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When is your child's fever really a fever?

We tested baby thermometers and found that taking your child's temperature isn't as simple as you might think. 

person using ear thermometer to test babies temp
Last updated: 31 July 2019

Need to know

  • Temperature readings are affected by a number of different factors, including age, time of day, and which body part you're measuring
  • It's hard to tell which particular body temperature a thermometer is showing
  • CHOICE experts share dos and don'ts for taking an accurate temperature reading

Taking a child's temperature is pretty straightforward, right? You put the thermometer in their mouth, or ear, or on their forehead, keep it there for as long as needed, then read what it says. Simple!

Unfortunately our testing has found that finding out your child's temperature is more tricky than you'd expect, due to a number of factors. 

Some thermometers convert the reading to what might be either a core temperature or an oral equivalent, adding to the confusion

CHOICE baby and kids expert Kim Gilmour

"We found that it can be confusing to know what temperature constitutes an actual fever. It does depend on things like age, what part of the body you're measuring, and the environment," says CHOICE's baby and kids expert Kim Gilmour.

"Also, some thermometers convert the reading to what might be either a core temperature or an oral equivalent, adding to the confusion, and they don't always make this clear in the instructions."

Where are you taking the temperature from?

If you use a digital probe thermometer to take an oral temperature, you'll get a different reading than if you take an axillary (armpit) temperature. That's because different parts of the body are slightly different temperatures.

Let's take oral temperature as a baseline. 

Here are the differences in temperature you're likely to find if you take a temperature from a different part of the body:

  • Ear: 0.3°C to 0.6°C higher than oral
  • Armpit: 0.3°C to 0.6°C lower than oral
  • Forehead: 0.3°C to 0.6°C lower than oral

Calculating oral equivalent temperature

To figure out the equivalent oral temperature when you're taking a forehead reading, you'll need to add 0.3°C to 0.6°C. 

To calculate the oral temperature from an ear reading, you'll need to subtract 0.3°C to 0.6°C.

Is it a fever or not?

Imagine that you've taken your child's temperature using an ear thermometer, and it comes back with a reading of 38.2°C – they have a fever, right? Well, maybe not. The equivalent oral temperature could be anywhere from 37.6°C to 37.9°C – warm, but maybe not a fever. 

BUT is the temperature readout the actual ear temperature, or the oral equivalent temperature? If the thermometer is displaying the oral equivalent temperature, then your child does have a fever. 

Confusing, right?

Which temperature is which?

When we tested baby thermometers, it was sometimes difficult to find out what kind of temperature reading the thermometer was giving: was it giving the core temperature, the oral temperature, the oral equivalent temperature, or the temporal artery temperature?

Even the instruction manuals often weren't clear, and in some instances we had to contact the manufacturers directly to clarify which temperature their thermometers display. 

"Parents buying a thermometer off the shelf expect to know what type of temperature the thermometer is displaying, but many of the manuals don't make it clear," says Kim. 

Accuracy

Knowing whether your child has a fever can be even trickier if the thermometer you're using is inaccurate. 

We found that good digital probe thermometers can meet their claimed accuracy to within 0.1°C. 

Ear and forehead thermometers are less accurate, but they don't generally claim to be as accurate as digital probe thermometers. Our testing has found that they're generally accurate to within 0.2°C. 

How to take your child's temperature more accurately 

Dos:

  • Find out what your child's 'baseline' temperature is when they're healthy. 
  • Always take your child's temperature in the same way and in the same location: oral, rectal, armpit, ear, forehead, etc.
  • Read the thermometer instructions so you know how to use it and what kind of body temperature the thermometer is displaying – is it the tympanic (ear) temperature, or the oral equivalent?
  • Make sure your child stays still while you're taking their temperature. (Easier said than done, we know!)
  • Make sure the ear thermometer is placed correctly in the ear, otherwise you may get an inaccurate reading. A build-up of earwax can also interfere with accuracy.
  • Any baby under three months of age who has a fever should be taken straight to the nearest emergency department. 

Don'ts:

  • Don't take your child's temperature straight after they've bathed or showered, or after they've been active. It can raise their core temperature, so you may not get an accurate reading. If they're sweating, it can also affect the reading of a forehead thermometer. 
  • Don't take your child's temperature straight after they've had food or drinks, if you're using an oral thermometer. Hot or cold foods could influence the reading. Wait 15 to 30 minutes before taking their temperature.
  • Don't use ear thermometers on infants under six months – they're generally not recommended due to the size of babies' ear canals.
  • Don't use forehead thermometers on infants under three months; they're generally not considered accurate for babies this young.
  • Don't bother using strip-type thermometers. Our testing has found that they're very easy to use, but not very accurate. 
  • Don't use mercury or alcohol thermometers. If they break, they can cause injury or poisoning. 

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