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Temperature measurement is a method to take a person's temperature and determine whether it is within a normal range. A high temperature is a fever.

In the past, a glass thermometer filled with mercury was used to measure body temperature. However, the American Academy of Pediatrics (AAP) recommends against using mercury thermometers because the glass can break, and mercury is poisonous.

Electronic thermometers are most commonly used and recommended. The temperature is displayed on a digital readout. Follow the directions that come with the thermometer. Electronic probe thermometers can be placed in the mouth, rectum, or armpit.

Plastic strip thermometers change color to indicate the temperature. This method is the least accurate. Place the strip on the forehead and read it after 1 minute. Read it while the strip is in place. Plastic strip thermometers for the mouth are also available.

Always clean the thermometer before using. You can use cool, soapy water or rubbing alcohol. If you are using a glass thermometer, grip the end opposite the bulb and shake the thermometer downwards until it reads 95°F or less.

You can measure the temperature on three body locations:

• Mouth -- place the thermometer under the tongue and close the mouth. Breathe through the nose, and use the lips to hold the thermometer tightly in place. Leave the thermometer in the mouth for 3 minutes or until the device beeps.

• Rectum -- this method is for infants and small children who are not able to hold a thermometer safely in their mouths. Place petroleum jelly on the bulb of a rectal thermometer. Place the small child face down on a flat surface or lap. Spread the buttocks and insert the bulb end of the thermometer about 1/2 to 1 inch into the anal canal. Be careful not to insert the thermometer too far. Prevent the child from struggling, since this can accidentally push the thermometer in further. Remove the thermometer after 3 minutes or when the device beeps.

• Armpit -- place the thermometer in the armpit, with the arm pressed against the body for 5 minutes before reading. This is the least accurate method for using a glass thermometer.

Electronic ear thermometers are common and convenient, but some users report that the results are less consistent than probe thermometers.

Digital thermometers have easy-to-read displays. To read a glass thermometer, gripping the end opposite the bulb so that the numbers are facing you. Roll the thermometer back and forth between your fingers until you see a silver or red reflection in the column. Compare the end of the column with the degree marking in the lines on the thermometer.

Wait at least 1 hour after vigorous exercise or a hot bath before measuring body temperature. Wait for 20 to 30 minutes after smoking, eating, or drinking a hot or cold liquid.

There is very little discomfort.

The measurement of body temperature determines whether a person has a fever. It may be helpful in monitoring to see if a person is ill or whether a treatment is working -- especially in antibiotic treatment of infections.

The normal temperature varies by person, age, time of day, and where on the body the temperature was taken. The average normal body temperature is 98.6°F (37°C).

Your body temperature is usually highest in the evening. It can be raised by physical activity, strong emotion, eating, heavy clothing, medications, high room temperature, and high humidity.

Daily variations change as children get older:

• In children younger than 6 months of age, the daily variation is small.
• In children 6 months to 2 years old, the daily variation is about 1 degree.
• By age 6, daily variations gradually increase to 2 degrees per day.
• Body temperature varies less in adults. However, a woman's menstrual cycle can raise temperature by one degree or more.

For information on when to call a doctor due to specific temperatures and ages, see the article on fever.

If the reading on the thermometer is more than 1 to 1.5 degrees above the patient's normal temperature, the patient has a fever. Most fevers are a sign of infection and occur with other symptoms. Abnormally high or low temperatures can be serious, and you should consult a health care provider.

There is essentially no risk. There is a rare risk of bowel perforation if the rectal thermometer is not carefully inserted.

Mackowiak PA. Temperature regulation and the pathogenesis of fever. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 6th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2005:chap 47.

Powell KR. Fever. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 174.

Updated by: Linda Vorvick, MD, Family Physician, Seattle Site Coordinator, Lecturer, Pathophysiology, MEDEX Northwest Division of Physician Assistant Studies, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.