The most reliable core temperature is obtained by inserting a thermometer into the rectum (rectal measurement). This measurement is accurate and has low possibility for errors in the results. The normal range is approximately: 36.2°C – 37.7°C.
This is also a very reliable method of measuring core temperature. In this case a thermometer capable of detecting the infrared heat given off by the ear drum (tympanic membrane) is used. This measurement is accurate and has low possibility for errors in the results. The normal range is approximately: 35.5°C - 37.5°C.
The oral measurement can be performed as ‘in the cheek’ measurement (called buccal) or as a ‘under the tongue’ measurement (called sublingual). Both measurements underestimate the rectal temperature by approximately 0.3° C – 0.8° C, with the ‘under the tongue’ measurement being preferable to ‘in the cheek’.
Body surface temperature measurements used in clinical practice are in the arm pit (axillary measurement) and in the groin. In both cases the respective limb is pressed against the body in order to reduce any ambient temperature influence. However, this is successful only to a limited extent with the disadvantage that the measurement time is long. In adults, the axillary measurement is lower than the rectal by as much as 0.5°C – 1.5°C! In infants, these differences, in comparison with the rectal temperature, are much smaller.