19
Reproducibility of Readings
Reproducibility in Temperature Measurement
Multiple temperature readings in the same area, mouth, rectum, axilla, ear or temporal artery, make for
variability with each separate measurement. This can be confusing for clinicians, since they expect the
same number with each measurement. The non-reproducibility of the readings, however, is not a func-
tion of the devices, but simply a function of physiology. The human body is a myriad of small gradients,
and variability of readings will occur on every method of temperature measurement. In addition, ther-
mometers are at room temperature, nearly 30°F (17°C) cooler than the tissue being measured. That
said, it is then easily recognized how time of insertion, probe placement, and tissue cool down all affect
reproducibility of temperature readings, no matter what device is employed.
Oral T
Oral T
Oral T
Oral T
Oral Tem
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emperature
perature
perature
perature
perature:
By far, the most common method of temperature measure-
ment is sublingual measurements. Placement of the probe
under the tongue, however, can result in substantial
differences caused by just a slight repositioning of the
probe. The standard heat chart commonly used by manu-
facturers of electronic thermometers on the right illustrates
a difference of nearly 2°F (1°C) depending on exactly what
area is being touched by the probe.
Differences from repeated oral temperatures can vary even
further, as they can superimpose artifactual errors over the
thermal gradients. Patient activities also affect the reading,
these varying by individual and activity. In fact, one large
manufacturer cautions waiting at least 15 minutes after
ingesting hot or cold food or drink, after exposure to
extremely hot or cold weather, and after smoking.
Ear T
Ear T
Ear T
Ear T
Ear Tem
em
em
em
emperature
perature
perature
perature
perature
The journals abound with citations addressing the lack of reproducibility of ear thermometers. In fact,
some manufactureres instruct the user to take three separate temperature measurements, and to select
the highest of the three. While much of this has to do with the device, physiology also plays a large part.
In such a small area, the difference of 30°F (17°C) between the room temperature probe and the
temperature of the ear being measured results in a noticeable tissue cool down. Geriatric patients
typically have a lower rate of perfusion than a younger individual, and it can take several minutes for the
ear to equilibrate following the use of an ear thermometer.
R
R
R
R
Rectal T
ectal T
ectal T
ectal T
ectal Tem
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em
em
emperature
perature
perature
perature
perature
Time and placement are critical for rectal temperature measurement. It has long been recommended
that the measurement be taken for at least five minutes or more for accuracy. The measurement is also
dependent on the depth of insertion, and just a few centimeters can result in a noticeable difference.
TTTTTem
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emporal Ar
poral Ar
poral Ar
poral Ar
poral Arttttter
er
er
er
ery T
y T
y T
y T
y Tem
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emperature
perature
perature
perature
perature
Because of the expanse of area being measured, and the normally strong perfusion of the artery in
particular, temporal artery temperatures are at least as reproducible as any other method. Of interest,
the temporal artery area will equilibrate in the shortest period of time compared to any other site. For
absolute accuracy, however, it is recommended to wait 30-60 seconds before repeating a temperature on
the same side, although, depending on the individual, the time involved can be much shorter. The
limitation in time is almost entirely the behind-the-ear measurement, as the perfusion rate per tissue
mass is not quite as high as the temporal artery. Since the method employs the area behind-the-ear with
every measurement, this area is the time limitation.
