Temperature measurement

Definition

Taking a person’s temperature assesses whether it is within a normal range. A high temperature is a fever.

How the test is performed

In the past, the traditional method for taking a person’s temperature used a glass thermometer filled with mercury. However, the American Academy of Pediatrics (AAP) now advises against using mercury thermometers because they are toxic if the glass breaks.

Electronic thermometers are most commonly used and recommended. The temperature is displayed on a digital readout. Follow the directions that come with the particular model you use. Electronic probe thermometers can be placed in the mouth, rectum, or armpit. Electronic ear thermometers are common and convenient, but some users report that the results are less consistent than probe thermometers.

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.

To use a glass thermometer (if that is all you have available), first clean the thermometer with cool, soapy water or rubbing alcohol. Grip the end opposite the bulb and shake the thermometer down 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 using the lips to hold the thermometer tightly. The patient must breathe through the nose. Leave the thermometer in the mouth for 3 minutes.  
  • Rectum - for this method, use a rectal thermometer. This method is for infants and small children who are not able to hold a thermometer safely in their mouths. Lubricate the bulb of the thermometer with petroleum jelly. 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. Remove the thermometer after 3 minutes.  
  • 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.

Read the thermometer by 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.

How to prepare for the test
Wait at least 1 hour after vigorous exercise or a hot bath. Wait for 20 to 30 minutes after Smoking, eating, or drinking a hot or cold liquid.

How the test will feel
There is very little discomfort.

Why the test is performed
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.

Normal Values

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 six 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 six, daily variations gradually increase to 2 degrees per day .  
  • Body temperature varies less in adults. However, a woman’s menstrual cycle can elevate 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.

What abnormal results mean

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.

What the risks are

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

Johns Hopkins patient information

Last revised: December 3, 2012
by Gevorg A. Poghosian, Ph.D.

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