Body temperature may not really gauge covid-19

TBILISI, GEORGIA - MARCH 28: A fan has their temperature checked before entering the stadium prior to the FIFA World Cup 2022 Qatar qualifying match between Georgia and Spain at the Boris Paichadze Dinamo Arena on March 28, 2021 in Tbilisi, Georgia. 30% of the stadium capacity have been allowed into the match as a Covid-19 precaution. (Photo by Levan Verdzeuli/Getty Images) (Levan Verdzeuli via Getty Images)
·5 min read

I went to get a coronavirus test after Thanksgiving, and the nurse took my temperature - 97.7 degrees Fahrenheit. This is not unusual for me, even though it was lower than what we think of as normal.

Normal body temperature is one health-related number that most everybody knows - 98.6 degrees. It's even easier in Celsius - a flat 37 degrees.

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Despite the exactitude of the widely accepted number, down to one-tenth of a degree, body temperature is not that fixed.

A recent study compiled data from 150,280 adult outpatient visits to Stanford Health Care facilities over a 10-year period. The average temperature was 98.0 degrees for men and 98.2 degrees for women.

Another recent study, of 96 adults, found an average temperature was 97.0 degrees. And a 2017 study, of 35,488 adults, came up with an average of 97.9.

These results lead to two key observations: Temperature is pretty variable. And if anything, average human body temperature is typically less than the long-accepted 98.6.

"The number comes from a mid-19th-century study," says Julie Parsonnet, an infectious-disease physician at Stanford University School of Medicine. In that study, a German physician, Carl Wunderlich, collected a million temperature readings from many thousands of patients and published this average: 98.6 degrees.

"Wunderlich was a giant in the field," says Philip Mackowiak, professor emeritus at the University of Maryland School of Medicine and an expert on fever and body temperature. He points out that while 98.6 was the "average," Wunderlich never called it "normal."

Indeed, most doctors know that a person's body temperature varies across the day and over the life span. "Your temperature is likely to be lowest in the morning," Parsonnet says. "Women tend to be warmer than men, and temperature tends to drop with age."

Body temperature can fluctuate in very cold or warm conditions and with activity such as exercise, Mackowiak says. In addition, he says the very term body temperature is a misnomer, as it depends on what part of the body one measures: mouth, ear, armpit or rectum.

Still 98.6 has persisted.

"People want simple answers even when the question is complex," Mackowiak says.

The variability in real-life temperature readings is one reason 99 degrees is not generally considered a fever. Most organizations use 100 degrees (or higher) as a threshold temperature at which students shouldn't go to school and employees shouldn't go to work.

Parsonnet knew that modern studies consistently found average body temperatures that were lower than 98.6 and she wondered what accounted for the difference. Better thermometers? That today's doctors tend to use oral readings rather than the under-the-arm method that Wunderlich used in 1851? Or have we humans changed in some fundamental way?

Parsonnet and her team studied three data sets: one from Civil War veterans up to 1930, another from a U.S. survey in the early 1970s, and her own data from Stanford clinics starting in 2007.

She documented a steady decline in average body temperature over the decades.

People are certainly healthier today than those in the past century; we live longer, we grow taller, and we have more tools to prevent and treat and infectious-disease.

"We don't have a lot of infections that used to be widespread, such as tuberculosis, syphilis and childhood diseases," Parsonnet says.

Body heat is a byproduct of metabolism, and there's evidence that, on average, metabolic rates have declined in the past 100 years. Parsonnet theorizes that improved living conditions and medicine, along with better sanitation and dental hygiene, has led to a decrease in overall inflammation, which explains our lower metabolic rates and our lower body temperatures.

I have seen my primary care doctor seven times in the past three years, and my average temperature from those visits is 97.4 degrees. Adele Diamond, a cognitive neuroscientist at the University of British Columbia, says her normal temperature is 97.2. She was curious about that, as well as why the women in her lab preferred a warmer room than the men, so she embarked on a study of individual differences in body temperature.

Participants took their temperature twice a day for two weeks; 77 percent of people reported their personalized normal body temperature was at least 1 degree below the standard of 98.6.

Diamond is surprised that as medicine becomes increasingly personalized, doctors still hold on to a fixed - and outdated - number.

And she wonders, as I do, whether having a low body temperature means her threshold for fever should also be lower. If she has a temperature of 99 degrees, which is 1.8 degrees higher than her normal, she asks, "Does that mean I have a fever?"

Her doctor is on board with that thinking, she said.

Parsonnet and Mackowiak both say that fever is simply a temperature above the normal range. They also point out that fever is only one sign of illness.

"My general dogma is if your temperature is normal and you feel sick, you're sick," Parsonnet says.

In a medical setting, providers are probably going to look for other signs of illness, such as a throat culture, a urinalysis or a blood count.

These days, I get my temperature taken all the time, for a very rough covid check - at the dentist, the physical therapist, the state vaccination site.

The forehead readers are not that accurate, however. "If you were wearing a hat, or just ran up the stairs, the forehead could give a high reading," Parsonnet says. Also many people with covid-19 don't develop fevers.

Mackowiak detailed the limitations of forehead thermometers in a 2020 paper.

The day I tested positive for the coronavirus, my temperature was 98.6 degrees. Perfect, by an 1800s standard.

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