Home testing of temperature, is it reliable? What is normal temperature?

Everyone knows normal body temperature is 98.6°F or 37°C… or is it? Our understanding of normal body temperature and fever has changed a bit since 37°C was declared normal body temperature. The value may be a bit different depending on where you take the temperature, i.e., orally, rectally, in the ear, etc. and what thermometer you use. Given this confusion, what is the best type of thermometer to use for at-home body temperature monitoring?

Normal body temperature, redefined

The body regulates its own temperature as much as possible through sophisticated thermoregulatory centre in the front of the hypothalamus in the brain. Body temperature varies considerably among different people and even in the same person during the course of a normal day. In fact, it is considered completely normal for body temperature to change by as much as 0.6°C (1.0°F) during the course of the day. 

Researchers measured body temperature using oral thermometers in 148 healthy men and women between the ages of 80 and 40. Temperatures ranged from 35.6°C (96.0°F) to 38.2°C (100.8°F) with an average of 36.8 ± 0.4°C (98.2 ± 0.7°F). Based on this data, the researchers declared that the upper limit of normal body temperature should be 37.2°C or 98.9°F in the early morning and 37.7°C or 99.9°F when randomly measured. Anything above this, according to these researchers, should be considered a fever.

Body temperature changes depending on where you measure it

As you probably realise, the surface of the skin is cooler than the centre of the heart, rectum, or brain. Therefore, measuring body temperature by placing a probe on the skin will not give the same reading as you would obtain by placing a thermometer into the rectum.

• Rectal temperatures are about 0.6°C (1.0°F) higher than oral readings at any given time.
• Tympanic (ear) temperatures are also about 0.6°C (1.0°F) higher than oral readings, the range of readings is wider for measurements taken in the ear. Tympanic measurements can vary by up to 2°C.
• Axillary (armpit) temperatures are 1.0°C (1.8°F) lower than oral temperatures.
• Forehead (skin) temperatures are 1.0°C (1.8°F) lower than oral temperatures and readings can vary widely.

What is the best way to measure body temperature?

For adults, a perfectly valid way to measure body temperature is to compare your oral thermometer reading with “normal” body temperature readings taken orally. In other words, an oral thermometer is certainly good enough for diagnosing a fever at home if you use oral temperatures as the measure of normal body temperature.

For young children who cannot use an oral thermometer, rectal temperature measurement is still the most accurate method. Tympanic membrane (ear) temperature varies too widely to be useful or accurate, especially for home use, where correct technique is critical to obtaining accurate results. 

Newer devices allow measurement of body temperature using infrared energy from a device that does not contact the skin. Infrared digital forehead thermometers measure surface temperature of the skin. The results from these devices have been mixed, but mostly negative. While infrared digital forehead thermometers can measure temperature within 2 seconds and are completely non-invasive (even non-contact), the measurements they provide vary widely enough that readings may not be reliable.

What is the best device to take temperature?

Mercury thermometers are certainly accurate and reliable, but they are no longer the best method for measuring temperature. Mercury is encased in glass, which can break, and the mercury itself can be toxic when handled.

Devices that use infrared energy to measure temperature are the newest devices on the market and parents love them because they can take their children's temperature without the need for rectal thermometry. Unfortunately, neither the infrared tympanic (ear) thermometers nor the digital infrared forehead thermometers are very precise. Not only do they measure skin temperature rather than oral or rectal temperature, but their readings can vary widely - up to 2°C (3.6°F) or more.

The best device for measuring body temperature and diagnosing fever in the home is an electronic digital thermometer

These devices can provide a temperature measurement relatively quickly, beep when they have stabilised on a reading, and automatically shut off when not in use. These devices need to be thoroughly cleaned between uses to minimise the risk of passing infection from one user to another. It also makes good sense to have separate devices for oral and rectal thermometry.

The best technique for taking temperature at home

It is reasonable to use the amended cutoffs (37.7°C or 99.9°F) for diagnosing fever at home using an oral thermometer. When a person's temperature exceeds this value, then the person has a fever. However, some people may not be satisfied with using standard values for fever when temperature can vary from person to person and within the same person over time. If someone truly wants to take the most accurate temperature at home, a little background work is required. Because several measurements are required, we suggest using this approach only if you are an adult and you can measure temperature orally.

Firstly

First, start by obtaining a high quality, battery-powered, digital electric thermometer that can take temperature measurements both rapidly and accurately. The device should be waterproof (at least the end used to perform measurements should be watertight) and should be able to spend up to repeated washings with soap, water, and alcohol.

Next

When the person is feeling healthy, obtain and record several oral temperature measurements throughout the day. Temperature be should be taken soon after waking, in the midmorning, after dinner, and before bed. Ideally, one would also measure body temperature once in the middle of the night, but this may be more hassle than many people are willing to endure.

Remember, temperature can change rather significantly in women when they are menstruating or ovulating, so readings taken during these times should not be used as normal values.These temperature measurements will give you an indication of your personal, afebrile (non-fever) body temperature. If your body temperature ever exceeds 1.0°C (1.8°F) beyond your personal normal, then you very likely have a fever, regardless of the absolute temperature.

References

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