Taking an oral temperature is a common way to estimate the body’s internal, or core, temperature. Accurate measurement depends entirely on the correct placement of the thermometer within the mouth. If the thermometer is not positioned properly, the reading will reflect the local temperature of the mouth rather than a true representation of the body’s condition.
The Ideal Placement Site
The most accurate location for an oral temperature reading is within the posterior sublingual pocket, often called the “heat pocket.” This area is located beneath the tongue, slightly off-center to the left or right of the frenulum. Positioning the thermometer here is important because this spot is close to major blood vessels, specifically the lingual arteries. These arteries circulate blood that has recently passed near the body’s core, meaning the local temperature closely mirrors the body’s internal temperature.
To achieve a true reading, the tip of the thermometer must be fully covered by the mucous membrane and surrounding tissue. This creates a sealed, insulated cavity when the mouth is closed around the device. Placing the thermometer tip too far forward, under the tip of the tongue, can result in a falsely low reading. The sublingual pocket provides a consistent, warm environment that minimizes interference from external air.
Step-by-Step Procedure for Accurate Reading
The process begins by ensuring the thermometer is clean and ready for use. If using a digital thermometer, secure a disposable probe cover over the tip and turn the device on. The patient should then be instructed to lift their tongue toward the roof of their mouth.
The thermometer tip is gently inserted deep into one of the posterior sublingual pockets, nestled securely against the tissue. The patient should then lower their tongue and close their lips firmly around the device without biting down. Keeping the lips closed prevents cool ambient air from entering the mouth.
The thermometer must remain in place until the device signals completion, typically a beep. Digital thermometers take only a few seconds, but glass thermometers require three minutes or more. After the reading is noted, the thermometer is removed, and the probe cover is discarded to maintain hygiene.
Factors That Invalidate Readings
Even with perfect placement, certain external factors can temporarily alter the temperature of the oral cavity, leading to an invalid reading. A person should avoid taking an oral temperature immediately after consuming hot or cold foods or drinks. The direct contact of these substances with oral tissues can artificially raise or lower the surface temperature.
Similarly, smoking or chewing gum immediately before a measurement can cause inaccuracies. Smoking introduces warm air and affects circulation, while chewing increases blood flow and muscle activity, both of which can raise the local temperature. Vigorous physical activity or breathing through the mouth for an extended period can also cool the oral cavity, resulting in a deceptively low reading.
To ensure the reading reflects the body’s true core temperature, a mandatory waiting period is required after these activities. Most healthcare guidelines recommend waiting at least 15 to 30 minutes after eating, drinking, chewing gum, or smoking. This interval allows the oral cavity to return to its baseline thermal state, ensuring the reading is accurate.