About fever measurement

The location and method of measuring body heat can lead to frequent and high inaccuracies and consequent error (131). Therefore, it is important to know where and what we are actually measuring.

Therefore, it is important to know where and what we are actually measuring.

The actual temperature of the body (the so-called central or core temperature) corresponds to the value measured in the mouth, ear or rectum.

Most digital thermometers available in Hungary are calibrated to accurately represent the temperature measured in the armpit (which is half a degree below the core temperature). When measured with these instruments under the tongue or rectum, the actual body temperature is half a degree higher than indicated. Digital thermometers have an accuracy of +/- 0.5-1 degrees (130). The price of thermometers is not necessarily in direct proportion to their accuracy.

The data in the ear measurement devices devices are for information purposes only, because the temperature sensor thread is not completely enveloped in the body, so the value obtained may differ by a few tenths of a degree (128). For other problems see below.

Infrared non-contact and skin contact (e.g. forehead) thermometers are for information only (36-39). Measured values may deviate from the actual core temperature unreliably; falsely lower or even higher. The skin type, sweating, and the broadness of the blood vessels in the skin strongly distort the measurement.

The most accurate are the classic glass thermometers that contain silver ("pseudo-mercury") gallium fibre or (red or blue) alcohol. They have an accuracy of +/- 0.2 degrees and are precisely calibrated. Some people have a hard time shaking them down: the bottom line is that the tools don't have to be shaken. Instead, they should be placed in a sock 1-2 minutes after measuring with the tank at the top and spun a few times, the centrifugal force taking the fibre down.

Mercury thermometers have not been permitted on the EU market since 2009. We do NOT recommend mercury-containing thermometers. When broken, mercury vapours can cause poisoning.


What is normal and what is elevated body temperature?

The measured values can be evaluated according to the professional recommendation as follows (123):

For values measured under the arm (axillary temperature) (without subtraction or addition):

36-37°C normal body temperature

37-38°C rise in temperature

38-39°C moderate fever

39-40.5°C high fever

Above 40.5°C very high fever


For rectal values, half a degree higher, i.e. (without subtraction or addition):

36.5-37.5°C normal body temperature

37.5-38.5°C rise in temperature

38.5-39.5°C moderate fever

39.5-41°C High fever

Above 41°C very high fever


Where and how to measure?

The recommended place of measurement is

in the rectum below the age of 1 year,

in the rectum or under the arm between the ages of 1-3 and

under the arm (armpit) of children over 3 years.

Some international recommendations differ in some respects.


Measurement taken in the mouth (under the tongue) we are measuring core temperature. This is half a degree higher than taken in the armpit. Do not expose the patient to cold or hot food or beverages 10 minutes prior to measurement. Place the end of the thermometer under the tongue on the inside of the dental lamina. It should only be used on children who are sure not to bite the thermometer.

If you are already using an ear thermometer, measure both ears and consider the higher one. However, larger amounts of earwax, sleeping on the ear, ear canal infection or otitis media may give false high values due to localized heat congestion.

For a reliable axillary measurement, in a child of over 6 month the arm is held to the chest with the thermometer tip kept resting in the armpit for 3 minutes. Adding half a degree to the measurement we obtain the core temperature.

Rectal measurement is most accurate. Calm mood and gestures are important. This should be done with a lubricated thermometer. Gently support the thermometer taking the measurement. Insert the thermometer 1.5 cm deep keeping in position for 3 minutes.

Body temperature can be influenced by certain factors, like normal daily fluctuations. In the morning the body temperature is 0.5-1°C lower than in the evening. Thus, in infants and the elderly, a temperature of 37.7°C in the morning is already considered as a rise in temperature, but not in the evening (20). The daily fluctuation of the body temperature is 0.5-1.0°C. It is lowest 1-4 a.m. and reaches its peak at 4-7 p.m.


How many times a day should we measure?

3 daily measurements are a good starting point.

Measure more often in young children and children in poor general condition.

If the child's limbs are still cold and pale, fever is probably still rising. If the child's limbs are warm and flushed, fever has probably reached the target temperature.

It is worth measuring at different times of the day and taking the daily fluctuation of optimally running fever into account. Body heat in the evening is usually higher and goes down by itself at dawn.

Do not disturb a child sleeping peacefully to take its temperature!


You can find the corresponding numbered references here: References

Version update: 1 March 2024