Axillary temperature measurement, often called the underarm method, is a common and non-invasive way to check for fever, particularly in infants and young children. This technique records the skin’s surface temperature, which is a localized reading rather than a direct measurement of the body’s internal core heat. Because the armpit is an external location, the recorded temperature is typically lower than the body’s core temperature, requiring a specific adjustment factor to estimate the true internal reading.
The Standard Axillary Correction Factor
The reading taken from the armpit is a surface temperature that does not fully reflect the body’s internal core temperature. This difference exists because the axilla is an open cavity subject to ambient air temperature and peripheral circulation, which cools the skin. Therefore, a standard adjustment must be made to the raw measurement to approximate the internal temperature.
The widely accepted adjustment is to add 1.0 degree Fahrenheit (F) or 0.5 degrees Celsius (C) to the temperature displayed on the thermometer. For example, an axillary reading of 98.6°F (37.0°C) is adjusted to an estimated core temperature of 99.6°F (37.5°C). This correction helps translate the cooler surface reading into a number closer to what would be found via a rectal or oral measurement.
The axillary temperature is generally found to be about one degree lower than an oral temperature and almost two degrees lower than a rectal temperature. Applying the standard adjustment is necessary to interpret the measurement accurately against standard fever thresholds.
Proper Technique for Axillary Temperature Taking
Achieving an accurate axillary reading depends on standardized technique, beginning with preparation of the measurement site. Ensure the armpit is completely dry before placing the thermometer, as moisture can cause evaporative cooling and lead to a falsely low reading. If the person has recently bathed or been heavily bundled, wait 20 to 30 minutes for the body temperature to stabilize before measurement.
The tip of the thermometer must be placed high in the center of the armpit, directly onto the skin. Crucially, the thermometer’s sensor must be surrounded by skin contact on all sides, not touching clothing. Once positioned, the arm should be lowered and held snugly against the side, often folded across the chest, to seal the cavity and trap the heat.
The duration of the measurement affects the accuracy of the reading. For modern digital thermometers, the device should be left in place until it signals completion with a beep, which is typically a minute or less. If using an older glass or non-beeping digital thermometer, the measurement can take substantially longer, often requiring five to ten minutes to ensure the sensor has fully registered the maximum temperature of the area. A secure hold throughout the entire duration prevents the thermometer from shifting and exposing the sensor to cooler air.
When to Use and When to Avoid Axillary Measurement
The axillary method is frequently chosen for newborns and young children because it is non-invasive and presents no risk of injury. However, its reliability for detecting a true fever is lower than that of rectal or oral measurements, as it represents a peripheral temperature influenced by external factors. When a high temperature is suspected, especially in infants, a more accurate method like a rectal or temporal artery measurement is often recommended to confirm the reading.
The need to manually add the correction factor depends on the device used. The 1.0°F (0.5°C) adjustment is applied to the raw surface temperature obtained from traditional or basic digital thermometers. Many advanced digital thermometers automatically calculate and display the adjusted core temperature, eliminating the need for manual addition; users must always check their device’s specific instructions.
Certain conditions can significantly compromise the accuracy of an axillary temperature, prompting the use of an alternative method. The reading may be unreliable if the person is shivering, has been recently active, or is in an unusually hot or cold room. These factors drastically skew the surface temperature, making it a poor proxy for the body’s core temperature.