A system for sensing and measuring ammonia in a breath sample is described. The system includes a sampling means for capturing and directing a breath sample from a subject to an ammonia sensor, the ammonia sensor including a conducting polymer polyaniline sensor. The sampling means includes a breath sample capture chamber, 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.