Parents often ask about the best way to check for fever. Rectal temperatures are the most reliable and accurate. This method is most reliable for infants under 3 months and toddlers when an accurate reading is crucial.
Oral temperature are okay for older children who can keep a termomenter under their tongue with their mouth closed for 30 seconds. Axillary temps are the least accurate.
Mercury thermometer (though harder to find in stores) are more accurate than the non-mercury ones. The electronic ear thermometer can be fairly accurate if used correctly and there is no ear wax. Temporal thermometers, which use infrared technology, are less prone to inaccuracies than the ear thermometers.
It should be remembered that readings differ by route. Rectal, temporal, and ear readings are all similar but oral is 0.5 to 1 degree lower than rectal and armpit is 1 to 1.5 degrees lower than rectal.