Breath test methods and apparatus for increasing accuracy and reducing the time taken to achieve diagnostically useful results. In order to determine when an increase in isotopic ratio of the exhaled breath is clinically significant, methods are described for the use of a variable and multiple threshold level; for reducing the time taken to determine an accurate baseline level; and for avoiding the effects of oral activity when making measurements. To increase measurement accuracy, methods are described, using the results of the breath tests themselves, of continuous and automatic self-calibration to correct for drifts in the gas spectrometer absorption curves. A method for increasing the spectral stability of cold cathode discharge infra-red light sources for use in breath test instrumentation is described. Calibration checking devices and methods of mandating their use at regular time intervals are described, to ensure maintenance of the accuracy of breath tests.