A fever in an infant can be the first sign of an illness. While a rise in body temperature above 100.4 degrees Fahrenheit is part of a healthy immune system response, it does signal potential danger and need for further evaluation. Since a reading may lead to a call or visit to the child’s doctor or emergency room, accuracy is key. What is the best type of infant thermometer?

A digital rectal thermometer.

This is according to such authorities as the American Academy of Pediatrics, Consumer Reports, and the American Academy of Family Physicians.

The definition of a fever is important as well. According to the AAFP:

A normal temperature is about 98.6°F (37°C) when taken orally (in your child’s mouth) and 99.6°F (37.5°C) when taken rectally (in your child’s bottom). Many doctors define a fever as an oral temperature above 99.5°F (37.5°C) or a rectal temperature above 100.4°F (38°C).

Fever in infants less than 4 weeks old can be an emergency, and most likely will require the baby to be hospitalized for evaluation and treatment. Fevers in babies 4 weeks to 3 months old are also considered an emergency requiring prompt medical attention. Always call your doctor for advice regarding fever.

The old-fashioned mercury thermometer has been phased out due to potential toxicity if the thermometer breaks and leaks mercury.

Consumer Reports recommends purchasing a rectal thermometer that can also be used orally as the child gets older, perhaps above age 3.

Expensive thermometers are not necessarily any better, but brands such as Omron have passed muster. Consumer Reports also lists BD Digital, Bebe Sounds, Safety 1st, Summer Infant, The First Years, and Vicks.

Be patient. Thermometers advertising instant reads sound great, but a reading in 20-60 seconds is sufficient and perhaps more precise.

Given the greater precision of rectal thermometers, my office has begun an ambitious initiative to check the temperatures of all our adult patients using the rectal technique as well. So far the response has been mixed. Some patients refuse the more painstaking measurements, while others seem to easily dismiss the added indignity as “just more of the same” they’ve come to expect from American healthcare. Just kidding.

But there’s no kidding about rectal thermometers being the best type for infants.

“Dr. Charles” is a family physician who blogs at The Examining Room of Dr. Charles.

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