The BIOMED I programme Stable Isotopes in Gastroenterology and Nutrition (SIGN) has focused upon evaluation and standardization of stable isotope breath tests using13 C labelled substrates.The programme dealt with comparison of13 C substrates, test meals, test conditions, analysis techniques, and calculation procedures. Analytical techniques applied for13 CO2 analysis evaluated by taking an inventory of instrumentation, calibration protocols, and analysis procedures. Two ring tests were initiated measuring13 C abundances of carbonate materials.
Evaluating the data it was found that seven different models of isotope ratio mass spectrometers (IRMS) were used by the participants applying both the dual inlet system and the continuous flow configuration.Eight different brands of certified13 C reference materials were used with a13 C abundance varying from delta13 CPDB -37.2 to +2.0 [per mil]. CO2 was liberated from certified material by three techniques and different working standards were used varying from -47.4 to +0.4 [per mil] in their delta13 CPDB value.
The standard deviations (SDs) found for all measurements by all participants were 0.25 [per mil] and 0.50 [per mil] for two carbonates used in the ring tests. The individual variation for the single participants varied from 0.02 [per mil] (dual inlet system) to 0.14 [per mil] (continuous flow system). The measurement of the difference between two carbonates showed a SD of 0.33 [per mil] calculated for all participants. Internal precision of IRMS as indicated by the specifications of the different instrument suppliers is <0.05 [per mil] for dual inlet systems and <0.3 [per mil] for continuous flow systems. In this respect it can be concluded that all participants are working well within the instrument specifications even including sample preparation. Increased overall interlaboratory variation is therefore likely to be due to non-instrumental conditions. It is possible that consistent differences in sample handling leading to isotope fractionation are the causes for interlaboratory variation. Breath analysis does not require sample preparation. As such, interlaboratory variation will be less than observed for the carbonate samples and within the range indicated as internal precision for continuous flow instruments. From this it is concluded that pure analytical interlaboratory variation is acceptable despite the many differences in instrumentation and analytical protocols. Coordinated metabolic studies appear possible, in which different European laboratories perform13 CO2 analysis. Evaluation of compatibility of the analytical systems remains advisable, however.