The core body temperature, measured internally, is approximately 37°C (98.6°F) and fluctuates approximately 1°C over the course of the day. It is lowest in the early morning and highest in the afternoon and evening. Women have a wider range of normal temperature than men.

Although the research gold standard for core body temperature is the blood temperature in the pulmonary artery, clinical practice relies on noninvasive oral, rectal, axillary, tympanic membrane, and temporal artery measurements.44 Tympanic membrane and temporal artery temperatures use infrared thermometry.

■ Oral and rectal temperature measurements remain common. Oral temperatures are generally lower than the core body temperature. They are also lower than rectal temperatures by an average of 0.4 to 0.5°C (0.7 to 0.9°F), and higher than axillary temperatures by approximately 1°. Axillary temperatures take 5 to 10 minutes to register and are considered less accurate than other measurements.

■ Tympanic membrane temperatures can be more variable than oral or rectal temperatures. Studies vary in methodology, but suggest that in adults, oral and temporal artery temperatures correlate more closely with the pulmonary artery temperature, but are about 0.5°C lower.

Oral Temperatures

For oral temperatures, options include electronic or glass thermometers. Due to breakage and mercury exposure, glass thermometers are being replaced by electronic thermometers. If using an electronic thermometer, carefully place the disposable cover over the probe and insert the thermometer under the tongue. Ask the patient to close both lips, and then watch closely for the digital readout. An accurate temperature recording usually takes about 10 seconds.

For glass thermometers, shake the thermometer down to 35°C (96°F) or below, insert it under the tongue, instruct the patient to close both lips, and wait for 3 to 5 minutes. Then read the thermometer, reinsert it for a minute, and read it again. If the temperature is still rising, repeat this procedure until the reading remains stable. Note that hot or cold liquids, and even smoking, can alter the temperature reading. In these situations, delay taking the temperature for 10 to 15 minutes.