years, 37.2 °C (95% CI 37.1-37.3 °C) for 75-84 years and 36.9 °C (95% CI 36.8-37.0 °C) for ≥85 years. AUC, sensitivity, specificity, and oral thresholds were similar between frailty groups.

Lower predictive performance for rectal fever prediction in older adults supports using an oral threshold of 37.2 °C in high-risk hospitalized adults aged ≥75 years. Adjusting oral thresholds in older adults may improve infection detection and management.