Temperature examination

Taking a patient's temperature is an initial part of a full clinical examination.

Core body temperature is normally carefully controlled within a narrow range so that essential enzymatic reactions can occur. Prolonged temperature elevation (hyperthermia) or depression (hypothermia) are incompatible with life. There is some natural variation in body temperature with the time of day and body conditions (see thermoregulation).

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Normal body temperature

Normal temperature is generally quoted as that for oral measurement and taken as 36 to 37 °C (degrees Celsius) which equals 96.8 to 98.6 degrees Fahrenheit. Rectal temperature is generally 0.5 °C (0.9 °F) higher than orally and that at the armpit a similar amount lower. The oral temperature is recorded by placing a thermometer under the tongue for 3 to 5 minutes. Elevations in temperature above 38 °C (100 °F) are considered to be fever. It should be noted that hot or cold drinks can alter the measured temperature for up to 30 minutes.

Methods of temperature measurement

Direct measurement of core body temperature would require invasive insertion of a probe and is not clinically possible, so a variety of indirect methods are used. Whilst the rectal temperature is generally considered to give the most accurate assessment of core body temperature, its recording is disliked by patients and medical staff alike.

There is a risk of injury from cracking the original glass thermometers if too much force is applied by the teeth to hold them in place and the alcohol or mercury contents are poisonous. This is avoided by the use of electronic thermometers which are made from solid plastic and use a metal (thermocouple) sensor.

The color change of plastic thermometer strips placed on the forehead gives an approximate local reading which depends to a great extent on ambient air temperature and local circulation effects. Using a thermometer to record the temperature under the armpit is less affected by surrounding air temperature, but is still prone to diverge from true core temperature if there are alterations in blood circulation.

Recently small ear thermometers have become available and it is thought that the eardrum closely mirrors core temperature values. These work by detecting the infrared heat emission from the tympanic membrane and a measurement is quickly taken within one second making them popular for use with children.

Rectal temperature measurement was used prior to ear monitors if the patient could not cooperate with holding the thermometer in the mouth. It remains though the gold standard test, particularly in cases of hypothermia.

Problems of various measurements

It is claimed that changes in core body temperature are reflected sooner and more accurately in the ear than at other sites. Whilst the electronic display of the temperature value is easier to read than interpreting the graduation marks on a thermometer, there are some concerns for the accuracy of ear thermometers in home use^[1].

Influencing factors on other areas where temperatures are taken:

• Oral temperatures are influenced by drinking, eating and breathing.
• Rectal temperatures lag behind changes in core body temperature and there is a risk of cross-contamination.
• Skin temperatures, measured under the arm or at the forehead, are not always reliable indicators of core body temperature, especially during those critical times when core body temperature is increasing or decreasing. This is because the skin is a tool the body uses to control core body temperature. For example, when fever is increasing people are likely to react by shivering and drawing in heat from the increase core body temperature. Skin temperatures are further influenced by factors such as fever lowering medication, clothing and external temperature.

References

1. ^  Accuracy of parents in measuring body temperature with a tympanic thermometer. Joan L Robinson , Hsing Jou and Donald W Spady. BMC Family Practice 2005, 6:3 doi:10.1186/1471-2296-6-3 Full PDF

See also

• Fever, for a more detailed explanation of normal and abnormal human body temperatures