Defining Usual Oral Temperature Ranges in Outpatients Using an Unsupervised Learning Algorithm

Abstract

Importance: Although oral temperature is commonly assessed in medical examinations, the range of usual or "normal" temperature is poorly defined.

Objective: To determine normal oral temperature ranges by age, sex, height, weight, and time of day.

Design, setting, and participants: This cross-sectional study used clinical visit information from the divisions of Internal Medicine and Family Medicine in a single large medical care system. All adult outpatient encounters that included temperature measurements from April 28, 2008, through June 4, 2017, were eligible for inclusion. The LIMIT (Laboratory Information Mining for Individualized Thresholds) filtering algorithm was applied to iteratively remove encounters with primary diagnoses overrepresented in the tails of the temperature distribution, leaving only those diagnoses unrelated to temperature. Mixed-effects modeling was applied to the remaining temperature measurements to identify independent factors associated with normal oral temperature and to generate individualized normal temperature ranges. Data were analyzed from July 5, 2017, to June 23, 2023.

Exposures: Primary diagnoses and medications, age, sex, height, weight, time of day, and month, abstracted from each outpatient encounter.

Main outcomes and measures: Normal temperature ranges by age, sex, height, weight, and time of day.

Results: Of 618 306 patient encounters, 35.92% were removed by LIMIT because they included diagnoses or medications that fell disproportionately in the tails of the temperature distribution. The encounters removed due to overrepresentation in the upper tail were primarily linked to infectious diseases (76.81% of all removed encounters); type 2 diabetes was the only diagnosis removed for overrepresentation in the lower tail (15.71% of all removed encounters). The 396 195 encounters included in the analysis set consisted of 126 705 patients (57.35% women; mean [SD] age, 52.7 [15.9] years). Prior to running LIMIT, the mean (SD) overall oral temperature was 36.71 °C (0.43 °C); following LIMIT, the mean (SD) temperature was 36.64 °C (0.35 °C). Using mixed-effects modeling, age, sex, height, weight, and time of day accounted for 6.86% (overall) and up to 25.52% (per patient) of the observed variability in temperature. Mean normal oral temperature did not reach 37 °C for any subgroup; the upper 99th percentile ranged from 36.81 °C (a tall man with underweight aged 80 years at 8:00 am) to 37.88 °C (a short woman with obesity aged 20 years at 2:00 pm).

Conclusions and relevance: The findings of this cross-sectional study suggest that normal oral temperature varies in an expected manner based on sex, age, height, weight, and time of day, allowing individualized normal temperature ranges to be established. The clinical significance of a value outside of the usual range is an area for future study.

Figure 1.. Distribution of Temperature in Both the Analysis and the Outlier Sets

The LIMIT (Laboratory Information Mining for Individualized Thresholds) algorithm determined the analysis and outlier (excluded) groups. To convert temperature to degrees Fahrenheit, multiply by 1.8 and add 32.

Figure 2.. Temperature in Men and Women, by Time of Day and Demographic Characteristics

Time of day is measured using a 24-hour clock. Shading marks the 95% CI. Each graph uses unadjusted (raw) data plotted with smoothing. To convert temperature to degrees Fahrenheit, multiply by 1.8 and add 32.

Figure 3.. Temperature Throughout the Day, in Men and in Women, by Demographic Characteristics

Each graph uses output from mixed-effects modeling using age, sex, time of day (on a 24-hour clock), height, and weight. For these specific graphs, temperatures are for an average US person weighing 88.8 kg if male or 76.4 kg if female (A and C), aged 30 years (B and C), and with height 1.75 m if male or 1.62 m if female (A and B). To convert temperature to degrees Fahrenheit, multiply by 1.8 and add 32.