In our earlier discussion of the science behind greenhouse gases we pointed out that all objects radiate electromagnetic radiation, doing so at a peak wavelength dependent upon their surface temperatures. That means two things. One is that things at the usual temperatures in our world are radiating EM radiation at wavelengths characteristic of the far infrared region. The other is that by measuring the intensity of infrared you can also measure the surface temperature of the body without touching it. Commercial devices are touted as highly accurate. Clinicians use them to measure body “core temperatures” by sticking an infrared sensor in your ear and measuring the infrared (IR) coming off your eardrum. Public health officials have talked about using the same technique at airports or other public places to detect people with fevers as part of trying to stop the spread of infectious diseases. Think of screening passengers remotely (and perhaps covertly) as they embark on a plane. Those with fevers could be pulled aside for a more thorough look. Sounds like a good idea, right? But how good are these devices for this purpose, i.e., detecting a person with a fever by measuring the temperature on some easily visualized body surface like the forehead? A team of French researchers set out to find out.
An IR thermometer (Raynger MX; Raytek, Berlin, Germany) was used to measure forehead temperature on 2026 patients seen in the emergency room of a large Paris teaching hospital and compared to their eardrum (tympanic membrane) temperature, used as a reference or “gold standard.” Here’s what the device looks like in this application:
57% were men and 43% women. Average age was 46 but the age range went from 6 to 103 years old. The average IR skin temp was 36.7°C [“normal” body temperature is 37°C or 98.6°F.], exactly the same as the average tympanic membrane temperature. So far so good. But the same average temperatures doesn’t mean that the skin and tympanic membrane temps matched for individuals. In this case they didn’t:
Correlation between cutaneous and tympanic measurements was poor, and the infrared thermometer underestimated body temperature at low values and overestimated it at high values. Multiple regression analysis showed that 3 variables (tympanic temperature, outdoor temperature, and age) were significantly (p<0.001) and independently correlated with the magnitude of the difference between cutaneous and tympanic measurements. (Hausfater P, Zhao Y, Defrenne S, Bonnet P, Riou B. Cutaneous infrared thermometry for detecting febrile patients. Emerg Infect Dis. 2008 Aug; [Epub ahead of print])
Here’s a scatter plot of the two measurements, the IR on the vertical axis and the tympanic membrane on the horizontal axis. The lack of correlation is pretty dramatic:
The bottom line is that this kind of remote fever sensing had poor positive predictive value, meaning that the proportion of people correctly identified as having fever was low, ranging from 10% to 16%. Thus there were a lot of false positives. Negative predictive value, the proportion of people classified by the IR device as not having fever who in fact did not have fever was high (97% to 99%), so not many people with fevers will be missed with the IR device. Predictive values depend not only on the accuracy of the device but also how prevalent fever is in the screened population. In the early days of a pandemic, fever prevalence will be very low, leading to low positive predictive value. The false positives produced at airport security would make the days of only taking off your shoes look good.
The idea of airport fever screening to keep a pandemic out has a lot of psychological appeal. Unfortunately its benefits are also only psychological: pandemic preparedness theater. There’s no magic bullet for warding off a pandemic. The best way to prepare for a pandemic or any other health threat is to have a robust and resilient public health infrastructure.
I think I’ve said that before. Well I’m saying it again.