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Research Applied to Clinical Practiceimgtchr1.gif (1235 bytes)
by Robert C. Knies, RN MSN CEN
Section Editor
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Temperature Measurement in Acute Care:
The Who, What, Where, When, Why, and How?

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Obtaining the patient’s core temperature has been a significantly controversial issue of the past several years.
This article examines obtaining core temperature to provide perspective and a clinician’s view of the issues.


    Patients present to Emergency Departments, Urgent Care’s, and primary care offices with various complaints, one of the more common is a febrile condition. Obtaining a patient’s body temperature is a routine part of data collection. It is a well-known fact that accurate measurement provides useful clues about the severity of the illness.

    Body temperature is not fixed, but is responsive to cyclical changes, such as circadian rhythms and menstrual cycles (see table 1). Body temperatures increase during the day reaching a maximum point by evening and then lowers to a minimum point in early AM. Remember, these descriptions are for people who live and work normal daytime shifts. Those who work off-hours will have significant differences (DiBenedetto, 1993).

    To understand temperature assessment (today called "thermometry"), requires an understanding of physics and bio-science. Thermal energy is released by every object animate or inanimate, and is measured by a device that can record it on a readable scale. Thermal energy is transferred by physical objects in four ways: evaporation, convection, conduction, and radiation. Conduction and radiation are measurable by devices such as glass thermometers and infrared thermometers respectively. However, evaporation and convection can affect the outcome measurements of the aforementioned (Severine & McKenzie, 1998).

Where is the most accurate site to obtain body temperature?

    The answer is the hypothalamus. It is located in the base of the brain and acts as the body’s thermostat. It functions by monitoring heat sensors throughout the body and adjusting the temperature based on the body’s needs (Severine & McKenzie, 1998). The goal of the hypothalamus is to maintain the body’s core temperature (the temperature of the heart, lungs, liver, kidneys, brain, etc.) between 96.9° to 100.4° F (36° to 38° C). Temperature sensors in the hypothalamus have been identified as the prime site for fever-producing pyrogenic action (DiBenedetto, 1993). But access to the hypothalamus is quite inconvenient. DiBenedetto (1993) describes body temperature as an estimate of the average temperature of the core portions of the body as reflected by the temperature of the blood in the major vessels.

    Traditionally, body temperature has been measured by contact thermometers in the oral, rectal or axillary sites. These sites are choices of convenience rather than correctness, because they often do not represent internal (core) body temperatures with the necessary accuracy (Bayham, Fucile, McKenzie, & O’Hara, 1996). The pulmonary artery is an ideal site because in the heart the blood is mixed from the viscera and the skin. Other core accessible sites are the esophagus, tympanic membrane and urinary bladder. These sites are used when accurate knowledge of core temperature is critical. However, none of these sites are convenient for routine acute care temperature measurement.

Who should do the measurement?

    Anyone can, as long as they understand the importance, the theory, and the technique. That is the key! Each point of meaurement has its own intricacies and controversies and are discussed in this article.

What equipment is out there to measure temperatures?

    The truest, accessible, core temperature (the "Gold Standard") is measurement of the pulmonary artery temperature with a thermistor catheter. Similar thermistor catheters can be placed in the esophagus and bladder for core temperature measurement, but other factors may affect their accuracy and will be discussed later.

    Many people consider the glass-mercury thermometer the standard against which all other devices are measured due to it’s ease of use, portability, cost and availability. However, there are accuracy issues related to this "ideal" device. Glass is porous and the mercury can evaporate within six months which may affect accuracy. These devices need to be "shaken down" before use, which is a competency issue. Also, a chance of mercury exposure due to breakage related to the high risk shaking action (Severine & McKenzie, 1998).

    Electronic thermometers have similar advantages as glass-mercury. However, the method of measurement is faster because it is calculated. This is performed in a process called "predictive mode" where the metal sensor in the tip of the thermometer takes sequential readings and the final number is calculated (Severine & McKenzie, 1998). This is one of the controversial points in this article. For example, a nurse may obtain a temperature on a patient with an electronic thermometer and question the reading, Then he/she takes the patient's temperature again with a different thermometer and obtains a different reading, sometimes significantly different, and questions the accuracy of the first thermometer. The potential issue exists in the calculation of the temperature due to variance in blood flow to that area, or the patient moved the thermometer after it was placed by the nurse.

    Single or Multi-use chemical thermometers are very popular with the general public, but they have some accuracy issues for clinical use. The two basic types are:

A) Flexible strips that have dot-like depressions filled with a series of chemicals that melt and change color in relation to the patient’s temperature.
B) Re-usable strips of plastic, where the chemical within the strip is activated by the patient’s temperature and a numerical reading is displayed. The accuracy issues related to both are based on where on or in the body the measurement is obtained, because vaso-motor activity and ambient temperatures will affect the results.

    The last device is infrared radiation readers (IR’s) such as those used in tympanic thermometers. These devices in contrast to the previously mentioned devices that rely on conductive heat transfer, measure naturally emitted electromagnetic radiation. Infrared radiation is the electromagnetic radiation just beyond the red end of the visible spectrum (>1mm wavelength) (Bayham et al., 1996). These devices function without contact to the point of measurement, and for effective functioning of any non-contact-IR thermometer it must contain the following components:

  • An IR sensor to measure the net thermal radiation flux.
  • A reference sensor to measure the ambient temperature of the IR sensor.
  • An optical system to focus the thermal radiation.
  • A computational means to calculate the target temperature.

    The intensity of the IR varies with the temperature from the surface of the object it is measured from. The warmer the temperature the greater the IR intensity. A "black-body" (the inner ear for example) is considered the ideal IR radiator since it emits all the IR it absorbs (Bayham et al., 1996).


    As discussed earlier there are several sites besides the inaccessible hypothalamus for measuring temperature. How do you choose which site to use? The issues of accessibility and accuracy are the critical thinking points of contention. You would not put in a central monitoring device (Swan-Ganz catheter, or urinary bladder sensor catheter) just to measure temperature. Other sites are affected by treatments and environmental factors, for example the mouth and forehead are affected by ambient air and oral medications or topical lotions. The rectum is affected by stool content, and disease states such as hemorrhoids or colitis (Severine & McKenzie, 1998).


    The most commonly used site is the sublingual area. It is considered a fairly accurate site due to its close proximity to the lingual and external carotid arteries (DiBenedetto, 1993). However, on average it runs lower than core temperature by approximately 0.8° F (0.5° C). Correct placement of the oral thermometer is important for accuracy. Differences in readings can vary by as much as 1.7° F (0.95° C) from the rear sublingual pocket to beneath the frenum in front of the floor of the mouth (Erickson, 1976). Dwell time is also important, especially with glass-mercury thermometers, which require a three to five minute dwell time. The electronic thermometers give an audible signal when the reading is obtained Severine & McKenzie (1998), explain that readings can be affected if:

  • The patient ate, drank, chewed gum, or smoked within 15 minutes of the reading.
  • The patient does not keep the thermometer properly placed under the tongue.
  • The patient is an oral breather.
  • The patient talks during the reading.

    DiBenedetto (1993) explains that changes in oral temperature reflect changes in blood flow not necessarily changes in core temperature. Houdas & Ring (1982) state that because of variable conditions oral temperature should not be considered equivalent to core temperature, unless studies are performed under strict controls.


    For many years rectal temperature measurement was considered the "gold standard" especially in pediatric patients. Many studies have shown that rectal temperatures fail to track rapid changes in body core temperature because the rectum has no thermoreceptors (Heidenreich & Giuffre, 1990; Howie, 1991). In fact, because of the delayed response, core temperature may be changing in the opposite direction, and the lag time may be up to one hour. DiBenedetto (1993), states that heat passes from the rectum into the blood, not vise versa. Dwell time is significant here also, the glass-mercury thermometers require a five to seven minute dwell time. Rectal temperature is a good approximation of body core temperature only if the patient is in thermal balance (Houdas & Ring, 1982).

Severine & McKenzie (1998) discuss other possible causes of inaccurate rectal readings, which include:

  • Heavy exercise of the large muscles in the buttocks and thighs.
  • The insulating effect of fecal material in rectum
  • Coliform bacterial action.
  • Improper depth of probe.
  • Shortened dwell time.


    These two sites are popular with the lay public due to their non-invasiveness and accessibility. Glass-mercury, electronic or single-use-chemical thermometers can be used, but the clinical accuracy at these sites is suspect. With glass-mercury and single-use-chemical thermometers the device must remain in position between 8 and 11 minutes (Severine & McKenzie, 1998). These sites are not located near a major artery or thermoreceptor and may not reflect temperature fluctuations. These factors may alter readings as much as 2.2° F (1.2° C) lower than actual core temperature (Severine & McKenzie, 1998). Lastly, if the patient is in shock the peripheral vaso-constriction will adversely affect the reading.


    This highly accurate device is considered invasive and is commonly used during surgery or in critical care areas. Due to the length of the esophagus the placement of the sensor is critical. If it is too high in the esophagus the reading will be affected by tracheal air (DiBenedetto, 1993). Proper placement is in the lower 1/3 of the esophagus which will allow the sensor to be closer to the heart and aorta, and that it will accurately reflect the core temperature. It also indicates changes in core temperature significantly faster than peripheral sites (Severine & McKenzie, 1998).


    Studies have shown a strong correlation between bladder and other core temperatures, because the urine is a filtrate of the blood and the kidney’s receive 20% of cardiac output (Mravinac, 1989; Erickson & Kirklin, 1993; Nierman, 1991; Earp & Finlayson, 1991). This method is considered minimally invasive, as it requires a urinary catheter with a thermistor tip to be inserted into the bladder. Mravinac (1989) explained that bladder temperatures track core temperature changes better than rectal. The readings may be altered due to urinary volume or if the patient is receiving bladder irrigations.


    Obtaining a patient’s temperature by tympanic temperature probes became available in the 1960’s. Originally, this was done by anesthesiologists during surgery with a probe placed directly against the membrane. Only since 1986 has a non-contact infrared device been available (Severine & McKenzie, 1998). However there has been continuous debate over their accuracy. Some of the latest case reviews or studies that support tympanic accuracy include; Talo, Macknin, & Medendorp, 1991; Rotello, Crawford, & Terndrup, 1996. Those opposed include; Staven, Saxholm, & Smith-Erichsen, 1997; Romanovsky, Quint, Benikova, & Kiesow, 1997.

    When infrared ear thermometers were first introduced, the manufacturers thought that clinicians would be more familiar with other body site temperatures (rectal, axillary) so they gave the option of displaying these other site calculations. Which meant that more calculations were made by the device, and consequently further confused the interpreting clinician. Many studies compare rectal and tympanic thermometers in an attempt to explain the "falsely low" readings of tympanic thermometers (Rotello, Crawford, & Terndrup, 1996). Previously mentioned authors (and I might say many clinicians) have concluded that infrared ear thermometers are inaccurate and insensitive in identifying patients with a "rectal-based" fever. But, as suggested earlier, rectal temperatures may over-estimate body core temperature.

    The tympanic site was chosen because it is located in close proximity to the internal carotid artery, which supplies the hypothalamus (DiBenedetto, 1993). Since the tympanic membrane and auditory canal are relatively devoid of metabolic activity, the primary determinant of the temperature is that of the carotid artery (Rotello et al., 1996).Tympanic temperatures are more effective in tracking changes in temperatures than rectal which can lag up to 60 minutes or more behind. The temperature of the tympanic membrane is also protected from the influence of ambient temperatures and is unaffected by smoking, respirations, eating or drinking.

    Recognition of the issues that may affect alterations in readings can supply the clinician with the ammunition to reply to the detractors of the use of this device. The three major issues are 1)size of the ear canal, 2)technique of directing the IR reader, and 3)metabolic occurrences that affect the attenuation of the IR signal.

    It is very important that the operator direct the IR reader at the tympanic membrane. Otherwise the device will obtain mixed signals from the ear canal, which can be affected by the ambient air. This will affect the calculated temperature, most likely displaying a lower than true reading. Bayham et al., (1996) describe the various temperatures in the ear, ranging from 97.52° F (36.4° C) on the ear canal wall to 99.14° F (37.3° C) in the center of the tympanic membrane. This a good example of how the mixed reading will affect the interpretation of the true temperature. The other issue is using the tympanic device on the correct size ears. Children less than two years old have ear canals with an average size of 5 mm or less, whereas most device’s probe covers are six to eight mm (Staven, Saxholm, & Smith-Erichsen, 1997). This "size" controversy will affect where the IR reader obtains its information from, and supports the non-use of these devices on children less than two years old.

    There have been several studies including Kiesow & Hurley, (1995); Talo, Macknin, & Medendorp, (1991) that have discussed how fluid, exudate and cerumen can affect the readings from IR tympanic thermometers and ultimately the outcomes of their studies. These naturally occurring conditions affect how the IR reader receives and interprets the information from the tympanic membrane. Therefore, clinicians need to be aware of how these conditions may affect the results of temperatures on patients presenting for care and should consider an alternative means for obtaining temperatures if temperature is considered critical data.

    Lastly, the issues of documentation need to be addressed. Whatever location you use to obtain the patients temperature, document that location and the data obtained! Do not adjust for location based on the old system of up one degree if axillary or down one degree if rectal. Just document the measurement obtained and the location, for example, 99.2° F tympanic or (T); or 100.5° F rectally or (R).

    In summary, research has supported the use of the pulmonary artery as the gold standard for invasive thermometry and tympanic membrane as the gold standard for non-invasive thermometry. As clinicians we must keep current with research and apply it to clinical practice. We must "kill the sacred cows" that exist, especially in nursing. To increase our professionalism we need to have the information to support changes in practice. There is still much controversy on the use of the tympanic thermometers and their accuracy. The best defense of their use is the knowledge base of critical thinking. This article supplied you with the current data on all the venues of obtaining temperature. It is your job to take this knowledge and apply it clinically, defend the research against its distracters and use research to improve care for our patients every day.


Table 1

Things That Affect Body Temperature
Circadian Rhythms
Menstrual Cycles
Ambient temperature
Disease states
Head Trauma
"Brain Attack" affecting the hypothalamus


Table 2

Summary of Data Presented


Variance from Core Temperature




< 0.8° F (0.5° C)

Affected by placement

Recent foods or fluids adversely affect the reading


> up to 1° F (0.6° C)

Reading may be delayed from core temperature change

-Fecal material

-Improper placement in children can perforate the rectum


<2.2° F (1.2° C)


Dwell time important for accurate reading


<2.2° F (1.2° C)


Dwell time important for accurate reading



Placement is key

Needs to be in lower 1/3 of esophagus



Affected by urine volume or bladder irrigations



Technique is key

Affected by cerumen or fluid behind tympanic membrane



    Bayham, E., Fucile, F., McKenzie, N., & O’Hara, G. (1996). Clinical considerations for use of FirstTemp̉ & FirstTemp Genius̉ Infrared tympanic thermometers. Sherwood Davis & Geck. Sherwood Medical Company: St. Louis, MO.
    DiBenedetto, L. (1993). Core Temperature. IVAC Corporation: San Diego, CA.
    Earp, J.K., & Finlayson, D.C. (1991). Relationship between urinary bladder and pulmonary artery temperatures: A preliminary study. Heart & Lung, 20 , 265-270.
    Erickson, R.S. (1976). Thermometer placement for oral temperature measurement in febrile adults. International Journal of Nursing Studies, 13: 199-208.
    Erickson, R.S., & Meyer, L.T. (1993). Comparison of ear-based, bladder, oral and axillary methods for core temperature measurement. Critical Care Medicine, 10, 1528-34.
    Heidenreich, T., Giuffre, M. (1990). Postoperative Temperature Measurement. Nursing Research, 89(3), 153-155.
    Houdas, Y. & Ring, E.F.J. (1982). Human Body Temperature. Plenum Press: New York and London. 57-141.
    Howie, J.N. (1991). Hypothermia and rewarming after cardiac operation. Focus on Critical Care, 18 (5), 414-418.
    Mravinac, C.M., Dracup, K., & Clochesy, J.M. (1989). Urinary bladder and rectal temperature monitoring during clinical hypothermia. Nursing Research 88, (2), 73-76.
    Nierman, D.M. (1991). Core temperature measurement in the intensive care unit. Critical Care Medicine, 19(6), 818-823.
    Romanovsky, A.A., Quint, P.A., Benikova, Y., & Kiesow, L.A. (1997). A difference of 5° C between ear and rectal temperatures in a febrile patient. American Journal of Emergency Medicine, 15(4), 383-385.
    Rotello, L.C., Crawford, L., & Terndrup, T.E. (1996). Comparison of infrared ear thermometer derived and equilibrated rectal temperatures in estimating pulmonary artery temperatures. Critical Care Medicine, 24 (9), 1501-1506.
    Severine, J.E., & McKenzie, N.J. (1998). Advances in temperature monitoring: A far cry from "shake and take". The Nursing Institute & Sherwood-Davis & Geck. Sherwood-Davis & Geck: St. Louis, MO.
    Stavem, K., Saxholm, H., & Smith-Erichsen, N. (1997). Accuracy of infrared ear thermometry in adult patients. Intensive Care Medicine, 23, 100-105.
    Talo, H., Macknin, M.L., & Medendorp, S.VB. (1991). Tympanic membrane temperatures compared to rectal and oral temperatures. Clinical Pediatrics (supplement, 1991), 30-33.

"Research Applied to Clinical Practice: Temperature Measurement in Acute Care"

is a webarticle by Robert C. Knies, RN MSN CEN [[email protected]]
©Robert C. Knies, RN MSN CEN
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