A breath analysis device into which a user exhales a breath sample is capable of venting an initial portion of the breath sample from the device, and routing a second portion of the breath sample into a disposable cartridge containing an interactant. The device may include a sensor, such as a pressure sensor, for detecting the initiation of exhalation, and may include a controller that switches a valve during the exhalation process to route a desired portion of the breath sample into the cartridge. After the exhalation process, an LED/photodiode arrangement, or another type of optical sensor, may be used to measure a color change produced by a chemical reaction in the cartridge, to thereby measure a concentration of a ketone or other analyte in the breath sample.
