How Accurate Is Your Child's Underarm Temperature?

I have been raising children for 26 years and have always thought that when you take a child's temperature rectally you subtract one degree, if taken axillary, you add one degree, and if taken orally, it is as the thermometer reads. Recently, my one-year-old granddaughter was sick and my daughter was watching as I took her temperature under the arm. She brought me a magazine article that said you add 1.8 degrees to an underarm reading. Have I been doing it wrong all these years or is this something new?

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Robert Steele

Robert W. Steele, MD, is a board certified pediatrician at St. John's Regional Health Center in Springfield, MO. He graduated from medical... Read more

You have raised such a great question. The idea of how to take a temperature and interpret it has be become so confusing. There are mercury thermometers, digital thermometers, ear thermometers, ones that read differently depending upon where you stick the thermometer, and some that are supposed to work by attaching them to the forehead. Who would have thought simple child care could get so complicated?

The easy answer to your question is that in children the rectal temperature is the standard by which all other methods are judged. However, the background and significance of this a bit more complicated. The first medical studies looking at the relationship of fever to illness were done by rectal temperatures. Most studies since that time have also used this same method to document temperature. In other words, rectal temperatures have become the standard by nothing more than historic convention. This makes a reasonable amount of sense because rectal temperatures are much more easy to perform in a squirming child as compared to an oral or axillary temperature where the thermometer can easily be dislodged during the process.

Therefore, the true temperature in a child is one taken rectally. A well done oral temperature should be the same as a rectal one; however, as any parent knows, getting a good oral temperature in younger children is challenging at best. The axillary temperature is the least accurate, and while adding one degree to an axillary temperature is typically what is suggested, it is by no means an accurate conversion. Another way to look at it is an axillary temperature is a nice screen to confirm that a fever exists, but the true temperature can only be ascertained by taking the temperature rectally or orally.

But you know, all the hoopla over the addition and subtraction of degrees between axillary, oral, and rectal temperatures has caused most people (parents and doctors alike) to lose perspective on what significance fever has on an illness. Fever is a good thing when it is present and not causing the child to feel miserable. The body develops a fever as a defense mechanism against infection by viruses and bacteria. Therefore, when a child has a fever, he likely has an infection of some type. But fever should not be viewed with alarm. It helps fight this infection and alerts us for other symptoms. So, if the temperature taken axillary reads 101F, does it matter if it is truly 102F or 102.8F? Not really. In either case, the fever should be treated only if the child isn't feeling good because of it.

There are only a couple of exceptions about being accurate about temperatures. A rectal temperature should be taken if you suspect:

  • Your child who is under two months of age has a temperature greater than 100.5F
  • Your child between two months and three years has a temperature greater than 103.5F.

These two exceptions exist because they may possibly be associated with more serious infections, and it alerts you to notify your doctor.

It sounds to me like you have been doing an excellent job of interpreting temperatures for 26 years. So, whether you add 1 degree or 1.8 degrees to an underarm temperature, it still remains an inaccurate reading. The precision of the temperature reading is not necessarily as important as how the child is doing while he has the elevated temperature. But, if you are truly concerned about the exact temperature of your one year old grandchild, taking a rectal temperature is the only way to go.

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