Determination of (CO2)-C-13/(CO2)-C-12 ratio by IRMS and NDIRS

Breath tests using C-13-labelled substrates require the measurement of (CO2 )-C-13/(CO2)-C-12 ratio in breath gas samples. Next to isotope ratio mass s pectrometry (IRMS), which is very sensitive but also complex and expensive, alternatively isotope selective nondispersive infrared spectrometry (NDIRS ) can be used to determine the (CO2)-C-13/(CO2)-C-12 ratio in expired breat h. In this study we compared NDIRS- with IRM S-results to investigate wheth er the less expensive and very simply to operate NDIRS works as reliable as IRMS. For this purpose we applicated 1-C-13-Phenylalanine to patients with advanced liver cirrhosis and healthy volunteers and took duplicated breath samples for IRMS and NDIRS at selected time points. Our data show a good c orrelation between these two methods for a small number of samples as requi red for simple breath tests. Longer series, where repeated measurements are required on the NDIRS instrument lead to a decreasing correlation. This in dicates the superiority of IRMS concerning (CO2)-C-13-kinetics over longer time periods.