The underarm temperature measurement, known as the axillary method, is a common, non-invasive way to check for fever, especially in infants and children. This technique involves placing a thermometer under the armpit and is favored for its convenience and safety. However, the reading obtained is often confusing regarding whether it accurately reflects the body’s true internal temperature. The central debate is whether a numerical adjustment is necessary to determine if a fever is present.
Historical Practice vs. Current Guidelines
Historically, medical professionals often advised caregivers to add one full degree Fahrenheit (or 0.5 to 0.6 degrees Celsius) to an axillary reading to estimate the core body temperature. This practice originated because older mercury-in-glass thermometers were slower and less precise. The axillary site was known to read lower than the more accurate rectal or oral sites, making the adjustment necessary to approximate internal temperature.
The traditional advice to “add a degree” is now often discouraged by many major medical organizations, especially when using modern digital thermometers. Current guidelines tend to favor interpreting the unadjusted axillary reading directly against a specific, lower fever threshold for that site. Using the unadjusted reading prevents the overdiagnosis of fever, which can occur if a degree is added to a reading that is already close to the fever range. For instance, the Canadian Paediatric Society recommends the axillary method as an alternative screening tool, while the American Academy of Pediatrics suggests it for neonates due to safety concerns with rectal thermometers.
Why Axillary Readings Differ from Core Temperature
The difference in temperature readings between the armpit and internal sites is rooted in the type of temperature being measured. The axilla measures peripheral or surface skin temperature, which is naturally lower than the core body temperature. Core temperature, best represented by a rectal measurement, is the temperature of the body’s internal organs and is tightly regulated by the hypothalamus.
Factors such as ambient room temperature, how tightly the arm is held against the chest, and the presence of clothing can all influence the surface reading. Because the thermometer is only in contact with the skin and not enclosed within a body cavity, heat loss to the environment is greater. Axillary temperatures can be significantly lower than rectal temperatures, with mean differences often exceeding half a degree Celsius.
Interpreting Axillary Results and Defining Fever
When using the axillary method, interpret the reading based on the threshold specific to that site, without adding a degree. An unadjusted axillary temperature reading of 99.0°F (37.2°C) or higher is typically considered a fever, particularly for infants and young children. For comparison, a rectal or oral temperature is generally defined as a fever at 100.4°F (38.0°C) or higher. If the armpit temperature is elevated, it serves as a reliable indicator that a fever is present, making further confirmation unnecessary in many common situations. The most reliable approach is to follow the specific guidance provided by a healthcare provider or the instructions accompanying the thermometer.