What rectal temperature indicates a fever?
Nov 14, 2025
Source: Cainiu Health
Dr. Li Jingxiang
Introduction
In general, a rectal temperature above 38°C indicates fever, with 38°C–38.9°C considered low-grade fever, 39°C–40°C high fever, and above 40°C very high fever. Rectal temperature measurement closely reflects core body temperature and provides more accurate results, making it especially suitable for infants, young children, and unconscious individuals. If high fever persists or is accompanied by convulsions or impaired consciousness, seek medical attention promptly.
Under normal circumstances, a rectal temperature above 38°C indicates fever, with 38°C–38.9°C classified as low-grade fever, 39°C–40°C as high fever, and above 40°C as hyperpyrexia (extremely high fever). Rectal temperature measurement closely approximates core body temperature and provides more accurate results, making it especially suitable for infants, young children, and unconscious individuals. If high fever persists or is accompanied by convulsions or altered consciousness, prompt medical attention is recommended.
The normal adult rectal temperature ranges from 36.5°C to 37.7°C, while for infants and young children it ranges from 36.5°C to 37.8°C. Because the thermoregulatory center in infants is not fully developed, their rectal temperatures tend to fluctuate slightly more than those of adults. A rectal temperature exceeding the normal upper limit for the respective age group may indicate potential abnormalities such as infection or inflammation.
When measuring rectal temperature, insert the thermometer 3–4 cm into the anus (1.5–2 cm for infants and young children) and leave it in place for 3–5 minutes before reading the result. Lubricate the tip of the thermometer before insertion, and perform the procedure gently to avoid damaging the intestinal mucosa. Rectal temperature readings are less affected by ambient temperature and measurement timing, thus providing a more accurate reflection of internal body temperature.
After measurement, clean the thermometer promptly and disinfect it with an alcohol swab. If fever is detected, reduce clothing to aid heat dissipation and encourage increased fluid intake. During measurement, comfort the individual to minimize anxiety and prevent movement that could lead to inaccurate readings or injury.
