Body temperature reflects the complex balance between the heat your body generates and the heat it loses. The body maintains a relatively stable core temperature, which is the operational heat of internal organs like the brain and heart. Temperature readings taken at the body’s surface are considered peripheral and can fluctuate more easily based on external conditions. This difference between internal operating temperature and external skin temperature is the reason why readings vary significantly depending on the measurement site.
Different Ways to Measure Body Temperature
Measuring body temperature involves placing a thermometer at sites that either closely mirror the body’s internal heat or sites on the surface of the skin. Core temperature is most accurately assessed through a rectal reading, which directly measures heat within a closed internal cavity. The tympanic measurement, taken in the ear canal, is also close to core temperature, detecting infrared heat radiating from the eardrum near the brain’s temperature-regulating center.
Peripheral measurements include the axillary, temporal, and oral methods. The axillary method involves placing a thermometer in the armpit, a skin-surface site easily influenced by the surrounding air and blood flow. The temporal method involves scanning the forehead, which measures the temperature of the temporal artery just beneath the skin. The oral reading is taken under the tongue, a semi-closed cavity with mucous membranes and a rich vascular supply.
The Standard Difference Between Oral and Axillary Readings
The clinical norm is that an axillary temperature reading is lower than an oral reading, typically falling approximately 0.5°F to 1.0°F lower. This consistent difference exists because the oral cavity is a protected space with a high density of blood vessels, allowing the thermometer to readily detect heat carried by the blood.
The armpit, or axilla, is a surface measurement site placed against the skin and covered by the arm. This location is more susceptible to heat loss through evaporation and ambient air temperature. The lower temperature at the skin’s surface compared to the deeper, mucosal tissue of the mouth accounts for the expected numerical gap. When an armpit reading appears higher than one taken orally, it contradicts this established physiological pattern.
Common Causes for Axillary Temperature Overestimation
When an axillary reading registers higher than an oral reading, the discrepancy is usually due to factors that artificially elevate the surface temperature or depress the oral temperature.
Measurement Technique and Device Error
Poor measurement technique is the most frequent cause, such as not centering the thermometer tip in the deepest part of the axilla. If the thermometer is loosely placed or not held firmly against the skin, it may pick up external warmth or fail to insulate the area properly. Device error is another possibility, where an improperly calibrated or malfunctioning thermometer provides a falsely high reading.
Environmental and Activity Factors
Environmental factors or recent activity can cause localized warming of the armpit skin that is not a true reflection of systemic body temperature. Wearing heavy or tight clothing, being in a warm room, or having recently engaged in physical activity can temporarily trap heat in the axilla. This trapped heat warms the skin, leading to an inaccurately high surface reading that can exceed a concurrent oral temperature.
Artificially Low Oral Readings
An artificially low oral reading can result from drinking a cold beverage immediately before measurement, which cools the mouth’s tissues. If the oral measurement is depressed by a cold drink, even a slightly inflated axillary temperature will appear higher in comparison.
When to Be Concerned About a Fever
A fever is a body temperature of 100.4°F (38°C) or higher, regardless of the measurement site. To confirm a fever, it is best to rely on measurements from more accurate sites, such as rectal or temporal readings. If an axillary reading suggests a fever, it should be confirmed with one of these reliable methods, especially when the reading is unexpectedly high compared to an oral measurement.
You should seek medical attention if a fever reaches 103°F (39.4°C) or higher in an adult, or if a lower fever is accompanied by severe symptoms like confusion, neck stiffness, or difficulty breathing. For infants under three months of age, any temperature of 100.4°F (38°C) or higher warrants an immediate call to a healthcare provider. A persistent fever lasting more than two or three days should also be discussed with a doctor.