From the Guidelines

Rectal temperatures are more accurate for detecting fever in elderly patients.

The evidence suggests that rectal temperatures are a more accurate method for measuring body temperature compared to oral, axillary, or tympanic membrane measurements 1.
A study published in the Clinical Infectious Diseases journal found that rectal measurements of temperatures are more accurate than either the oral or axillary method and that electronic techniques are better than standard mercury thermometry 1.
Another study published in the same journal found that rectal temperatures are a more accurate predictor of infection in LTCF residents, with a sensitivity of 70% for predicting infection when the criterion is set at 100°F (37.8°C) 1.

Oral temperature measurements can be distorted by mouth breathing or hot or cold fluids or gases in or near the mouth, making them less reliable 1.
Tympanic membrane thermometers are not accurate if there is inflammation or blockage of the auditory canal, and their readings can be influenced by environmental temperature and sweating 1.
Axillary measurements, temporal artery estimates, and chemical dot thermometers should not be used in the ICU due to their low accuracy 1.

The choice of temperature measurement method should be based on the clinical circumstances of the patient, with rectal thermometers being a suitable option for elderly patients 1.
However, rectal thermometers should be avoided in neutropenic patients due to the risk of infection 1.
It is essential to maintain and calibrate temperature measurement devices properly and use them in a manner that does not facilitate the spread of pathogens 1.

From the Research

Rectal temperatures are more accurate for detecting fever in elderly patients, as they can identify fevers that may be missed by oral or tympanic temperatures 2, 3, 4.
A study found that rectal temperature is clinically the most useful temperature measurement in elderly patients, detecting fever in approximately 86% of febrile patients 3.
Another study found that rectal thermometry identified fevers in 14.7% of patients who were afebrile orally and in 12.2% of patients who were afebrile tympanically 4.

Oral temperatures may not be accurate in patients with dry mucous membranes and tactile warmth, with one study finding that fever or hypothermia was missed in 49.5% of patients if only oral temperatures were obtained 2.
Tympanic membrane and temporal artery temperatures have been found to have lower sensitivity and specificity compared to rectal temperatures in detecting fever 5, 6.
A study found that the diagnostic accuracy for infection was comparable among rectal and tympanal thermometry, but significantly lower in temporal artery thermometry 6.

Clinicians should be attentive to elderly patients' vague clinical presentation and consider using rectal thermometry to identify fevers that may be missed by other methods 4.
Different cut-off points should be used to identify infection using tympanal or rectal thermometry, with a cut-off point of 37.3°C for tympanal thermometry and 37.9°C for rectal thermometry 6.
Temperature measurement is an insensitive method to identify geriatric patients with infection, and clinicians should consider other clinical and laboratory findings in addition to temperature measurement 6.