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An exhaustive, 24-page article released Tuesday in the Annals of Internal Medicine shows that taking body temperature rectally is most accurate, short of actually sticking a needle in a vein.

That finding confirms that these types of thermometers are more accurate than so-called peripheral thermometers: those that are held under the tongue, laid across the forehead or against the temple, tucked into armpits, and gently placed in ear canals. But how important is it to get super-precise temperatures?

In the latest study, researchers reviewed 75 published studies comparing peripheral thermometers with central ones — mostly either temperatures taken from a vein or from the rectum.


They found that rectal thermometers were highly accurate. Peripheral thermometers were further off, with oral thermometers doing best among the runners-up.

The peripheral thermometers were especially inaccurate in patients with very high fevers. At those extremes, thermometers in armpits and ears could read “as much as one to two degrees higher or lower than actual body temperature,” the researchers wrote.


But does a degree or two one way or the other really matter? More importantly, does it matter enough to warrant sticking a thermometer where the sun don’t shine?

Lead researcher Dr. Daniel Niven of the University of Calgary said the one- to two-degree difference is not a problem for most people.

“If you have a fever from a cold or something like that, then you can use an oral thermometer, which is a bit more accurate than the (armpit or ear thermometers).

“But there are select circumstances where accuracy really matters,” he added.

The researchers determined that peripheral thermometers are especially bad at detecting low-grade fevers. Catching these more subtle rises in body temperature is important in early detection of infection or illness in particularly vulnerable patients — the elderly, the immunocompromised, or patients with tumors or with connective tissue disease.

The temperatures of such patients tend to be closely monitored already. But Niven said that the most accurate thermometers are not necessarily the most used.

“There’s not data to show what thermometers are being used where, but anecdotally, the primary method used in ICUs are axillary (armpit thermometers),” said Niven, an intensive care physician.

But for regular consumers, Niven has just this one piece of advice: Don’t use armpit thermometers. “They are the least accurate,” he said.

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