‘Let’s say someone normally runs an oral temperature of 96.6°. What causes this?‘.
Nothing abnormal if this person’s baseline temperature is lower than the consensus average of 36.8 +/- 0.4 oC (98.2 +/- 0.7 oF). As well, gender, age and time of reading greatly contribute to temperature variability.
In a 2002 meta-analysis of several oral, rectal, tympanic (ear-canal) and axillary (armpit) temperature studies (), normal oral temperature ranged from 35.7 to 37.7 oC (96.26 to 99.86 oF) for men, and from 33.2 to 38.1 oC (91.76 to 100.58 oF) for women (see figures below).
‘If that person has a temperature of 99°, is that then a fever?’ Is it similar to someone with an average oral temperature of 98.6° being at 101°?‘.
Could be, provided temperature stayed elevated for a while and was accompanied by changes in other vital signs, i.e., respiratory rate, pulse rate and blood pressure.
History of Average Body Temperature
For long, the notion prevailed that 37 oC (98.6 oF) was the average healthy body temperature. How did we arrive at this? Astonishingly enough for such a basic human health vital sign, mainly from one 19th century study. In his 1868 study,apparently analyzed >1 million axillary (armpit) temperature readings from ~25000 subjects (2). Doing so, he concluded that
- 37 oC (98.6 oF) was the mean temperature of healthy adults.
- It had a range from a low of 36.2 oC (97.2 oC) to 37.5 oC (99.5 oF).
- It had a circadian rhythm, being lowest between 2 and 8AM and highest between 4 and 9 PM.
- Women generally have slightly higher temperatures and greater variability compared to men.
- There might be racial differences in body temperatures.
- Temperatures in the old tend to be ~0.5 oC (~0.9 oF) lower than in the young.
Flaws of Wunderlich’s Average Body Temperature Measurements
Since Wunderlich’s time, thermometer precision has vastly improved while temperature readings have expanded from axillary to oral, rectal and tympanic (ear canal). Do these changes impact this vital sign average? They do and how!
Shockingly, a 1992 re-appraisal (3) states that very few studies tried to replicate Wunderlich’s data. It further states such studies had several flaws,
- Done >40 years prior, i.e., prior to 1992.
- Small numbers of subjects (4, 5, ).
- Took only single temperature readings from a large number of subjects (7, 8).
This 1992 study also highlights the many drawbacks of Wunderlich’s data, namely,
- Cumbersome thermometers.
- Needed to be read in situ, i.e., while still in contact with the body.
- When used for axillary (armpit) measurements, they took 15 to 20 minutes to equilibrate.
This 1992 study instead took repeat oral temperature readings at 1 to 4 prescribed times per day in 148 subjects, aged 18 through 40 years. Cohort had 122 men (88 black, 32 white, 1 Hispanic, 1 Oriental) and 26 women (17 black and 9 white) (see data in figure below).
- ‘Thirty-seven degrees centigrade (98.6 oF) should be abandoned as a concept relevant to clinical thermometry; 37.2 oC (98.9 oF) in the early morning and 37.7 oC (99.9 oF) overall should be regarded as the upper limit of the normal oral temperature range in healthy adults aged 40 years or younger, and several of Wunderlich’s other cherished dictums should be revised’.
- That 36.8 +/- 0.4 oC (98.2 +/- 0.7 oF) is the normal range of oral temperature.
- Most importantly and with special relevance to this question, that individual variability ‘limits the application of mean values derived from population studies to individual subjects‘, and ‘maximum oral temperature, like the mean temperature, exhibited by any population varies according to time of day‘.
A 1998 review (9) further asserts that Wunderlich’s thermometers may have been calibrated as much as 1.4 to 2.2 oC (0.72 to 2.16 oF) higher than modern-day thermometers. In other words, it’s normal for average body temperatures to trend lower than what conventional wisdom suggests.
1. Sund‐Levander, Märtha, Christina Forsberg, and Lis Karin Wahren. “Normal oral, rectal, tympanic and axillary body temperature in adult men and women: a systematic literature review.” Scandinavian journal of caring sciences 16.2 (2002): 122-128.
2. Wunderlich C. Das Verhalten der Eiaenwarme in Krankenheiten. Leipzig, Germany: Otto Wigard; 1868.
3. Mackowiak, Philip A., Steven S. Wasserman, and Myron M. Levine. “A critical appraisal of 98.6 F, the upper limit of the normal body temperature, and other legacies of Carl Reinhold August Wunderlich.” Jama 268.12 (1992): 1578-1580.
4. HORVATH, STEVEN M., H. Menduke, and GEORGE MORRIS PIERSOL. “Oral and rectal temperatures of man.” Journal of the American Medical Association 144.18 (1950): 1562-1565.
5. LINDER, FORREST E., and HUGH T. CARMICHAEL. “A biometric study of the relation between oral and rectal temperatures in normal and schizophrenic subjects.” Human Biology 7.1 (1935): 24-46.
6. Tanner, J. M. “The relationships between the frequency of the heart, oral temperature and rectal temperature in man at rest.” The Journal of physiology 115.4 (1951): 391-409.
7. Whiting, Madeline H. “On the association of temperature, pulse and respiration with physique and intelligence in criminals: a study in criminal anthropometry.” Biometrika 11.1-2 (1915): 1-37.
8. Ivy, Andrew C. “What is normal or normality?.” Quarterly Bulletin of the Northwestern University Medical School 18.1 (1944): 22.
9. Mackowiak, Philip A. “Concepts of fever.” Archives of Internal Medicine 158.17 (1998): 1870-1881.