A system and method for sensing and measuring ammonia in a breath sample comprises a sampling means for capturing and directing a breath sample from a subject to an ammonia sensor, the ammonia sensor comprising a conducting polymer polyaniline sensor (e.g. PANI-silver composite, PANI-carbon nanotube composite). The sampling means (120) comprises a breath sample capture chamber (125), the chamber having an inlet and outlet, the inlet having a first valve through which a breath is exhaled into the sample capture chamber, the outlet having a second valve through which breath surplus to the volume of the chamber is expelled, to provide capture of a breath sample of predefined volume. The system may be used as a point of care testing device. A sensor comprising a membrane layer for controlling the humidity response of the sensor is also shown.
