Axillary Temperature Equivalent for Rectal 100.4°F
A rectal temperature of 100.4°F (38°C) corresponds to an axillary temperature of approximately 99.4°F (37.5°C), using the standard conversion of subtracting 1°F (0.6°C) from the rectal reading.
Temperature Conversion Between Sites
The relationship between rectal and axillary temperatures has been well-established in clinical studies:
The mean difference between rectal and axillary temperatures is approximately 1.0°C (1.8°F), with axillary readings consistently lower than rectal measurements 1.
In patients older than 1 month, the standard adjustment is to add 1°F to the axillary temperature to estimate the rectal temperature (or conversely, subtract 1°F from rectal to estimate axillary) 1.
Therefore, a rectal temperature of 100.4°F would translate to an axillary temperature of approximately 99.4°F 1.
Important Caveats About Axillary Measurements
Accuracy Limitations
Axillary measurements have wide limits of agreement (ranging from 0.32°C to 1.98°C difference from rectal), meaning individual readings can vary substantially 2.
Axillary thermometers demonstrate good sensitivity (95%) but limited specificity (75%) for detecting fever, making them adequate for screening but requiring confirmation for critical decisions 2.
Rectal measurements remain more accurate than oral or axillary methods when precision is essential 3, 4.
Clinical Context Matters
In long-term care facility residents and frail elderly, a rectal temperature of 100.4°F already meets the threshold for fever (≥99.5°F rectal on repeated measurements) and warrants clinical evaluation 3, 5.
Electronic thermometry is more accurate than mercury thermometry regardless of measurement site 3, 4.
Practical Application
Use axillary temperature of 99.4°F as a screening estimate for a rectal reading of 100.4°F 2, 1.
Confirm borderline or critical readings with rectal measurement when clinical decisions depend on precise temperature determination 2, 4.
Consider that the standard deviation of ±0.45°C means the actual axillary reading could range from approximately 98.6°F to 100.2°F for a true rectal temperature of 100.4°F 1.