The measurement of body temperature can help detect illness. It can also monitor whether or not treatment is working. A high temperature is a fever.
How the Test is Performed
The American Academy of Pediatrics (AAP) recommends against using glass thermometers with mercury. The glass can break, and mercury is a poison.
Electronic thermometers are most often recommended. The temperature is displayed on an easy-to-read panel. The probe can be placed in the mouth, rectum, or armpit.
Mouth -- place the probe 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 mouth. 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 about 1/2 to 1 inch into the anal canal. Be careful not to insert it too far. Struggling can push the thermometer in further. Remove after 3 minutes or when the device beeps.
Armpit -- place the thermometer in the armpit, with the arm pressed against the body. Wait for 5 minutes before reading.
Plastic strip thermometers change color to show the temperature. This method is the least accurate.
Place the strip on the forehead and read it after 1 minute while the strip is in place.
Plastic strip thermometers for the mouth are also available.
Always clean the thermometer before and after using. You can use cool, soapy water or rubbing alcohol.
Electronic ear thermometers are common and easy to use. However, some users report that the results are less accurate than with probe thermometers.
How to Prepare for the Test
Wait at least 1 hour after heavy 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.
The average normal body temperature is 98.6°F (37°C). The normal temperature can vary due to:
Age (in children over 6 months, daily temperature can vary by 1 to 2 degrees)
Time of day (often highest in the evening)
Where on the body the temperature was taken
Body temperature can be raised by:
Being in a high temperature or high humidity
Feeling strong emotions
Menstruating (in women)
Taking certain medicines
Teething (in a young child -- but no higher than 100°F)
Wearing heavy clothing
What Abnormal Results Mean
If the reading on the thermometer is more than 1 to 1.5 degrees above your normal temperature, you have a fever. Fevers may be a sign of:
Certain types of arthritis, such as rheumatoid arthritis or lupus
Diseases in the intestines, such as Crohn's disease or ulcerative colitis
Infection (both serious and non-serious)
Many other medical problems
Body temperature that is too high or too low can be serious. Call your health care provider if this is the case.
Often, older people do not run a high temperature, even if they are sick.
Mackowiak PA. Temperature regulation and pathogenesis of fever. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2009:chap 50.
Nield LS, Kamat D. Fever. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 19th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 169.
Linda J. Vorvick, MD, Medical Director and Director of Didactic Curriculum, MEDEX Northwest Division of Physician Assistant Studies, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.