Oral (Mouth) Temperature is Higher Than Axillary (Armpit) Temperature
Oral temperature measurements are consistently higher than axillary measurements, with oral readings averaging approximately 0.5-1.2°C (0.9-2.2°F) higher than axillary readings. 1, 2
Temperature Measurement Hierarchy in Clinical Practice
Core vs. Peripheral Measurements
Rectal measurements provide the most accurate approximation of core body temperature and correlate most closely with pulmonary artery catheter readings (the gold standard), followed by oral, then ear-based, and finally axillary measurements 3, 2
Axillary temperatures are the least reliable method for temperature assessment and should be avoided when accurate measurements are needed 3, 1
Specific Temperature Differences
The mean difference between oral and axillary temperatures is 1.17°C ± 0.72°C (approximately 2.1°F), with oral readings being higher 1
In febrile intensive care patients, axillary temperature showed the poorest correlation with core body temperature compared to all other measurement methods 2
Axillary measurements have a sensitivity of only 46% for detecting fever, making them inadequate for clinical decision-making 1
Clinical Implications for Elderly Patients
Measurement Accuracy Concerns
In elderly patients, unassisted axillary temperature assessment with gallium-in-glass thermometers is particularly inadequate, with cognitive impairment leading to significant underestimation of body temperature (mean difference of -0.40°C ± 0.42°C from nurse-assisted measurements) 4
Electronic axillary thermometers provide better accuracy than traditional glass thermometers in elderly patients, but oral measurements remain preferable when feasible 4
Fever Definitions in Elderly Populations
For elderly patients in long-term care facilities, fever is defined as a single oral temperature ≥100°F (37.8°C) or repeated oral temperatures ≥99°F (37.2°C), while repeated rectal temperatures ≥99.5°F (37.5°C) also meet criteria 3
Axillary measurements require higher cut-off points to detect fever - approximately 37.6°C (99.7°F) compared to 37.2°C (99°F) for oral measurements 5
Practical Recommendations
Preferred Measurement Sites
Use oral temperature measurements over axillary when the patient is alert, cooperative, and able to keep their mouth closed 3
Avoid axillary measurements when accurate temperature assessment is critical for diagnosis and management, as they are often 1-2 degrees different from actual core body temperature 3
If oral measurement is not feasible, rectal temperature is the next best alternative, though it may be impractical in many clinical settings 3, 2
Common Pitfalls to Avoid
Do not rely on axillary temperatures for fever screening in clinical settings - they have unacceptably low sensitivity (46%) and poor correlation with core temperature 1, 2
Oral temperature measurements can be distorted by mouth breathing or recent consumption of hot or cold fluids, so ensure proper technique and timing 3
In elderly patients with cognitive impairment, never allow unassisted temperature measurement with traditional thermometers - the underestimation can be clinically significant 4
When establishing temperature trends, maintain consistency in measurement method, as switching between sites can create misleading patterns 2