What’s the Difference Between a Rectal Thermometer and a Regular Thermometer?

Reviewed on 9/9/2021
difference between rectal thermometer and oral thermometer
Temperature readings vary depending on the type of thermometer used. Learn about the pros and cons of rectal vs. oral thermometers

Temperature readings vary depending on the type of thermometer used, and it’s important to get accurate temperature to determine whether a fever is present. Keeping a working thermometer in your home is recommended for a quick fever diagnosis. But which type of thermometer is best?

Differences between rectal thermometers and oral thermometers are in the bulb shape, color coding, and temperature indicators:

Bulb shape

  • Since rectal tissues are fragile and easily ripped, rectal thermometers have a short, blunt, slightly rounded tip that reduces the risk of a rectal puncture or tear.
  • Oral thermometers have longer, thinner tips as a long and slender bulb exposes as much mercury surface area as possible. Exposing a larger surface area aids in obtaining an accurate temperature more quickly.

Color coding

  • Rectal thermometers are red, whereas oral thermometers are blue.
  • This color difference helps prevent a rectal thermometer from being mistaken for an oral thermometer, preventing the spread of fecal bacteria into the mouth.

Temperature indicators

  • Rectal and oral thermometers have different lines or indicators showing normal temperature, and usually differ by 1 degree:
    • The temperature indicator in a rectal thermometer should be about 99.6°.
    • The temperature indicator in an oral thermometer should be about 98.6°.
  • Thermometers that are dedicated to specific uses make sure to note these differences, allowing the user to obtain the most accurate readings.

What are different types of thermometers?

You can take your temperature using oral, rectal, axillary, or tympanic, or temporal thermometers. 

The average normal oral temperature is 98.6 degrees F (37 degrees C):

  • Rectal (anus) temperature is generally 0.5-1 degrees F higher than oral temperature. 
  • Axillary (armpit) temperature is usually 0.5-1 degrees F lower than oral temperature. 
  • Tympanic (ear) temperature is 0.5-1 degrees F higher than oral temperature.
  • Temporal (forehead) temperature is 0.5-1 degrees F lower than oral temperature.

Rectal thermometers can be used to measure oral and axillary temperatures, but due to significant differences in their shapes, oral and axillary thermometers should not be used to measure rectal temperature. 

Any thermometer that has been in the rectum, regardless of the design, should not be used to measure oral temperature due to the danger of spreading harmful bacteria.

What are the pros and cons of different thermometers?

Rectal temperature readings: High accuracy

  • Many doctors recommend using a rectal thermometer for children aged 3 and younger because it gives the most accurate readings.
  • To use, dab petroleum jelly on the thermometer's bulb. Place your baby on their stomach on your lap, a bed or a changing table and gently insert the bulb half to one inch into your child's rectum. With two fingers, hold the thermometer loosely in place until it beeps.
  • Pro: Best choice for kids up to 3 years of age or whenever an exact reading is critical.
  • Con: May be uncomfortable for older children.

Oral temperature readings: Good accuracy

  • In children over 3, using an oral thermometer can also provide fairly accurate temperature readings, as long as the child keeps their mouth closed for 2-3 minutes and refrains from drinking hot or cold liquids 15-20 minutes before the thermometer is inserted.
  • Pro: Easier to use in kids over age 4 and doesn’t require removing clothing. 
  • Con: Reading may be thrown off if your child doesn't keep the thermometer under their tongue until it beeps.

Axillary temperature readings: Adequate accuracy

  • Temperature readings taken in the armpit are the least accurate. However, if your child refuses to allow you to insert a thermometer anywhere else, using a mercury thermometer under the arm for 4-5 minutes can provide you with reasonably accurate results. For higher accuracy, many doctors recommend adding 1 degree F to the reading.
  • Pro: Very easy to use.
  • Con: May get a false reading if the room temperature is too cold.

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What are ear and forehead thermometers?

Ear thermometers

Ear thermometers, or tympanic thermometers, work quickly but are typically costly and may be inaccurate if you don't insert them precisely into your child's ear canal or if there is a buildup of earwax. Since this type of thermometer is typically small and has an ear canal probe at the end, the tip may be difficult to insert in a baby’s ear.

Here are tips for using a tympanic thermometer correctly:

  • Pull your child's ear slightly up and backward before inserting for a more precise fit.
  • Remove earwax before inserting.

Forehead thermometers

Forehead thermometers detect the temperature of the temporal artery over the forehead. Just swipe the thermometer across your baby’s forehead, and you will get a temperature reading that is displayed on the device. 

Many studies claim that this is the second best way to measure temperature in babies (the best being rectal).

What to do for best results

If you're using a digital thermometer, keep it in place for at least 30 seconds. To get the best results, avoid eating or drinking anything for at least 20 minutes before taking your temperature. When purchasing a new thermometer, make sure to read the package to learn how to use it.

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References
What is the difference between an oral thermometer and a rectal thermometer? https://brooksidepress.org/vitalsigns/lessons/lesson-2-temperature/2-15-what-is-the-difference-between-an-oral-thermometer-and-a-rectal-thermometer/

Comparison of Oral and Rectal Temperatures during Work in Heat: https://journals.physiology.org/doi/pdf/10.1152/jappl.1956.8.4.406

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