The purpose of this proposal is to evaluate the ANDROS infrared heterodyne ratiometer (IHR) in the context of a state-of-the-art gas isotope radio mass spectrometry (GIRMS) laboratory dedicated to an on-going clinical program for the application of 13C-labeled compounds to research and diagnosis. This side-by-side comprison will allow us to establish: 1. The relative sensitivity, precision and accuracy of IHR vs GIRMS measurements. 2. The ability of IHR to detect baseline variations of respiratory 13C abundance in fasting patients, to detect the change in abundance induced by meals with different natural abundances of 13C, and finally to detect the excretion of 13CO2 generated from the administration of 13C labeled fats, carbohydrates, amino acids and protein. These values will allow us to construct a figure of merit for the IHR, relating its precision and accuracy to the limiting changes occurring in the patient 13C signal. This permits the costs of 13CO2 breath test analyzed by this system to be compared with existing GIRMS system costs. The same figure of merit will be used to calculate the manner in which the clinical discrimination capability of 13CO2 breath tests is affected by instrument performance. This will be carried out using newly developed compartmental analysis techniques. Finally, the clinical convenience, reliability and suitability of the ANDROS system for routine 13CO2 breath tests will be established in daily operations. The results of this assessment will be fully reported to both clinical and instrumental audiences by publications in the appropriate journals.