|1800s Wunderlich Thermometer (the thermometer is over 12 inches long)|
Dr. Wunderlich took the temperatures of over 25,000 patients under his care and determined the temperature of a normal human being was 98.6°F or 37°C.
This value has been parroted back ever since...
But two problems... He used an archaic, one foot long, thick thermometer that registered a temperature 2°C higher compared to modern thermometers. See picture above. You can see one at the Mutter Museum in Philadelphia as well.
Given that, what IS the normal temperature of a human being using the best thermometer modern medicine has to offer?
In 1992, nearly 120 years after Dr. Wunderlich's declaration of 98.6°F, researchers at University of Maryland determined that the TRUE temperature was... drum roll please...
What they also discovered was that the "normal" temperature varied in a given healthy individual by as much as 1 degree F throughout the day being lowest around 6AM and highest around 5PM. That said, the upper limit of normal remained consistently at 99.9°F or 37.7°C (Dr. Wunderlich stated 100.4°F or 38°C was the upper limit). Lower limit of normal was 96.0°F. Females also had a slightly higher normal temperature compared to males and there was a trend toward higher temperatures among black compared to white individuals.
In spite of these "more true" temperature values, 98.6°F has both the weight of history and momentum going for it resulting in perhaps an alternative fact that will stick with us for years to come.
Physicians (including myself) will continue to disregard any temperature below 100.5°F as a fever and will continue to consider 101.5°F to be the cut-off that would be considered clinically significant. Why? Because much of medical knowledge and protocols that have been developed over decades are based on the 98.6°F value and changing the defined temperature (even if it is more correct) would result in changes to numerous and varied protocols that have been already established.
This may certainly be one truth that coexists with an alternative truth, but at least these truths are better understood for what they are.
A Critical Appraisal of 98.6°F, the Upper Limit of the Normal Body Temperature, and Other Legacies of Carl Reinhold August Wunderlich. JAMA. 1992;268(12):1578-1580. doi:10.1001/jama.1992.03490120092034
On the Temperature in Diseases: A Manual of Medical Thermometry. Published in 1871. The New Sydenham Society. London.