What is a Rectal Thermometer?

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  • Written By: wiseGEEK Writer
  • Edited By: O. Wallace
  • Last Modified Date: 09 February 2019
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A rectal thermometer is one means of taking body temperature. It is usually thought to give an extremely accurate reading, but since it is inserted into the anus, not all people prefer this method of taking temperatures. Most often, it’s used on children, especially infants, and animals, because they generally can’t hold a thermometer in their mouths. Squeamish parents can take an axillary temperature (in the armpit) or use ear or forehead temperatures if they prefer to avoid taking a child's temperature rectally.

Up until recently, most rectal thermometers were made of glass and contained mercury. These are now considered unsafe for use, and old ones should be disposed of properly. People can find other newer and safer thermometers, including digital or plastic ones. Given where this thermometer rests when taking a temperature, most healthcare professionals recommend that it be reserved only for rectal use. People might consider buying a separate thermometer for oral and axillary use.

The basic means of taking the rectal temperature is to first thoroughly wash the thermometer in warm and soapy water. If the reading is not digital, the thermometer should be shaken down so the reading is well below 98°F (36.67°C). The thermometer should not be inserted into the rectum without receiving lubricant first. Some people recommend petroleum jelly and others suggest water-based ones.


The person having his temperature taken with a rectal thermometer is typically most comfortable lying on the stomach, though sometimes people can take the temperature with babies if they lift up the legs while the baby lies on his back. The thermometer should be inserted to about half its length; there may be a guide marking for insertion. Standard plastic or glass thermometers without a digital reading usually take two minutes for temperature reading to be accurate.

If people are used to oral temperature readings, an initial rectal read may suggest a person has a fever. This may not be the case, however, as rectal readings tend to be almost 1° higher than oral readings, and 2° higher than axillary ones. A normal rectal temperature is approximately 99.6°F (37.56°C). Higher readings begin to suggest presence of fever.

There is one more step to taking the temperature: users should thoroughly clean the thermometer and wipe it down with alcohol to make certain no germs from the rectum remain on it. Essentially, washing the thermometer before and after use is good sanitary practice.

As mentioned, there are alternatives to the rectal thermometer for those who prefer not to use them. Thermometers that fit in the ear have been popular for many years, and ones that are plastic strips that sit on the forehead and take a reading are also available. Some people prefer the classics, though, and rely on the accuracy of temperatures taken rectally.


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