Fever may be an indicator of illness. The three best ways to measure body temperature are rectally, orally, or with a tympanic thermometer (ear). In infants under 3 months old, only take a rectal temperature. For older kids, oral temperatures are fine as long as the child can hold the thermometer under his tongue.

Tympanic thermometers work for kids older than 3 months who won't tolerate an oral temperature. Temperatures that are taken under the arm (axial temperatures) take several minutes using an oral thermometer and are not recommended unless there's no other choice. To take a rectal temperature, you'll need a digital thermometer and lubricant.

Never use a glass mercury thermometer. If you have one in your home, contact your garbage carrier for proper disposal advice.

Steps to Take a Rectal Temperature

Put petroleum jelly or water-soluble lubricant on the bulb end (thin, shiny end) of the thermometer. Water-based lubricants, such as KY or Surgilube, are better than petroleum jelly if you have them.
Lay the child face down and spread the buttocks apart.
Insert the bulb end of the thermometer into the anal canal no more than one inch. Just make sure the shiny metallic part is inside.
Keep the child from struggling as you don't want him to accidentally push the thermometer in too deep.
Keep the thermometer in place until the thermometer beeps or at least one minute.
Remove the thermometer and read the digital result.

Infants Under 3 Months

For infants under 3 months old, call the doctor or go to the emergency department if the temperature is higher than 100.4 F. Kids who are listless, weak, lethargic, unusually irritable, or have stiff necks or headaches warrant a call to the doctor no matter how high their temperatures are. If you can't wake your child, call 911.

Older Than 3 Months (Even for Adults)

Kids with fevers of less than 102 F should get rest and all the clear fluids they care to drink. Fevers over 102 F can be treated with Tylenol or Motrin. Never give kids aspirin. Follow the label or call your doctor to get the proper dosage for your child's age.

Call the doctor for fevers over 102 F that last more than a day in kids under 2 years old and more than three days with kids over 2 years old. Rather than just telling the doctor your child has a temperature, give the actual numbers and the way you took the temperature (oral, rectal, in the ear or under the arm). Never add or subtract numbers from the temperature regardless of which way you take it.

Adults with fevers of more than 102 F can take Tylenol, Motrin, or aspirin if they're uncomfortable, but it's not necessary. Adults with any fever should call the doctor for headaches or stiff necks, or if the fever lasts more than three days.