A system is provided for sensing an analyte in breath of a user. The system comprises a base； a breath input operatively coupled to the base that receives the breath； a cartridge coupled to the base and in fluid communication with the breath input to receive the breath, wherein the cartridge comprises an interactant subsystem that is selected to undergo a reaction with the analyte when the analyte is present in the breath and to undergo an optical change corresponding to the reaction； and an optical subsystem coupled to the base and configured to sense the optical change, wherein the optical subsystem generates an output comprising information about the analyte in response to the optical detection.
