What is the Most Accurate Way to Take a Temperature?

When an immune system is fighting an infection whether viral, bacterial, or otherwise, the result is an elevated body temperature.

In the realm of Covid-19, the CDC has recommended monitoring an individual for signs of elevated body temperature, to gauge the possibility of being positive for the disease.

Taking a temperature is the simplest measure available.

What is a Fever?SmartXcan Alternate E

A fever is when a rectal, ear or temporal artery temperature reaches 100.4ºF (38ºC) or higher. If you are using an oral or oral pacifier thermometer, many doctors would say that a fever is also over 100.4ºF (38ºC), and a fever in a young patient to be an oral temperature of 100ºF or 37.8ºC. Talk to your doctor about what temperature range they consider a fever, and about the recommended treatment. Using the axillary (armpit) temperature, a fever is present if the reading is 99.4ºF or higher (over 37.2ºC), but others consider 99.0ºF to be a fever. Keep in mind that in systemic autoinflammatory diseases, it is not just a fever. With the rise in temperature comes inflammation in various parts of the body. You or your child may “only” be at 99.5 degrees Fahrenheit but feel miserable. Therefore, your symptoms and how you or your child feels need to be considered when making treatment decisions. If you are on prescribed medications such as NSAIDS, your body may not manifest a high temperature, a “fever”, but you may still be symptomatic.

Which Temperature Taking Method Should You Use?

The key to deciding how to take a temperature is to choose a method that you and/or your doctor are most comfortable with and use it every time. You want to be consistent when taking yours or your child’s temperature. Switching thermometer types and trying to compare different methods each time you take a temperature will only get confusing. Use the same type of measurement every time and get to know what a normal temperature range is for you or your child when not in a flare.

Certain household thermometers use batteries, so it is important to be aware that if it is starting to be inconsistent with readings, it may be time to change the battery. Replace the thermometer it no longer works right. You need a reliable, accurate and consistent method of taking temperatures.

Medical research has not determined an exact correlation between oral, rectal, ear, armpit, and forehead temperature measurements. Generally, the correlation of temperature results are as follows:

The average normal oral temperature is 98.6°F (37°C).
A rectal temperature is 0.5°F (0.3°C) to 1°F (0.6°C) higher than an oral temperature.
An ear (tympanic) temperature is 0.5°F (0.3°C) to 1°F (0.6°C) higher than an oral temperature.
An armpit (axillary) temperature is usually 0.5°F (0.3°C) to 1°F (0.6°C) lower than an oral temperature.
A forehead (temporal) scanner is usually 0.5°F (0.3°C) to 1°F (0.6°C) lower than an oral temperature.
Reference: Jennifer Tousseau (author), Karen Durrant RN (editor).

It is important to note that the inner ear, the sinus cavity including eye duct area and along your forehead; are the best touch point areas, as well as under your tongue; to obtain an accurate fever measurement. Rectal measurements are also accurate as an inner body core measurement and is still widely used in pediatrics.

Temp Measurement Ear

Oral thermometers have been the medical standard for years. Placing a thermometer under the tongue and leaving it in place for 3-5 minutes yields consistent measured values very accurately. Eating and drinking can affect the accuracy when taking an oral temperature. It is recommended that one wait 15 to 30 minutes after eating and drinking before taking an oral temperature. Because of concerns with breakage and mercury, public health facilities do not use these types any longer. Rectal thermometers are also not utilized, as tears and injuries frequently can occur in the youngest and oldest patients. Doctors’ offices typically use a digital thermometer that fits into your ear.

Studies show ear thermometer temperatures are comparable to rectal temperatures. Tympanic (ear) thermometers readings average about the same as rectal. The temperature is taken in both ears and the highest reading is used, unless you or your child has been laying on that ear. Several things can affect the reading in the ear including laying on one ear, an ear infection, wax build up, and angle of the thermometer. For some individuals, this method is not the best. According to the Mayo Clinic, those with a “small, curved ear canal” will have inaccurate readings. Temporal artery thermometers, another type of thermometer, provide readings that average about the same as rectal and ear thermometers in some studies, but lower in others. These thermometers seem to have the most inconsistencies in how they compare to other methods.

When monitoring daily temperature for your symptom journal, it is best to take the baseline temperature at the same time of day, each day, generally in the morning. You can take it more often, and at other times of the day as needed, but consistency is the key for daily monitoring and between flares. If you get a high or low reading, re-take the temperature! It may be correct, but it is best to confirm the temperature if it is unusual. The doctor will ask you if you took it again to verify it, as there are sometimes errors with the thermometers, or the techniques used. You want to have the most accurate information for your daily symptom logs. If you need to take it more than once to get an accurate reading, or repeat it again later, make sure to record these too. Most of the time, you will not need take your temperature more than once a day if no disease flares or symptoms are present, but you may need to do more frequent temperatures on days with disease flares.

What if the Temperature Changes?

It is normal in any of these methods to get various readings, even when taking the temperature multiple times within just a few minutes. Our body temperature normally fluctuates, and you may see a difference of a full degree or more in just a few minutes or in the case of ear thermometers, between the left and right ears. This seems to happen often for some with fever disorders while in a flare. Know that this is normal, but if you are not confident that you are getting an accurate reading, let’s look at another method.

Thermal Imaging.

Why Thermal Imaging for elevated body temperature measuring?

Here’s how thermal imaging works: A special lens focuses the infrared light emitted by all of the objects in view. The focused light is scanned by a phased array of infrared-detector elements. The detector elements create a very detailed temperature pattern called a thermogram.

Everything Emits Light, Even if You Can’t See It

With science, you don’t always get what you want. But if you try sometimes, you get something even better. That is what happened to William Herschel in 1800. While testing some light filters, Herschel used a prism to split sunlight into its component hues. Then he set up some thermometers. He knew that light falling on an object would warm it up, but he wanted to measure the effects of each color separately.

Then he noticed something strange: A thermometer at the end, beyond the red color — one that was not even in the light — also warmed up. What the heck? Of course, the reason was that there was light hitting that thermometer, you just couldn’t see it with human eyes. That was the discovery of what we now call infrared light.

Alternate E Source TI pyramid

Most of the light is invisible, so you need an infrared camera for that. It’s basically just like a  digital camera, but instead of having a sensor that detects visible wavelengths, this sensor can “see” infrared wavelengths. Seriously, these sensor things are awesome.

How Accurate is Thermal Imaging?


Germanium for Thermal Imaging

Germanium is number 32 on the elemental chart and has amazing properties that lends itself to being used for thermal imaging. Because germanium is transparent in the infrared wavelengths, it is an important infrared optical material that can be readily cut and polished into lenses and windows. It is especially used as the front optic in thermal imaging cameras working in the 8 to 14 micron for passive thermal imaging and for hot-spot detection in military, mobile night vision, and firefighting applications.

The major end uses for germanium in 2007, worldwide, were estimated to be: 35% for fiber-optics, 30% infrared optics, with remaining in electronics and solar electric applications.

We can provide an extremely accurate body temperature measure by using Thermal Imaging technology however we need to be cognizant of where we take the measurement or “scan”.

We are not taking a temperature with a thermometer, we are taking a scan or looking at the thermal image feedback to detect an EBT, elevated body temperature; even in any early onset of fever. The eye duct region and forehead sinus area are the key area to scan for the best detection of interior body heat, so this is the best area to capture.

Not just Face Value

Consistency is really a key parameter that will affect accuracy with this methodology as distance from the scan interface can vary when using a handheld device. It is impossible to gauge for accuracy one person is 5 inches away vs 12 inches away and so on.

Industrial style thermal imaging guns which are handheld are not suitable for this application. There are designed to sense industrial heat activity or for firefighting detecting smolder, and thermal hotspots within a wall.

The thermal image capture with such an industrial handheld device will offer detection however it may be off target by a good percentage, so not suitable when looking for an early onset of elevated body temperature.

The Thermal Imaging scan must be designed specifically for the application of a human measurement, as opposed to wooden studs and a sheetrock wall.

Theory behind EBT

Experts have confirmed that there is a correlation between the eye canthus temperature (tear duct area) and core temperature. The tear duct is a small area and you need to put utilize enough pixels in the image to make a good measurement. The best results to capture EBT will always be consistency: same distance for the scan, best image quality (resolution), enhanced process for speed and focus on the sinus cavity -forehead area for an accurate thermal capture.

How does AI to help in measurement accuracy?

Adding “smarts” to a scan with the thermal imaging sensors assists in deriving the most repeatable accuracy using a reliable database. Advancements in technology now allow for machine learning so the more thermal images are taken and processed, the more accurate an algorithm can calculate the result. More importantly this type of processing allows for speed in the transaction, so multiple persons can be accurately scanned for early fever detection in short order. When you consider a large public venue, transportation hub, Broadway play; a brief stop for a thermal image facial scan makes sense. It is not perfect, but it could be a quick, inexpensive, noninvasive way of surveying large groups of people and, at the least, flagging those with a higher probability of being infected for follow-up questioning.

Sure, an oral thermometer is accurate. But can you imagine stopping each person outside a grocery store to stick a one in under their tongues? How do we make it better?

Accurate Temp Checking during COVID

As we are in a time that accurate temperature taking and fever detection will assume a more prominent role, it does not necessarily mean that all persons with a fever have COVID-19. Maybe they have an ordinary cold or flu. It is important to know that monitoring for EBT, elevated body temperature is a screen; not a diagnostic test. From a public safety viewpoint, a fast and safe accurate thermal image scan when going to the ball game makes the most sense, so those that do test positive can be directed to proper healthcare intervention.


Jennifer Tousseau (author) & Karen Durrant RN (editor) September 13, 2013 “What is the Most Accurate Way to Take a Temperature”: saidsupport.org

Royal College of Physicians April 7 2020 The RPC and COVID-19: https://www.rcpjournals.org

Wikipedia: https://en.wikipedia.org/wiki/Germanium

Wikipedia: https://en.wikipedia.org/wiki/Wien%27s_displacement_law

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