Core body temperature represents the internal heat of your body’s vital organs, such as the heart, liver, and brain. This internal temperature is different from the peripheral or skin surface temperature, which can fluctuate significantly based on external conditions. Monitoring core body temperature provides a reliable indicator of overall health, as the body works to maintain a stable internal environment for optimal organ function. This precise thermal regulation is managed by the hypothalamus in the brain, acting like a thermostat to ensure bodily processes operate efficiently.
Rectal Temperature Measurement
Rectal temperature is the most accurate method, especially for infants and young children, as it closely reflects the body’s internal temperature. To perform this safely, use a digital rectal thermometer and lubricate its tip with petroleum jelly. Position the child on their belly across your lap or on a firm surface, or on their back with legs bent towards their chest.
Gently insert the lubricated tip about 0.5 to 1 inch (1.3 to 2.5 centimeters) into the anal opening, or just until the tip is fully inside the rectum, being careful not to force it. Hold the thermometer in place firmly, steadying it with your hand against the child’s buttocks, until it beeps or signals completion. After removing the thermometer, read the temperature immediately, then clean the device thoroughly with soap and water or rubbing alcohol, and label it for rectal use to prevent cross-contamination.
Oral, Ear, and Forehead Measurements
Oral temperature measurement is a common method for older children and adults who can hold the thermometer correctly. Place the tip of a digital thermometer under the tongue towards the back of the mouth, instructing the individual to close their lips around it without biting. It is advisable to wait at least 15 to 30 minutes after eating or drinking hot or cold liquids to ensure an accurate reading.
Tympanic, or ear, thermometers measure infrared heat waves emitted from the eardrum. To use an ear thermometer, gently pull the earlobe up and back for adults, or down and back for children under three years old, to straighten the ear canal. Insert the thermometer tip into the ear opening, aiming towards the eardrum, and hold it until it signals the reading is complete, always using a new, disposable probe cover for each use.
Temporal artery thermometers, used on the forehead, measure the heat waves from the temporal artery, a blood vessel just beneath the skin. Place the sensor on the center of the forehead and, for some models, gently slide it across to the hairline, often finishing with a touch behind the ear. Other models may require holding the thermometer towards the center of the forehead at a specified distance, following the manufacturer’s specific instructions.
Accuracy and Appropriate Use for Different Ages
Rectal temperature is the most precise method for determining core body temperature, particularly for infants up to three months of age. Oral temperatures are 0.5 to 1.0 degree Fahrenheit (0.3 to 0.6 degrees Celsius) lower than rectal readings. Ear (tympanic) temperatures can be similar to rectal readings when taken correctly, but earwax or improper positioning affects them. Forehead (temporal) temperatures are also lower than oral temperatures by about 0.5 to 1.0 degree Fahrenheit (0.3 to 0.6 degrees Celsius).
In children aged three months to four years, rectal temperature remains the preferred choice, though temporal artery thermometers can also provide accurate readings. For children six months and older, ear thermometers may be used, but proper placement is necessary for accuracy. By age four and older, most children can cooperate for oral temperature measurement, which is a convenient and accurate option.
Normal Ranges and When to Seek Medical Attention
Normal core body temperature ranges from 97°F (36.1°C) to 99°F (37.2°C), with an average of 98.6°F (37°C). Individual temperatures can vary slightly based on factors like age, time of day, and activity level. A temperature reading above this normal range indicates a fever, while a temperature below 95°F (35°C) suggests hypothermia.
A low-grade fever is a temperature between 99.6°F (37.6°C) and 100.3°F (37.9°C). A significant fever is 100.4°F (38°C) or higher, particularly when measured rectally. For infants under three months, any rectal temperature of 100.4°F (38°C) or higher warrants immediate medical attention. For children aged three to twelve months, seek medical evaluation if the fever is higher than 102°F (38.9°C) or lasts longer than 24 hours. In older children and adults, a fever of 103°F (39.4°C) or higher, or a fever accompanied by symptoms such as a stiff neck, severe headache, confusion, persistent vomiting, or difficulty breathing, should prompt a call to a healthcare provider or a visit to an emergency room.