Using a probe thermometer

How to get the most accurate reading from a probe thermometer.

Not all fevers are the same

Knowing whether your child is running a fever can be important for diagnosing their illness. And finding out whether you're just a little unwell or in the throes of the flu can be helpful for an adult too. The trouble is, thermometers don't always seem to give the right reading, and what is too feverish for some may just be normal for others.

Thermometers are only accurate if the person using them has the right technique. So what do you need to know to use a probe thermometer correctly?

What is a fever?

Fever, an elevated temperature above the normal range, is a typical symptom of disease. It's often defined as an oral temperature over 37.5°C for mild cases, or over 38.2°C in more severe cases. Fever passes as the body fights off the disease, but babies with fever – especially those under three months of age – must be checked by a doctor in case the illness is serious. Always seek medical advice if you're concerned about your child's temperature or health.

Know your 'normal' temperature

Normal temperature ranges are slightly different for babies, toddlers, older kids, adults, and the elderly. They can vary a lot; the "normal" temperature of 37°C is only approximate and varies from person to person, and can also be dependent on time of day. Temperature also tends to be lower in the morning and higher in the evening – a variation of 0.5°C is typical.

The best method to determine an individual's normal temperature is to use the thermometer when the person is feeling well. Record readings twice a day (early morning and late afternoon) and take the average of the two temperatures.

How to take accurate temperature readings

In general, a person should be still when their temperature is taken. Avoid taking temperatures immediately after your child (or an adult) has exercised or taken a shower or bath, as these temporarily affect body temperature. How you take a reading will also depend on where you're taking the temperature.


The person whose temperature is being taken should not eat or drink for 15 minutes beforehand, as both can affect mouth temperature. Put the probe at the back of their mouth under the tongue and keep their mouth closed until the reading is complete.


Lay your child on their side or, for an infant, on their stomach with legs hanging, such as over your knees. Lubricate the thermometer tip with petroleum jelly and insert it gently about one centimetre into the rectum. Temperature readings are usually 0.5°C higher than for an oral reading.

Armpit (axillary)

Ensure the armpit is clean and dry. Place probe in the armpit with tip touching skin and position the arm next to the body to ensure room air doesn't affect the reading. You could gently hug your child to keep their arm in place. These readings are usually 0.5°C lower than for an oral reading.

Handling the thermometer after use

  • You should always sterilise a probe thermometer after use, especially when it has been used for rectal readings.
  • Check the instructions for specific advice on how to clean your thermometer.
  • Usually, you can wipe thermometers with a soft cloth and warm water/mild detergent. Do not immerse in water unless the instructions state otherwise. Sterilise the probe with alcohol but avoid alcohol contact with the battery/display end of the thermometer.
  • Use of a disposable sheath on the probe tip minimises spread of bacterial and viral infections.