Rapid and accurate 13CO2 isotopic measurement in whole blood: comparison with expired gas.

Determination of 13CO2 enrichment on the CO2 released from blood by acid has been used in situations in which breath sampling is difficult. This method can be improved by measuring this enrichment on the CO2 spontaneously released from blood. Therefore, simultaneous comparisons of 13CO2 content between breath and arterialized blood added with or without acid were performed in 51 samples from human studies, using the statistical method of Bland and Altman (J. M. Bland and D. G. Altman. Lancet 1: 307-310, 1986). Strong relationships exist between the methods (r > 0.99) expressed in atom percent excess (APE). Compared with breath, the acid method overestimates the 13CO2 enrichment (0.318 +/- 0.632 APE x 1,000, P < 0.001). The acid-free method shows similar enrichments to breath (0. 003 +/- 0.522 APE x 1,000, P = 0.97) with good precision and degree of agreement (95% confidence interval 0.15 APE x 1,000). The analysis can be performed up to 5 days after sampling with a good reproducibility. In conclusion, measuring 13CO2 enrichments on the CO2 spontaneously released from blood is feasible, gives identical results to the breath method, and lessens operator manipulations. It allows study of situations in which the breath sampling method is not feasible.