A pediatrician was entering her hospital during the early stages of the recent novel Coronavirus (COVID-19) epidemic and noticed that as part of the government-required employee entrance screening procedure the security guards were starting to set up an infrared technology screening camera. She was curious because her impression was that the technologies had some benefit but weren’t more widely used because of technical issues when used in the field. “I wonder how good they actually are at detecting a fever or more importantly missing one?” she asked herself.
The purpose of measuring temperature is to monitor the health of the individual and evaluate for a potentially abnormal physiological state such an ambient hyper/hypothermia or infectious disease. Rectal and esophageal temperature are considered the two most reliable measures for the gold standard of core body temperature. Oral, axillary and inguinal temperatures are common alternative measurement sites but do underestimate core body temperature. These sites often use digital/electronic temperature measurements and contact with the patient’s skin which is not helpful for high volume screening locations. Another option is a temperature measuring gun like a tympanic thermometer.
Infrared thermal imaging (ITI) uses infrared data collection taken from an appropriate body location that then uses a mathematical formula to convert the data into a temperature reading. There is no specific data correlating core body temperature to ITI temperature.
There are not too many ways to screen people for fever that don’t take a lot of time or that are not invasive. ITI has been used for remote sensing for human body temperature and fever screening since 2006 in international airports and is used in other locations such as hospital entrances and public places for crowd surveillance. While ITI can be accessible and affordable and efficient, it may not be used properly. ITI can be the first step in detection and therefore evaluation and management of individuals.
“Mass health screening during a pandemic will certainly be influenced by several other factors, including perceived and actual pandemic severity, as well as the potential consequences of illness detection, either negative or positive, which can affect the sensitivity of screening that uses self-report. If being detected as febrile is perceived as harmful, travelers may hide their symptoms. Alternative, during a pandemic with high mortality rates, incentives for reporting symptoms might be present such as access to scarce antiviral medication and medical care. In both situations, a comprehensive screening approach may be necessary, which uses [ITI] for fever screening and a health questionnaire to detect other symptoms or exposures that would increase specificity of the screening process. Finally, the usefulness of any infectious disease screening must take into account temperature fluctuations, use of antipyretic medication, transmission risks, prevalence of infections and asymptomatic infections.”
A 2015 study of ITI in hospitalized children found that screening of either eye (best location, see below) produced consistent results for children. ITI was also consistent with axillary temperatures for children who were febrile or nonfebrile, but forehead and tympanic measurements were not consistent with axillary temperatures. The error of the temperature measured by ITI was up to 0.4°C though, which makes it less sensitive for detection of true fever.
One study of emergency room patients in 3 different US locations using 3 different ITI technologies, found that asking patients if they felt febrile had a sensitivity of only 75% for fever detection, specificity was 84.7% and positive predictive value was 10.1% when compared to oral temperature. The ITI technologies (2 of them) did have higher correlations with oral temperature than self-report though. They also found that there may be gender differences where men had higher ITI temperatures than women which they felt may be because of facial hair, cosmetic use or subcutaneous fat composition.
Thermal imaging in medicine includes evaluation and management of inflammatory disease, regional pain syndrome, Raynaud’s phenomenon, and general temperature or disease monitoring in at risk populations (e.g. neonates). ITI is used in many other applications such as inspections in the power industry, manufacturing and material and biological research applications.
The International Organization for Standardization has documents for standard use of infrared thermal imaging for fever screening.
The standards for use include:
The optimal place to measure is the inner canthus of the human eye. The measurement area needs to be at least 9 pixels (pixel is .26 mm) in size. “…[T]he subject should be stationary and close to the camera without angular distortion to optimize the target site, ruling out the common practice of distant surveillance of a moving crowd of subjects.” The camera is best mounted close to the subject, at eye level and the face needs to fill the majority of the ITI image. The camera needs to have clear focus and resolution. The camera needs to be calibrated properly based on the manufacturer’s recommendations which can be several times a day depending on the specific technology used. Calibration is necessary as ambient air temperatures and relative humidity can change the skin surface temperature. An appropriate fever threshold also needs to be determined for what is considered a significant temperature difference A higher threshold has the advantage of detecting fewer false positive fevers, but may have more false negative results.
For mass screening, the appropriate facial area (i.e. inner canthus) has to be recognized by the technologies which can be difficult to do but is improving with facial recognition technology. Glass and plastic also do not transmit infrared, so wearing eyeglasses can also potentially interfere with the appropriate testing.
It is possible to have a proven fever without an increase in facial temperature.
Technologies improve every day. Therefore use of ITI in specific locations or for specific reasons hopefully has improved as some of the data above is older.
Questions for Further Discussion
1. What are the elements that you consider when thinking about the accuracy of temperature measurements?
2. What temperature measurements do you use and how are they affected by your local environment?
3. What are causes of fever? A review can be found here
4. How good are masks for preventing infection? A review can be found here
- Symptom/Presentation: Health Maintenance and Disease Prevention
- Specialty: Preventive Medicine and Health Maintenance
- Age: Young Adult
Evidence-based medicine information on this topic can be found at SearchingPediatrics.com – Infrared Thermography, and SearchingPediatrics.com – COVID-19 and the Cochrane Database of Systematic Reviews.
Mercer J, Ring EFJ. Fever screening and infrared thermal imaging: Concerns and guidelines. Thermology. 2009;19:67-69.
Nguyen AV, Cohen NJ, Lipman H, et al. Comparison of 3 infrared thermal detection systems and self-report for mass fever screening. Emerg Infect Dis. 2010;16(11):1710-1717. doi:10.3201/eid1611.100703
Ring EFJ, Ammer K. Infrared thermal imaging in medicine. Physiol Meas. 2012;33(3):R33-46. doi:10.1088/0967-3334/33/3/R33
Ring EFJ, Jung A, Kalicki B, ZUBER J, RUSTECKA A, Vardasca R. New standards for fever screening with thermal imaging systemss. J Mech Med Biol. 2013;13:1350045. doi:10.1142/S0219519413500450
Dwith Chenna YN, Ghassemi P, Pfefer TJ, Casamento J, Wang Q. Free-Form Deformation Approach for Registration of Visible and Infrared Facial Images in Fever Screening. Sensors. 2018;18(1). doi:10.3390/s18010125
Topalidou A, Ali N, Sekulic S, Downe S. Thermal imaging applications in neonatal care: a scoping review. BMC Pregnancy Childbirth. 2019;19(1). doi:10.1186/s12884-019-2533-y
How infrared thermography screens body temperature – cnTechPost. https://cntechpost.com/2020/02/15/how-infrared-thermography-screens-body-temperature/. Accessed April 3, 2020.
For COVID-19 Coronavirus, How Well Do Thermometer Guns Even Work? https://www.forbes.com/sites/brucelee/2020/02/16/for-covad-19-coronavirus-how-well-do-thermometer-guns-even-work/#65dce7a32af9. Accessed April 3, 2020.
Donna M. D’Alessandro, MD
Professor of Pediatrics, University of Iowa