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.

The actual temperature of the body (the so-called central or core temperature) corresponds to the value measured in the 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 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 off: 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 measured value down.

Mercury thermometers have not been permitted on the EU market since 2009. We do not recommend mercury-containing thermometers from distant countries (China, etc.). 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 valid Hungarian 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


According to the Hungarian professional recommendation the place of measurement is recommended in the rectum below the age of 1 year, in the rectum or under the arm between the ages of 1-3 and in the underarm of children over 3 years. Other international recommendations differ in some respects.

Measurement taken in the mouth (under the tongue) gives the same value as in the underarm. 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.

A standard thermometer inserted too deep into the ear may damage the ear canal or the eardrum. 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 axillary measurement, the child's arm is clamped to the torso from 6 months of age onwards is reliable. But during the measurement, we need to be there and fix the thermometer.

Rectal measurementis most accurate. Calm mood and gestures are important. This should be done with a lubricated thermometer. Support your hand with your little finger and hold the thermometer as long you are taking the measurement. Insert the thermometer 1.5 cm deep.

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). See the section on the meaning of fever. During intense exercise (wood cutting, vigorous exercise/sport, hot yoga, prolonged crying in children ...) or heat gut/sunstroke, the body heat can rise up to 39°C. This is more like heat congestion, because the body is only capable of releasing heat at a slower rate than it is being produced.


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.

It is important to know the child's natural fever in the long term and to estimate his or her temperature. This can be done by pre-measuring with our palms by placing them on the child's forehead.

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.

If fever is high in the morning and in the morning hours, it may be a sign of a more serious illness. Therefore, 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.

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


Refer to the literature by numbers in this document here: References

Version update: 08th March 2020