Calor?
“My normal temperature is 97, so 98.6 is a fever for me”.
I hear a version of this phrase at least once a week. But is it true. What is normal body temperature?
98.6. A Boy Band? Average IQ of the Bush administration? The marking on a mercury thermometer? You remember mercury thermometers? No? Back in the day we had thermometers that used mercury and had a red mark at 98.6, and if your temperature went above it, you got to stay home from school. We loved to crack them open and play with the mercury, which may explain why I more of a hatter than not.
Lets go back in time.
In1868 Carl Reinhold August Wunderlich published Das Verhalten der Eigenwärme in Krankenheiten (The Course of Temperature in Diseases). Wunderlich gave 37°C (98.6°F) special significance with respect to normal body temperature based on a million observations in about 25,000 people. He did that calculation to get 98.6 without an electronic calculator. Only those in Victorian times were devoted enough to spend that kind of time on calculating the average of a million numbers. Wunderlich gave 38°C (100.4°F) as the upper limit of the normal range so a temperature greater than 100.4 was the first quantitative definition of a fever.
Tests conducted with one of Wunderlich’s thermometers demonstrated they were mis-calibrated by as much as 1.4 to 2.2°C (2.6 to 4.0°F) higher than today’s instruments. So 98.6 is based on data collected with a bad thermometer.
So what is normal temperature?
A 1992 study using modern thermometers measured 700 baseline oral temperature in 148 healthy men and women between age 18 and 40. They showed there was a range of temperature from 35.6°C (96.0°F) to 38.2°C (100.8°F),
an overall mean of 36.8 ± 0.4°C (98.2 ± 0.7°F),
a median of 36.8°C (98.2°F),
a mode of 36.7°C (98.0°F);
Mean, median, mode. Does that you take you back? I had to look them up to help my son with his homework. But its 98.2, not 98.6, that is the average temperature.
Temperature varies during the day: a nadir at 6 a.m. (lowest measuresd temperature 36.0 °C (96.8 °F)) and a zenith at 4 to 6 p.m. (highest measured temperature 37.7 °C (99.9 °F). So like a stopped clock being right twice a day, people are 98.6 twice a day: once on the way up and once on the way down.
The big exception is ovulating females (but not ovulating men) are warm in the am. People use this fact called the rhythm method, as a form of birth control. The medical term for these people is ‘parents’.
“Women had a slightly higher (women are hotter than men?) average oral temperature than men (men are cooler than women?) 36.9°C [98.4°F] versus 36.7°C [98.1°F]. Black subjects exhibited a slightly higher mean temperature and slightly lower average diurnal temperature oscillations than white subjects (36.8°C [98.2°F] versus 36.7°C [98.1°F] and 0.51°C [0.93°F] versus 0.61°C [1.09°F], respectively); these differences approached, but did not quite reach, statistical significance (t test, p = 0.06, df = 698). Oral temperature recordings of smokers did not differ significantly from those of nonsmokers”.
So what’s a fever? Depends on the time of day. “Fever is most appropriately defined as an early-morning temperature of 37.2°C (99.0°F) or greater OR a temperature of 37.8°C (100°F) or greater at any time during the day”. The other way to judge a fever is to have my wife hold her hand against your forehead. At least where the children are concerned, she says it is more reliable than the thermometer.
This is of practical importance as I seen a couple of patients a year with the diagnosis of fever off unknown origin who, it turns out, took their temp at 4 in the afternoon and by golly is was 99.9, as it was on each subsequent day. Normal temperature is 98.6. They have a fever. Patients can get thousands of dollars wasted on the work up of their nonexistent fever.
Normal temperature 98.6 is one of many medical misunderstandings.
Future posts may cover rubor, dolor and tumor.
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http://www.worldwidewords.org/qa/qa-mad2.htm
JAMA Vol. 268 No. 12, September 23, 1992. A critical appraisal of 98.6 degrees F, the upper limit of the normal body temperature, and other legacies of Carl Reinhold August Wunderlich
Interesting, thanks. I write as a non-biologist - worse, an engineer :-) - and my field is control and dynamic systems so I tend to see things in those terms. Please forgive the naivety of the following.
There are two main ways in which regulating (homeostatic) systems maintain a constant or near constant value for the variable which is regulated: either there is a ‘reference point’ and a feedback mechanism (like the dial on your central heating system) or the target value is in some sense an ‘optimal’ value, i.e. is the bottom of an energy well and the system self-restores to the equilibrium point. (In fact this can also be reframed as an intrinsic feedback system, but let that pass).
I have never been able to figure out why the body’s homeostatic control of temperature regulates to the value of 37C (or whatever value is the actual average). Does the body have a ‘bio-sensor’ (in the hypothalamus?) with a ’setting’ of 37? If 37C is some kind of dynamic equilibrium (heat loss = heat gain), why don’t people have more widely varying temperatures? How is the regulatory mechanism affected by, e.g. viruses to allow/maintain a higher temperature during fever?
Answers, links, suggestions for textbooks all welcome.
To clarify what happens with women of reproductive age:
During the first half of a woman’s menstrual cycle, her temperature will bounce around in a lower range. After ovulation, this average temperature will rise about 0.5 to 1 degree F and bounce around in a higher range.
For example, my basal temp averages 97.5–97.9 during the follicular phase (first half of the cycle). During the luteal phase (second half of the cycle), it averages 98.2–98.7. (So yeah, on average, lower than 98.6.)
Many women use basal body temperature charting in order to get pregnant (though, when used correctly, it can be effective as contraception). Although temperature alone doesn’t predict ovulation, the temperature shift helps to confirm that ovulation has occurred. A woman who doesn’t see this shift should talk to her doctor; she may need treatment if she wishes to conceive.
Continuing to monitor temperature after ovulation can also indicate whether the luteal phase is long enough to sustain a pregnancy. If the LP is shorter than 10 days (some doctors consider 10–11 borderline), the uterine lining may begin to break down before a fertilized egg can implant. Again, this can be treated.
Women who are charting will often note a drop in temperature 1–2 days before the onset of menstruation, back into the usual follicular phase range (though some women don’t see this drop until after menses have started). Eighteen days of sustained high temperatures after ovulation is, with rare exception, indicative of pregnancy.
See Fertility Friend and Taking Charge of Your Fertility by Toni Weschler, for more info.