Rectal: The Gold Standard for Accuracy
For infants and young children (typically under 3 years old), the rectal method is considered the gold standard for measuring core body temperature with the highest accuracy. This is because it provides a direct measurement of the internal body temperature, which is the most reliable indicator of a true fever. This method requires a digital thermometer specifically for rectal use. While some may be hesitant about this method, it is safe when done correctly and can be the most crucial way to determine a fever in a very young child. For infants under three months, any rectal temperature of 100.4°F (38°C) or higher is considered a medical emergency and requires immediate medical attention.
How to take a rectal temperature:
- Prepare: Use a clean, digital thermometer specifically marked for rectal use. Apply a small amount of petroleum jelly or water-based lubricant to the tip.
- Position: Place your child belly-down on your lap or a firm surface. For an older child, have them lie on their side with one knee bent.
- Insert: Gently insert the lubricated tip about ½ to 1 inch into the anus. Do not force it if you feel resistance.
- Hold and Wait: Hold your child still and keep the thermometer in place until it beeps, usually within 10-20 seconds. Never leave the thermometer unattended while in the rectum.
- Read and Clean: Remove the thermometer, read the result, and clean it thoroughly with soap and water or rubbing alcohol before storing.
Temporal Artery (Forehead): The Convenient Alternative
Temporal artery thermometers, or forehead thermometers, measure the infrared heat from the temporal artery on the forehead. They are very quick, non-invasive, and convenient for all ages, and are often used in clinical settings. This method is the next most accurate after rectal, especially when used consistently. However, factors like sweating or being outside in the cold can affect the reading.
How to use a temporal artery thermometer:
- Prepare: Ensure the forehead is clean, dry, and not in direct sunlight. If the person has been sweating, wait 10-15 minutes or take the reading behind the earlobe.
- Position: Place the sensor on the center of the forehead.
- Scan: Press and hold the scan button, slowly sliding the thermometer across the forehead to the hairline near the top of the ear.
- Read: Release the button and read the temperature displayed on the screen.
Oral (Mouth): For Older Children and Adults
Oral temperatures are a reliable method for cooperative individuals over the age of 4 or 5. It is important to wait at least 15 to 30 minutes after eating, drinking, or smoking to ensure an accurate reading. Oral temperatures are generally slightly lower than rectal readings.
How to take an oral temperature:
- Prepare: Use a clean, digital thermometer. Wait at least 15 minutes after eating or drinking.
- Position: Place the tip of the thermometer under the tongue, towards the back on one side.
- Wait: Close the lips firmly around the thermometer and wait until it beeps.
- Read and Clean: Read the temperature and clean the thermometer after use.
Axillary (Armpit) and Tympanic (Ear): Less Reliable Options
Axillary, or armpit, temperatures are often used as a preliminary screen for fever but are the least accurate method. The reading can be affected by ambient temperature and is typically 0.5°F to 1°F lower than an oral reading. If a fever is suspected based on an axillary reading, it should be confirmed with a more accurate method.
Tympanic, or ear, thermometers measure infrared heat from the eardrum and are a fast option. However, they are not recommended for infants under 6 months old and can be less accurate than other methods due to inconsistent placement and earwax buildup.
Comparison of Thermometer Accuracy
| Method | Best for | Accuracy | Pros | Cons |
|---|---|---|---|---|
| Rectal | Infants and young children (under 3 years) | Highest | Most reliable measure of core temperature. | Invasive, requires cooperation, less convenient. |
| Temporal Artery (Forehead) | All ages, especially 3 months and older | High (if used consistently) | Non-invasive, quick, easy to use. | Reading can be affected by sweat or external temperature. |
| Oral | Cooperative children (over 4) and adults | Good (if done correctly) | Convenient and widely accessible. | Requires cooperation; affected by recent food/drink/smoking. |
| Tympanic (Ear) | Children over 6 months | Moderate | Very fast reading. | Can be less accurate due to placement or earwax. |
| Axillary (Armpit) | Screening for all ages | Lowest | Non-invasive, simple. | Least reliable; should be confirmed with another method. |
Important Considerations for Accurate Readings
Regardless of the method chosen, consistency and proper technique are essential. Avoid mixing different measurement methods when tracking a fever, and always inform your healthcare provider which method was used, as normal temperature ranges vary by site. Keep in mind that a single reading is less important than the overall trend. If a child's temperament changes or they develop concerning symptoms, always consult a healthcare professional regardless of the specific temperature reading. For more information on general fever guidelines, you can visit the CommonSpirit Health blog at https://www.commonspirit.org/blog/what-is-considered-a-fever.
Conclusion: Choosing the Right Method
While rectal remains the most accurate measure of core body temperature, the best method for you depends on age, comfort level, and the situation. For infants under three months, rectal is non-negotiable, while for older children and adults, a consistently used temporal or oral thermometer can provide reliable results. The key takeaway is to choose a method, stick with it, and learn what a normal baseline temperature looks like for that person and device. This understanding, combined with observation of other symptoms, provides the most accurate picture of health.
