Thermistors, which are also used in medical-grade thermometers, can’t directly measure a user’s body temperature while inside a smartphone, but they can be used to track the amount of heat energy that has been transferred between a user and the mobile device they’re making contact with.
During a clinical trial at the University of Washington’s School of Medicine’s Emergency Department, the app was tested by 37 participants, which included 16 with a mild fever, and the results were compared against readings from an oral thermometer. FeverPhone was able to predict a user’s core body temperature with “an average error of about 0.41 degrees Fahrenheit (0.23 degrees Celsius),“ which is on par with the accuracy of home use thermometers, including non-contact options.
It does sound quite good, especially seeing no extra hardware is required. The app is not yet available though for download, and it also sounds like other devices may also be explored e.g. holding a Fitbit to your forehead.