Temperature Measurement for Patients with Fever

US Pharm. 2008;33(3):14-17.

Temperature is one of the standard vital signs taken to help monitor a patient's well being. Patients and many caregivers generally accept that the normal body temperature is 98.6 ^oF. However, there are numerous misconceptions that surround fever. Several new products meant to facilitate temperature measurement have recently been introduced. Their accuracy is in question as they are compared against traditional methods, with varying results.

Core Body Temperature
The body's temperature (set point) is regulated by the hypothalamus.^1-3 It has the highest temperature in the body, but its temperature cannot be measured directly because of inaccessibility.^4,5 For this reason, devices designed to measure temperature are compared against a concept known as "core temperature."⁶ Core temperature is defined as the temperature within the pulmonary artery.⁴ Different devices have variable abilities to measure core temperature.

The human body is not uniform in its temperature; thus, measurements at different locations can yield varying results.⁴ This introduces another level of difficulty in estimating the patient's core temperature. Furthermore, older patients tend to have cooler temperatures, so the standard of 98.6^oF is not always applicable.⁷

Fever Measurement Sites
Patients can take temperatures orally, rectally, under the arm, in the ear, or on the surface of the skin. Each site has its own advantages and disadvantages. Rectal temperatures have long been thought to be the closest estimator of core temperature.⁴ Environmental temperature does not affect rectal readings and they can be carried out on patients of any age. However, these advantages are counterbalanced by its many problems. The site may be contaminated with bacteria and feces. The procedure is uncomfortable, embarrassing, and carries a risk of rectal perforation.⁴ Additionally, rectal measurements lag behind the actual core temperature. ⁸

Oral sublingual temperatures are simple and rapid to conduct, but their accuracy can be compromised by such activities as drinking hot or cold beverages.⁹ Bradypnea, breathing at a rate of eight breaths per minute, can create false temperature elevations. ⁹

Axillary measurements are comfortable for patients and present little safety risk, although the readings are affected by ambient temperature. Because of their questionable accuracy, many authorities recommend against axillary measurement to detect fever.⁴

Otic thermometers (tympanic thermometry) measure blood flow in the tympanic membranes, which are supplied by the carotids.⁴ Otic measurement is simple, carries no risk of infection, and is not sensitive to ambient air temperature.

Skin measurement of temperature is affected by the surrounding air temperature.⁴ It has been attempted with two radically different technologies. Some of the earliest models were crude, color-changing strips of questionable reliability. However, the recent introduction of a temporal artery thermometer has sparked renewed interest in skin measurements.

Fever Measurement Devices
Many pharmacists remember when glass mercury thermometers were the only devices available for home detection of fever.¹⁰ Glass thermometers are now considered hazardous to the environment because they contain mercury.¹¹ The Environmental Protection Agency warned that breakage allows mercury to enter the environment, where its evaporation leads to toxicity. While a national law to prohibit their use has not been enacted, several states and cities have banned them. Nevertheless, there may be millions of these dangerous devices in medicine cabinets across the nation. Pharmacists can initiate trade-in programs as a public service project to encourage people to purchase safer thermometers, perhaps giving a rebate for each glass thermometer that is turned in. The mercury-containing thermometers can then be carefully carried to a certified waste disposal site.

Several companies now market nonmercury glass thermometers. They may contain red or blue alcohol solutions of questionable accuracy. Others contain a gallium/indium/tin mixture known as galistan. These are not widely available at this time.

Alternatives to Glass Thermometers: The introduction of digital thermometers in the 1980s provided an alternative to older glass products. They provided results more rapidly and did not pose an environmental hazard. Several companies market full lines of digital thermometers, such as Vicks and Becton Dickinson (BD). Patients should be urged to purchase inexpensive probe covers (sheaths) to help prevent patient-to-patient contamination with these devices. The thermometers should only be used with a protective sheath, and the sheaths should be discarded after each use.

Vicks Wearable Thermometers are adhesive-backed devices that can be attached to a baby's axilla, providing continuous ability to read the temperature for 48 hours before they are discarded.¹² The device contains a grid of 55 dots that change color in a programmed manner as the temperature rises or falls, and a legend provides the parent with the child's temperature, as high as 104.8^o F, in 0.2-degree increments.

Otic (tympanic or ear) thermometers have become increasingly popular in the last decade. Their accuracy in those under the age of 3 has been cited as a problem in their use. ¹³ However, other studies have affirmed their reliability, although the findings are highly dependent upon the device and/or site to which they are compared (e.g., axillary or rectal measurements).^8,11,14 If studies continue to demonstrate that they are accurate, they may become the new gold standard for home detection of fever.⁴ Patients should follow all directions provided with the devices and also use the probe covers designed for the particular model.

Fever detection via temporal artery measurement at home is also possible with use of the Exergen TAT-2000C Temporal Thermometer.¹⁵ The thermometer takes 1,000 readings per second as it is stroked across the forehead, selecting the most accurate as the patient's temperature. The accuracy of these devices has also been studied. ¹⁶ One study compared temporal artery and axillary measurements to simultaneous rectal and (when available) pulmonary artery catheter measurements.¹⁷ The investigators found that temporal artery and axillary measurements were not accurate enough to recommend them as a replacement for rectal or other invasive methods. The investigators recommended temporal measurement only as a possible substitute for axillary measurement. The U.S. Department of Health and Human Services conducted a study that found temporary artery measurement to be no better, and possibly inferior to, the time-honored method of manually feeling the forehead, although the manufacturer questioned its findings.¹⁸ Possible inaccuracies may be due to the fact that the temporal artery is not located in the same place in each person, and may also lie at differing depths, either of which can affect the accuracy of the method.¹⁸ Moreover, skin temperature may not be reflective of core temperature, as many variables can alter skin temperature without affecting the core body temperature.

Measuring temperature in infants and children under the age of 2 has always been a challenge. The site that most accurately reflects core body temperature is the rectum, but rectal temperatures are uncomfortable and invasive.¹⁹ If done improperly, rectal measurements may result in perforations of the bowel wall. Axillary measurements do not have these drawbacks, but their ability to estimate core temperature is questionable. Digital oral thermometers and otic thermometers are possible options for infants. However, another alternative is the pacifier thermometer, with a digital readout that measures temperature as the child sucks on the proximal end. The major issue with these devices is reliability. Standard oral temperatures are always to be taken with the tip of the thermometer firmly placed in one of the "hot pockets" under the tongue. Pacifier thermometers do not reach the sublingual spaces but extend only to the supralingual areas traditionally occupied by a pacifier. One study measured their ability to accurately measure core body temperature in children aged between seven days and 24 months.¹⁹ Rectal temperatures were used as the standard for comparison. The investigators discovered that by adding 0.5^oF to the readings obtained from a six-minute pacifier temperature, there was a close agreement with the rectal temperatures. Therefore, it appears that these simple devices are safe and accurate for infants and young children.

A Recent Innovation in Thermometry
Fever InSight is a useful feature of some of Vicks' digital thermometers.¹² These models have a large circular readout at the distal end. The background color of the digital display changes as the temperature increases. The display appears green if there is no fever (i.e., 98.6^oF), yellow with a slight elevation (up to 99.2^oF), and red when fever is present (101.3^oF and above).

Fever Increases Risk When Using Certain OTC Products
In its comprehensive review of nonprescription products, the FDA and its expert panels identified several instances when patients asking the pharmacist about minor illnesses should be referred to a physician if fever is also present. ²⁰ For instance, parents may ask the pharmacist about fever in a teething baby, insisting that fever is normal when teething. The FDA stressed that fever is not a symptom of teething and may indicate the presence of infection. Labels caution that a physician should be consulted if the fever persists. Products for diarrhea should not be used if the patient has fever, unless a physician recommends it. Nasal solutions containing cromolyn sodium should not be used for allergic rhinitis without physician advice if the patient has fever. Oral nasal decongestants, topical and internal products for sore throat, oral and topical antitussives, and expectorants should not be used if the patient has a fever, without physician advice to do so. If patients have fever and stiff neck, they should not use a migraine product without consulting with a physician. Women with vaginal fungal infections should not use vaginal antimonilial products if they have fever in excess of 100^oF measured orally, as that may indicate an STD.

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