Is Fever More Than Just a Matter of Degree?

Over the past several years, I have encountered many patients in my practice with a basal temperature of less than the average 37°C (98.6°F). While I consider them to simply be part of a bell-shaped curve, these patients are convinced that they are febrile at a temperature of 37.2°C (99°F). In the past, I have dismissed such comments. However, as I continue to see patients with basal temperatures of 36.1°C (97°F) or so, my curiosity about the possible veracity of these claims has grown, and I have been unable to find literature to offer insight. Is there any expert opinion regarding low basal temperatures and a different set point for true fever?
—MD

Normal oral temperatures are likely to be lower than 37°C (98.6°F), particularly if measured in the morning, and individuals may vary by almost 0.5°C (about 0.9°F). In a study by Mackowiak and colleagues,¹ oral temperatures were measured at different times of the day in 148 healthy men and women, aged 18 to 40 years. Temperatures ranged from a low of 35.6°C (96.0°F) to a high of 38.2°C (100.8°F), with a mean of 36.8°C ± 0.4° (98.2°F ± 0.7°). Temperatures varied over the course of the day by 0.5°C (0.9°F), with the lowest temperatures occurring at 6 am and higher temperatures between 4 and 6 pm. A temperature of 37.2°C (98.9°F) represented the 99th percentile for am temperatures and 37.7°C (99.9°F), the 99th percentile for pm temperatures.

If your patient normally has an am oral temperature of 36.1°C (97°F), then an am oral temperature of 37.2°C (99°F) would be both at the 99th percentile of the normal range and a significant increase for that person. Although the patient's temperature might not meet formal criteria for a true fever, it would represent a large enough rise in temperature to be treated clinically as a fever.

— Gary Kelsberg, MD
   Faculty Physician
  Valley Family Medicine
  Residency Renton, Wash