Monitoring body temperature is a fundamental step in assessing health, especially when evaluating for fever in infants and young children. Accurate readings provide medical professionals with the necessary data to determine the severity of an illness and guide treatment. Because precision is important, the device and method used must be appropriate for the intended anatomical site. Using a thermometer designed for one area in a completely different location compromises the integrity of the reading and introduces health risks.
The Critical Risk: Contamination and Hygiene
The most significant prohibition against using a rectal thermometer under the arm involves the risk of microbial cross-contamination. Rectal thermometers contact fecal matter, which harbors various bacteria, including intestinal flora. Even with diligent cleaning, the minute crevices of a digital thermometer probe cannot guarantee complete sterilization suitable for cross-site application.
Transferring a thermometer from the rectal area to the armpit risks depositing these microbes onto the skin surface. If the thermometer were mistakenly used in the mouth later, it could lead to a serious gastrointestinal or systemic infection. Due to this high hygiene risk, healthcare guidelines recommend that any thermometer used rectally be permanently labeled for that purpose only. The potential for transferring pathogens outweighs any convenience of using a single device.
Calibration and Design Differences
Beyond hygiene concerns, rectal and axillary thermometers are designed and calibrated to measure different physiological temperatures, which affects accuracy. The rectum provides a measurement that closely approximates the body’s core temperature, the most stable indicator of fever. Conversely, the armpit measures the surface temperature of the skin, which is inherently lower and more susceptible to external influences.
The difference in readings between the two sites is substantial; an axillary temperature can be one to two degrees Fahrenheit (0.5°C to 1.1°C) lower than a true rectal temperature. Using a rectal thermometer under the arm will yield an inaccurately low reading, potentially masking a fever that requires medical attention. Furthermore, the physical design of a rectal thermometer often features a shorter, blunt probe tip optimized for safe insertion into the anus. Using this design in the armpit is less comfortable and less effective for measuring surface temperature compared to a standard axillary probe.
Correct Temperature Taking Methods
To ensure accuracy, it is necessary to use dedicated devices and follow site-specific procedures for measurement. For the axillary method, the thermometer tip should be placed directly in the center of the armpit, ensuring it touches skin and not clothing. The arm must then be held snugly against the body to trap heat and hold the probe securely. The reading should be taken once the digital thermometer signals completion, typically after about one minute.
The rectal method is considered the most accurate, particularly for infants up to six months of age. Before insertion, the digital thermometer tip should be lubricated with petroleum jelly or a water-soluble lubricant. The tip is then gently inserted into the rectum, no more than one inch (2.5 cm) for older children or about half an inch (1.25 cm) for infants. The thermometer must be held in place until the device beeps, and it should be thoroughly cleaned with soap and cool water immediately after use.