What to Know About Using a Rectal Thermometer

Checking body temperature is one of the first steps to take if you or someone you are taking care of is ill. There are different kinds of thermometers that measure body temperature on the skin, in the mouth, or in the ear. However, research shows that a rectal thermometer may provide the most accurate temperature reading, especially for infants.

What Is a Rectal Thermometer?

A rectal thermometer is a thermometer that is inserted in the rectum to check the body's internal temperature. It has a similar bulb-like shape as an oral thermometer but usually has a shorter and stubbier tip for easier insertion into the rectum.

Pediatricians recommend them for children under 3 months because they are generally more reliable and accurate than other thermometers. For example, temperatures from oral thermometers can easily be influenced by drinking cold or hot drinks, while the environment's temperature can influence skin thermometer readings.

Since rectal thermometers check the internal body temperature, they are less likely to be affected by such factors, which leads to a more accurate reading.

Also, other types of thermometers may be uncomfortable for an infant and hard to use. Temperatures that are taken under the arm (axial temperatures) take several minutes, which can be difficult to do on an infant. In addition, infants and even some older adults cannot hold an oral thermometer under the tongue long enough for a temperature reading.

Although not as accurate as rectal thermometers, temporal artery (forehead) thermometers are useful options for infants who won't stay still when checking their rectal temperature.

Steps to Take a Rectal Temperature

To take a rectal temperature, you'll need a digital thermometer and a lubricant.

Follow these steps:

  1. Put petroleum jelly or a water-soluble lubricant (such as KY-Jelly or Surgilube) on the end of the thermometer.
  2. Lay the child on their stomach and spread the buttocks apart, or place them on their back with their knees pulled up.
  3. Insert the bulb end of the thermometer into the anal canal no more than 1 inch.
  4. Keep the thermometer in place until it beeps, or for at least one minute.
  5. Remove the thermometer and read the result.
  6. Disinfect the thermometer with rubbing alcohol or an alcohol-based wipe.
When to Call a Pediatrician for a Fever
Verywell / Kelly Miller

Reading a Rectal Thermometer 

You may be wondering what's considered a normal or high temperature reading, especially if your child is an infant. The average normal body temperature is 98.8 degrees F, but that number can vary depending on how a body temperature is taken.

For instance, rectal and ear temperatures are usually 0.5 F higher than an oral temperature, while armpit and forehead temperatures are usually 0.5 F lower than an oral temperature.

There is no universal standard when it comes to normal body temperature in different age groups. Generally, a fever is considered to be 100.4 F or higher while a body temperature that is too low (hypothermia) is 95 F or lower.

Still, specific age groups may need prompt medical attention for certain temperature readings.

Babies Under 3 Months

If your baby is under 3 months old with a fever that's 100.4 F or higher, call your doctor or go to the emergency room. If the baby also has difficulty breathing, is crying uncontrollably, has a stiff neck, or has a seizure, go straight to the emergency room or call 911. There may be an underlying problem like meningitis that needs immediate medical attention.

If your baby has a rectal temperature reading of 97.7 F and lower, you should call your doctor to see if a visit to the hospital is necessary.

You should also call your child's doctor if:

  • The fever goes away but comes back.
  • Your baby does not act more alert or comfortable after their fever goes down.
  • They have fevers that come and go for up to a week or more.
  • Their fever lasts longer than 48 hours.

Children Older Than 3 Months (and Adults)

Babies and children older than 3 months with a rectal thermometer reading of up to 102.2 F should be monitored and can be treated with over-the-counter fever medications like acetaminophen (Tylenol). Just be sure to check and follow the product's dosage instructions.

Babies that are 3 to 12 months old with a rectal temperature reading of 95 F or lower or a fever that's 102.2 F or higher should be assessed by a doctor.

If a baby or child has a fever of 104 F and above, or a fever of 100.4 F or higher with symptoms like difficulty breathing, stiffness of the neck, loss of consciousness, and seizures, take them to an emergency room right away.

If you're an adult and have a temperature reading lower than 95 F or a fever that's over 104 F, you should see your doctor. You should seek medical help right away if you have a fever that's over 104 F with symptoms such as:

  • Loss of consciousness
  • Stiff neck
  • Trouble breathing
  • Swelling or inflammation of any body part
  • Confusion
  • Seizure

A Word From Verywell  

A rectal thermometer can be an accurate way of checking your own body temperature, but it is the standard when it comes to checking the temperature of your infant. As many parenting books don't thoroughly cover rectal readings, the idea of checking your child's temperature rectally may make you nervous or afraid to do so.

But no need to worry. By following the right steps, you can safely check your child's temperature and get a correct reading. If you need more advice on checking your child's temperature rectally, speak with your pediatrician, as they can give you more guidance on how to do it effectively.

Frequently Asked Questions

  • Will a rectal thermometer help my baby poop?

    It might. If your baby is constipated, inserting a rectal thermometer just as you would if you were taking their temperature may stimulate a bowel movement. However, It's important to speak with your pediatrician to address your baby's constipation and discuss if using a rectal thermometer is appropriate.

  • When can I stop using a rectal thermometer on my child?

    Although rectal thermometers can be used from infancy to adulthood, many older children may not like to check their temperature rectally. You can stop using a rectal thermometer on your child after three months, but make sure your child is cooperative enough and can tolerate other thermometers.

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Article Sources
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