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. Probably not, says Dr. Daniel Niven, an intensive care physician with the Peter Lougheed Center in Calgary, who headed a review of the effectiveness of different types of thermometers. “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). Niven and colleagues analyzed data gathered from 75 studies from around the world, involving more than 8,600 patients, both adults and children. 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.  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. They found peripheral thermometers could be off as much as 1.5 degrees. The more invasive measures were the gold standard, Niven says, enabling doctors to make diagnoses in patients without obvious symptoms. This is important because if a temperature measures as normal when it's not, then a serious infection could go undetected. 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.
– In what STAT describes as an "exhaustive" 24-page report, there's one type of thermometer that rises above the rest when it comes to gauging body temperature. Rectal thermometers remain the "gold standard" for getting the most accurate reading, HealthDay News reports, even though "it's no one's favorite method." For the study appearing in the journal Annals of Internal Medicine, researchers examined 75 published studies, comprised of more than 8,600 patients total, that compared "central" thermometers (i.e., either rectal ones or those that can get temps from a vein) with "peripheral" ones—the kind we stick under our tongues, in our armpits and ears, and on our foreheads, STAT notes. The results: Rectal thermometers were "highly accurate," while peripherals didn't perform as well, especially in detection of low-grade fevers and accuracy in measuring high-grade ones (the study found ear and armpit thermometers could be up to two degrees off in either direction). For most people with a minor cold, that isn't a big deal, lead author Dr. Daniel Niven tells STAT. But when you're dealing with certain groups—including older patients, those with immunity issues, and patients with tumors—it's critical to use the most accurate thermometers that can pick up even the slightest fever, the study notes. The least-accurate instruments for temp-taking? Armpit thermometers, Niven says. The study didn't include infants or toddlers, so the recommendation to use rectal thermometers can't be extrapolated to them based on this study, NPR notes, though it adds that the American Academy of Pediatrics gives a thumbs-up to rectal thermometers as the most accurate for kids under the age of 3. (Toucans don't need thermometers—they've got their own beaks to keep things in balance.)