Understanding a Fever and How to Accurately Measure It

Understanding a Fever and How to Accurately Measure It

By Dr. Joaquin Azpilicueta, Exergen Corp.


With cold and flu season about to kick off and kids back at school and life trying to resume, amid the COVID-19 pandemic evolution, it is highly likely that at some point, you will be reaching for your family thermometer. More than ever, you want it to be accurate.

Temperature accuracy is a crucial and time critical measurement in identifying any signs of illness or deterioration. So, as soon as your child starts complaining of low energy or if they feel a bit warm, you should immediately start measuring their body temperature.

Checking your body’s temperature with a thermometer is an easy way to see if a fever is present. Fever is a condition that goes with an abnormal raise of body’s temperature and is usually caused by an infection. It is a sign that the body is fighting off that infection.

Body temperature varies over the course of the day and varies among individuals – with women tending to have higher body temperatures than men, and younger people tending to have higher temperatures than older ones, with temperature being lowest in the morning and highest in the evening.

So, what kind of thermometer should you use to get the most accurate result, how should it be used and how to interpret the readings to know what temperature constitutes a fever?

Thermometers: understanding the options

There are many different types of thermometers you can use to measure temperature. When you are taking the temperature of your child, the ideal thermometer should be non-invasive, easy to use and it should give an accurate, consistent and relevant reading. Let’s see what are the options and what are their pros and cons.

First of all, please be aware that in most parts of the world the old mercury-based thermometer is no longer in use and selling them is forbidden. They can easily break, and their fluid is toxic. Do not use them.

Digital Thermometres

This type of thermometer uses an electronic heat sensor to record the body temperature. They can be used in the rectum, mouth or armpit. Using the armpit to measure the body temperature is seldom a good idea because the measurements from there is not very accurate.

Rectal measurements are an accurate option BUT, they have negative side-effects, such as being invasive and threatening to children and even adults. They’re very uncomfortable as you have to undress the patient and position them to make it less discomforting. Last but not least, you need to rinse it well, as it is invasive and, therefore, could be contagious if you use it on another patient without the appropriate cleaning beforehand.

Digital ear or tympanic thermometers

In-ear thermometers are fast, accurate and closer to central temperature than armpit or groin measurements. The main drawbacks are that they can be painful if not done properly and for a consistent and patient-comfortable reading they require a good technique to be performed. They might not be usable if a child has a cold and are not suitable for new-borns. Also, variations of the outer ear channel and presence of ear wax can add challenges to obtaining an accurate reading.

Temporal artery thermometer

Temporal artery thermometers read the central temperature in the body by capturing the blood temperature at the temporal artery level.

Scanning the forehead over the temporal artery with a temporal artery thermometer is fast and accurate. This type of thermometer is also not invasive or threatening to children or adults because the measurement is done by swiping the thermometer gently across the forehead of the patient. The technique can be performed within 2 seconds without disturbing a resting patient.

The pros of temporal artery thermometers are:

  • fast
  • easy
  • accurate
  • cost-effective
  • comfortable and sleep non-disturbing for all, children, teens and adults
  • highly predictive of fever as measured by rectal thermometry (1)

What is a normal body temperature?

Normal central body temperature is 98.6 degrees Fahrenheit (°F) or 37 degrees Celsius (°C). A normal temperature is usually lower in the morning and increases during the day. It reaches its high in the late afternoon or evening.

What temperature is considered a fever?

As the thermometry depends on the method used, the general cut-off value for fever is:

  • measured orally (in the mouth): 100°F (37.8°C)
  • measured rectally (in the bottom): 100.4°F (38°C)*
  • measured in an axillary position (under the arm): 99°F (37.2°C)
  • measured with temporal artery thermometry (forehead) : 100.4°F (38°C)*

*Central temperature

When should you call a doctor when a person has a fever?

In general, if the central temperature is 102°F (38.8°C) or higher and if fever is present for over 2 days but, along with temperature, you should seek for signs that can help you take the decision. Here is some advice:


–       0 – 3 months – if the body temperature is 38°C or 100.4°F or higher

–       3 – 6 months – if the body temperature is 39°C or 102°F or the child is unusually irritable, lethargic or uncomfortable

–    6 – 24 months – if the body temperature is higher than 39°C or 102°F and the fever lasts longer than one day but the infant shows no other symptoms.


–    24 months – 5 years – if the body temperature is higher than 39°C.

As per the NICE guidance, by observing the activity of an infant you can assess the level of severity and seek for medical care.

This table below shows the risk to your child dependant on their behaviour and symptoms. Green represents a low risk, yellow a moderate risk (seek for help) and red a high risk (seek for help immediately).

*A child’s response to social interaction with a parent or healthcare professional, such as response to their name, smiling and/or giggling.

Table 1. Excerpt from the NICE: Traffic light system for identifying risk of serious feverish illness (2)


If the body temperature is 39°C or 102°F or higher, if it lasts more than 2 days or if the person shows warning signs like (3):

  •  A change in mental function, such as confusion
  • A headache, stiff neck, or both
  • Flat, small, purplish red spots on the skin (petechiae), which indicate bleeding under the skin
  • Low blood pressure
  • Rapid heart rate or rapid breathing
  • Shortness of breath (dyspnea)
  • A temperature that is higher than 104° F (40° C) or lower than 95° F (35° C)
  • Recent use of drugs that suppress the immune system (immunosuppressants)

Fever amid the COVID-19 pandemic.

Fever is one of the most common signs of the COVID-19 virus so understanding what constitutes a fever, and how to measure it will help your family stay safe. But it is also important to remember that a fever can result from many other infections including the flu, hepatitis, malaria, and norovirus as well as inflammatory diseases such as rheumatoid arthritis or Crohn’s disease. It can also be the result of teething in infants.

The main symptoms of coronavirus (COVID-19) are:

  • a high temperature – this means you feel hot to touch on your chest or back (you do not need to measure your temperature)
  • a new, continuous cough – this means coughing a lot for more than an hour, or 3 or more coughing episodes in 24 hours (if you usually have a cough, it may be worse than usual)
  • a loss or change to your sense of smell or taste – this means you’ve noticed you cannot smell or taste anything, or things smell or taste different to normal

We can’t always stop ourselves getting ill. If only we could! But by educating our children and being aware ourselves of the preventative measures within the government guidelines, we can all play our part in reducing the spread of COVID 19, we well as other illnesses.

By following these measures and knowing how critically important accurate temperature measurement is, you can keep your children, your family and yourself safe during flu season and beyond.

Mummy H

References and further reading:

  1. Comparison of temple temperatures with rectal temperatures in children under two years of age. / Siberry, George Kelly; Diener-West, Marie; Schappell, Elizabeth; Karron, Ruth A. In: Clinical pediatrics, Vol. 41, No. 6, 01.01.2002, p. 405-414.
  2. N.I.C.E“Fever in under 5s: assessment and initial management NICE guideline [NG143]
  3. The Merck Manual
  4. WHO, Q&A on coronaviruses (COVID-19) 
We have posted this post at no fee, as we would like to share this with our readers for their information and knowledge.

4 Thoughts to “Understanding a Fever and How to Accurately Measure It”

  1. Susan B

    Without a doubt the best post I have ever seen on measuring fever. Thank you!

  2. Maya

    Thank you for all this information. It’s useful reminding ourselves of what exactly a fever is.

  3. ashleigh allan

    Helpful post and particularly at this time with COVID. Thanks

  4. Margaret Gallagher

    Excellent! Can be so confusing ! Loving your explanation

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