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Is 98.6F (37C) Wrong?

November 1, 2023

Written by Carmen Wolfe

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Normal oral temperature is hard to define, but this large cross-sectional study suggests that the average is not 37oC (98.6oF).

“But 99 is a fever for me.”
37oC is a normal temperature, right? Maybe not.

By evaluating all outpatient visits over a six-year period in a single healthcare system, authors sought to determine individualized temperature ranges based on age, sex, height, weight, and time of day. This cross-sectional study examined more than 600,000 encounters, and an unsupervised machine learning algorithm was used to filter out diagnoses that could be related to temperature (infectious disease accounted for 76% of these).

The mean overall oral temperature was 36.64oC (97.95oF), with a standard deviation of 0.35oC. Even with subgroup analysis based on the characteristics above, the mean temperature did not reach 37oC for any group. The main factors affecting temperature were age, sex, weight, height, and time of day.  Average temperatures are higher in women than in men, and decrease with increasing age. Average temperature is inversely related to height, and directly related to weight. Average temperature is lowest in the early morning and peaks in the late afternoon.

The study is limited by lack of information on race, ethnicity, ovulatory cycle status in women, and is limited to a single geographic distribution in the United States. Be cautious in your utilization of this information, as this study aims only to define normal oral temperature ranges and doesn’t address what defines a fever.

How will this change my practice?
While 37oC is a round number, the average oral temperature is actually a bit lower at 36.64oC. I’ll adjust my expectations of normal and remember that multiple patient variables could affect this vital sign.

Defining Usual Oral Temperature Ranges in Outpatients Using an Unsupervised Learning Algorithm. JAMA Intern Med. 2023 Oct 1;183(10):1128-1135. doi: 10.1001/jamainternmed.2023.4291.

What are your thoughts?