What type of thermometer is most accurate for detecting fever?

What the evidence says

The most trustworthy way to check for fever depends a bit on your child's age, but digital thermometers used the right way beat other options. The Canadian Paediatric Society explains that a rectal reading, taken in the bottom, is the most reliable way to measure body temperature in babies and young children.

For children old enough to hold a thermometer under their tongue without biting it, usually around age 4 or 5, an oral reading is considered the most accurate method. Armpit readings are easier and less invasive but tend to run lower and less precise than rectal or oral readings.

Ear (tympanic) thermometers are quick, but Canadian guidance notes they can give readings that are too low, even when used correctly, so they are not as reliable as rectal thermometers. Forehead and non-contact infrared thermometers are convenient too, but their readings can be thrown off by sweat, room temperature, sunlight, or how they are aimed.

Here is where Canada and the United States differ a bit. Some American sources rank forehead temporal artery readings as the second most accurate method after rectal. Canadian sources are more cautious, with one children's hospital resource stating that non-contact and temporal artery thermometers are not recommended because of how easily outside factors can throw off the reading. Fever strips, pacifier thermometers, and just touching your child's forehead are not considered reliable by any of these sources.

Bottom line: keep a digital thermometer at home, learn the rectal method for babies and young toddlers if a doctor or nurse has shown you how, and switch to oral once your child is old enough. If a quicker method like ear or forehead reads normal but your child still seems unwell, it is worth double-checking with a rectal or oral reading.