A noninvasive apparatus and method for measuring a subject's end tidal carbon monoxide concentration is disclosed, featuring the ability to (i) determine carbon monoxide concentration on a breath-by-breath basis, (ii) determine mean carbon monoxide concentration by averaging local carbon monoxide values, (iii) avoid the premature determination that an end tidal phase is over, and (iv) determine when breath variability or system variability will likely preclude accurate testing. The disclosed device and method allow for more robust determination of end tidal carbon monoxide concentration in subjects with turbulent or irregular breath patterns.