Thermometer basics: Taking your child's temperature

If your child feels warm or seems under the weather, it's probably time to take his or her temperature. Sounds simple enough — but if you're new to it, you may have questions. Which type of thermometer is best? Are thermometer guidelines different for babies and older children? Here's what you need to know to take your child's temperature.

Know your thermometer options

A glass mercury thermometer was once a staple in most medicine cabinets. Today, digital thermometers are recommended instead of mercury thermometers, which can break and allow mercury to vaporize and be inhaled.

Regular digital thermometers, which use electronic heat sensors to record body temperature, can be used in the mouth, armpit or rectum. Digital ear thermometers, also called tympanic thermometers, use an infrared ray to measure the temperature inside the ear canal. Other options include a digital pacifier thermometer and temporal artery thermometer — which uses an infrared scanner to measure the temperature of the temporal artery in the forehead.

If you want to get a single thermometer for the entire family, a regular digital thermometer is probably best. However, if you plan to use the digital thermometer to take a rectal temperature, get two digital thermometers and label one for oral use and one for rectal use. Don't use the same thermometer in both places.

Accuracy varies

The most accurate way to take a child's temperature is to use a digital thermometer rectally or orally. Rectal temperatures provide the best readings for infants. Ear thermometers are another option for babies and older children. However, earwax or a small, curved ear canal can interfere with the accuracy of a temperature taken with an ear thermometer. Armpit temperatures and temperatures measured with a pacifier thermometer are considered the least accurate methods. The reliability of temporal artery thermometers hasn't yet been verified.

Whatever the method, make sure you carefully read the instructions that came with your thermometer. After each use, clean the tip of the thermometer with rubbing alcohol or soap and lukewarm water. For safety — and to make sure the thermometer stays in place — never leave your child unattended while you're taking his or her temperature.

Age matters, too

The best type of thermometer — or the best place to insert the thermometer, in some cases — depends on your child's age.

• Birth to 3 months. For newborns, use a regular digital thermometer to take a rectal temperature. Turn on the digital thermometer and lubricate the tip of the thermometer with petroleum jelly. Lay your baby on his or her back, lift your baby's thighs, and insert the lubricated thermometer 1/2 to 1 inch (1.3 to 2.5 centimeters) into your baby's rectum. Stop if you feel any resistance. Hold the thermometer in place for about a minute or until the thermometer signals that it's done. Remove the thermometer and read the number.
• 3 months to 4 years. For older infants and toddlers, you can use a digital ear thermometer, a digital pacifier thermometer or a temporal artery thermometer. Carefully follow the instructions that came with your thermometer. You can also use a regular digital thermometer to take a rectal temperature or an armpit temperature. To take an armpit temperature, first turn on the digital thermometer. When you place the thermometer under your child's armpit, make sure it touches skin — not clothing. Hold the thermometer tightly in place for about a minute or until the thermometer signals that it's done. Remove the thermometer and read the number.
• 4 years and older. By age 4, most kids can hold a digital thermometer under the tongue for the short time it takes to get a temperature reading. Turn on the digital thermometer. Place the tip of the thermometer under your child's tongue and ask your child to keep his or her lips closed. Remove the thermometer when it signals that it's done and read the number. If your child has been eating or drinking, wait at least 15 minutes to take his or her temperature by mouth. If your child is too congested to breathe through his or her nose, you may need to take an armpit or rectal temperature — or use a digital ear thermometer.

When to see a doctor

A fever is a common sign of illness, but that's not necessarily a bad thing. In fact, fevers seem to play a key role in fighting infections. If your child is older than age 1 and is drinking plenty of fluids, sleeping well and continuing to play, there's usually no reason to treat a fever.

If you want to give your child medication to treat a fever, stick to acetaminophen (Tylenol, others) until age 6 months. If your child is age 6 months or older, ibuprofen (Advil, Motrin, others) is OK, too. Read the label carefully for proper dosage. Don't use aspirin to treat a fever in anyone age 18 years or younger.

In general, contact your child's doctor if your child:

• Is younger than age 3 months and has a rectal temperature of 100.4 F (38 C) or higher
• Is older than age 3 months, has a temperature up to 102 F (38.9 C) and seems unusually irritable, lethargic or uncomfortable
• Is older than age 3 months and has a temperature of 102 F (38.9 C) or higher that doesn't respond to over-the-counter medication or lasts longer than one day

When reporting a temperature to your child's doctor, give the actual reading and state how the temperature was taken. Don't add or subtract numbers from the reading depending on where the thermometer was placed.

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