A system for certifying a detection of a gaseous substance present in air exhaled by an individual. The system includes a sensor providing a measurement of the quantity of the gaseous substance, a portable terminal connected to the sensor and having a camera for taking an image of the individual. The terminal has a module for authenticating the individual and the sensor used for the exhaled air sample by analyzing the data of images using facial recognition and by analyzing a graphic marker for the sensor. The terminal produces an item of data representative of the quantity of gaseous substance measured in the sample and a piece of authenticated identification data of the individual who has exhaled the sample. The system can at the same time authenticate the identity of the individual, authenticate the sensor used, and associate the result of the value of the test with the individual.
