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Methods of recording temperature

Traditionally, the mouth, axilla and rectum have been the preferred sites for obtaining temperature readings, due to their accessibility. With the development of new technology the use of the tympanic membrane is increasingly popular, as it is less invasive and provides rapid results (Burke 1996). It has been suggested that tympanic membrane thermometers give a more accurate representation of actual body temperature, as the tympanic membrane lies close to the temperature regulation centre in the hypothalamus and shares the same artery (van Staaij et al. 2003).

Oral

To most accurately measure the temperature orally, the thermometer is placed in the posterior sublingual pocket of tissue at the base of the tongue (Torrance and Semple 1998). It is important that the thermometer is placed in this region and not in the area under the front of the tongue, as there may be a temperature difference of up to 1.7°C between these areas. This temperature difference is due to the sublingual pockets being protected from the air currents, which cool the frontal areas (Neff et al. 1989). This area is in close proximity to the thermoreceptors which respond rapidly to changes in the core temperature, hence changes in core temperatures are reflected quickly here (Carroll 2000, Stevenson 2004).

Oral temperatures are affected by the temperatures of ingested foods and fluids, smoking and by the muscular activity of chewing. It has been shown that oxygen therapy does not affect the oral temperature reading (Hasler and Cohen 1982, LimLevy 1982). A respiratory rate that exceeds 18 breaths per minute, together with a patient who smokes, will also reduce the core temperature values (Knies 2003).

Rectal

The rectal temperature is often higher than the oral temperature because this site is more sheltered from the external environment. Rectal thermometry has been demonstrated in clinical trials to be more accurate than oral or axillary thermometry; however, it is not advocated due to its invasive nature (Trim 2005). While other more precise methods can still detect fever, the rectal method offers greater precision. However, the presence of soft stool may separate the thermometer from the bowel wall and give a false reading, especially if the central temperature is changing rapidly. In infants this method is not recommended as it carries a risk of rectal ulceration or perforation (Jensen et al. 1994).

A rectal thermometer should be inserted at least 4 cm in an adult to obtain the most accurate reading.

Axillary

The axilla is considered less desirable than the other sites because of the difficulty in achieving accurate and reliable readings (Evans and Kenkre 2006) as it is not close to major vessels, and skin surface temperatures vary more with changes in temperature of the environment (Woollens 1996). It is usually only used for patients who are unsuitable for, or who cannot tolerate, oral thermometers, for example after general anaesthetic or patients with mouth injuries (Edwards 1997).

To take an axillary temperature reading, the thermometer should be placed in the centre of the armpit, with the patient’s arm firmly against the side of the chest. It is important that the same arm is used for each measurement as there is often a variation in temperature between left and right (Heindenreich and Giuffe 1990).

Whichever route is used for temperature measurement, it is important that this is then used consistently, as switching between sites can produce a record that is misleading or difficult to interpret.

Consideration is required when interpreting variations in 4–6 hourly observations, and when taking onceonly daily temperatures as the average person experiences circadian rhythms which make their highest body temperature occur in the late afternoon or early evening, that is between 4 pm and 8 pm. The most sensitive time for detecting pyrexias appears to be between 7 pm and 8 pm (Angerami 1980). Samples et al. (1985) found the highest temperature between 5 pm and 7 pm. These studies suggest that the most