How to mesure body temperature correctly?

Question

I own a Geratherm thermometer. And the instructions say to hold it under the armpit for 4 minutes.

When i follow the instructions, then my body temperature is 36,7 (not sick). I figured that holding the thermometer for 10 minutes should give even a more accurate reading. But then my body temperature is 37,0 (sick).

What's wrong? Is holding the thermometer for longer results in a more accurate or less reliable reading? I am confused.

Answer 1

Altogether, digital thermometers are known to be very inaccurate.². If you want the most accurate reading, a fever should be measured rectally, not axillary (in the armpit)¹.

So therefore, different limits apply to axillary temperature measurements:

In patients older than 1 month, the mean difference (SD) between the rectal and axillary temperatures was 1.04°C (0.45°C); thus, the axillary temperature was adjusted by adding 1°C, and no adjusted axillary temperature differed from the rectal temperature by more than 1°C.

^{Comparison of Rectal, Axillary, and Forehead Temperatures, Arch Pediatr Adolesc Med}

The basal body temperature varies from 36.5 up to 37.5 degrees celsius according to Elsevier, 2017³. Studies from 2008 allow values of <= 38.3 degrees.⁴.In your case, your first reading was 37.7 basal temperature (not really sick), and the second 38.0 (maybe slightly sick).

Both values aren't accurate for medical differential diagnosis (axillary measurements, and varying quite a lot), but body temperature really doesn't tell you a lot anyway. If you feel sick, consider yourself sick. I personally probably have above 37 degrees axillary in the morning after a cozy sleep, and unless I feel sick, I wouldn't consider myself sick if my temperature is 0.3 above guidelines and I lack other symptoms of any sickness.

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^{1: S. T. ZengeyaI. Blumenthal, Modern electronic and chemical thermometers used in the axilla are inaccurate, European Journal of Pediatrics}

^{2: An investigation into the accuracy of different types of thermometers, Nursing Times}

^{3: Luxem, Jürgen; Runggaldier, Klaus: Rettungsdienst RS/RH, Elsevier. 2017. German book for the paramedic education}

^{4: Laupland, B. Fever in the critically ill medical patient., 2008.}

See also:

^{Temperature Measurement for Patients with Fever, US Pharmacists, 2008.}