This Small Business Innovation Research (SBIR) Phase II project proposes to develop a carbon isotope ratio analyzer based on Off-Axis Integrated Cavity Output Spectroscopy to measure the ratio of the isotopic abundances of 13C to 12C in exhaled breath. The compact analyzer will serve as a medical diagnostic instrument and will operate in a point-of-care setting. The instrument combines robust telecommunications-grade diode lasers with Off-Axis ICOS, an innovative technology that provides extremely long optical paths (several kilometers typical) for ultrahigh sensitivity. The instrument will be inexpensive, portable and easy to use and report measurements of 13CO2/12CO2 with sufficient sensitivity and precision to replace mass spectrometry in 13C-labeled breath tests for diagnosis of several diseases. Prior Phase I work has successfully demonstrated a laboratory instrument with a precision of 0.24 per mil (0.024%) in less than 6 minutes. In Phase II, a prototype instrument capable of autonomous operation, will be developed and tested in on-going clinical trials. <br/><br/>The commercial impact of the project will be significant, as the proposed instrument will aid in quick diagnosis of gastrointestinal diseases at the doctor's office, thereby enhancing rates of patients' compliance with treatment regimens.