The axillary, or under the arm, temperature measurement is a common, non-invasive method used to check for fever, especially in infants and young children. This technique involves placing a thermometer in the armpit and holding the arm close to the body to capture the surface temperature. Axillary temperature is considered a surface reading, measuring skin temperature rather than the internal core body temperature. It provides a quick and accessible way to screen for temperature elevation.
The Fever Threshold for Under the Arm Readings
A fever is defined by different numerical thresholds depending on the measurement site used, and the axillary reading has a lower threshold than core methods. For an under the arm reading, a temperature of 99.0°F (37.2°C) or higher is generally considered a fever. This threshold is lower than the standard 100.4°F (38.0°C) used for rectal or forehead measurements.
The difference in the threshold exists because the armpit is a surface site and is naturally cooler than the body’s core temperature. Axillary readings typically measure approximately 0.5°F to 1.0°F lower than a rectal temperature. This discrepancy means the fever definition is adjusted downward to account for the difference between skin and internal temperature. Axillary temperature is often used as a screening tool, and if a high reading is obtained, a more accurate method may be recommended for confirmation.
Reliability Compared to Other Measurement Sites
The axillary method is valued for its safety and convenience, especially when measuring the temperature of a sleeping or uncooperative child. It carries virtually no risk of injury or discomfort, making it a preferred initial screening method in home settings. The process is simple, requiring only a digital thermometer and a short waiting period until the reading is complete.
However, the axillary method is considered the least accurate when compared to core temperature sites like the rectum. Rectal temperature is often referred to as the gold standard because it provides the closest estimate of the body’s true core temperature. Oral and temporal (forehead) readings also tend to be more reliable than axillary measurements in older children and adults.
The variability of axillary readings is a significant limitation, as the result can be easily affected by external factors. Environmental conditions, such as a cold room, or user error, like failing to ensure the thermometer tip is fully covered by the armpit skin, can lead to inaccurate results. The time it takes to stabilize an axillary reading, which can be up to four minutes, is longer than oral or rectal measurements, contributing to inconsistency.
Actionable Steps Based on Axillary Temperature
When an under the arm reading indicates a fever, the most important next step is to evaluate the age of the individual and any accompanying symptoms. For an infant under three months old, any axillary temperature reading of 99.0°F (37.2°C) or higher warrants immediate contact with a healthcare provider. Babies in this age group are at a higher risk for serious infections, and fever requires prompt medical evaluation.
For older children and adults, the severity of the fever and the presence of other symptoms guide medical attention. A persistent fever lasting more than 48 to 72 hours, regardless of the reading’s height, should be discussed with a doctor. Other concerning signs that necessitate immediate professional consultation include extreme lethargy, difficulty breathing, a stiff neck, or a strange rash.
While a mild axillary fever can often be managed at home with comfort measures and appropriate over-the-counter medication, the overall clinical picture matters more than the isolated number. If the person looks or acts very sick, or if you have any doubt about their condition, seeking medical advice is the most prudent action. Always inform the healthcare provider about the measurement site used when reporting the temperature.