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Fever, also known as pyrexia or a febrile response, is a frequent medical symptom that describes an increase in internal body temperature to levels that are above normal (the common oral measurement of normal human body temperature is 36.8°C or 98.6°F). Fever is most accurately characterized as a temporary elevation in the body’s thermoregulatory set-point, usually by about 1-2°C. Fever is not a disease but a symptom of disease.


When a patient has or is suspected of having a fever, that person´s body temperature is measured using a thermometer. At a first glance, fever is present if:

  • rectal temperature (in the anus) or otic temperature (in the ear) is at, or higher than 38 degrees Celsius (100.4 degrees Fahrenheit)
  • oral temperature (in the mouth) is at, or higher than 37.5 degrees Celsius (99.5 degrees Fahrenheit)
  • axillary temperature (in the armpit) is at, or higher than 37.2 degrees Celsius (99 degrees Fahrenheit)

There are different locations where temperature can be measured, and these differ in temperature variability. Tympanic membrane thermometers measure radiant heat energy from the tympanic membrane (=infrared). These may be very convenient, but may also show more variability.

Children develop higher temperatures with activities like playing, but this is not fever because their set-point is normal. Elderly patients may have a decreased ability to generate body heat during a fever, so even a low-grade fever can have serious underlying causes in geriatrics.

Temperature is ideally always measured the same moment of the day, in the same way, after the same amount of activity.


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