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Published on August 04, 2020

Is temperature a good marker for COVID-19?

COVID Temp Check

The next time you walk into a business, store or doctor’s office you might have your temperature checked to screen for COVID-19. But will temperature alone indicate whether you have the virus?

The answer is maybe ... or maybe not, said Christopher Butler MD a family medicine practitioner affiliated with Cape Cod Healthcare. It might depend on what’s “normal” temperature for you.

“It’s a very crude estimate to say that a normal temperature is 98.6 Fahrenheit,” he said. “It depends on the body area where you are checking the temperature. It depends on what device you’re taking the temperature with. And, in terms of interpreting it, it depends on the patient. Is this a young person? Middle aged? Is it an older person? There are a lot of variables involved.” 

Temperature is important to our health, although doctors aren’t entirely sure exactly what a fever signals to the body, Dr. Butler said. The part of the brain called the hypothalamus is “sort of a thermostat area,” he said. It responds to things that are in the body that aren’t supposed to be there, such as germs, and then generates a response. Fever, he said, may be a way to alert other body systems to fight whatever is going on, and kick the immune system into gear.

The U.S. Centers for Disease Control and Prevention (CDC) lists fever as one criterion for screening for COVID-19 and considers a person to have a fever if their temperature registers 100.4 or higher -- meaning it would be almost 2 degrees above what’s considered an average “normal” temperature of 98.6 degrees. That’s the guideline being used by many workplaces, along with asking employees about other symptoms, such as cough and shortness of breath.

But 98.6 was established as the average more than 150 years ago, by Carl Reinhold August Wunderlich, a German doctor, who, although he surveyed 25,000 patients, used foot-long thermometers far less accurate than modern digital tools.

Many Factors at Play

Recent findings published in the journal eLife by Stanford University researchers suggest that not only do most people run a bit lower, but average temperatures have been dropping in the last century, possibly due to improvements in overall health that decreased chronic inflammation. Other research says “normal” body temperatures can range from 97 to 99 degrees F.

And, as Dr. Butler noted, many factors -- age, hormones, time of day, activity level - can affect our body temperatures. “It’s true,” he said, “that older patients tend to have lower body temperatures than younger patients.”

That means a patient who has a baseline temperature of 97.5 could actually have a 2-degree temperature and register at lower than 100.4. And some COVID-19 patients might not have a fever at all. In one study, 44 percent of hospitalized patients did not have fevers when they were admitted, according to the CDC, although 89 percent eventually developed one. That makes temperature checks alone an “inefficient” screening tool, Dr. Butler said, and may offer a false sense of security.

“Is checking somebody’s temperature going to tell us they are infected? Is it going to tell us they aren’t infected? I guess the short answer is, in a few cases, yes. But there are going to be a lot of cases where people don’t have a fever, but they are infected, and people who have a fever, who end up having something else.”

But don’t give up yet, he said.

“We’re going to learn more about these tests [for COVID-19] and we’re probably going to come up with a better way of screening people as time goes on. But the state of the art right now is that I think temperature screenings are not the best way to screen. Unfortunately, there are not a lot of alternatives.”