Total lung capacity: single breath methane dilution versus plethysmography in normals

Methane is an inert tracer gas used to obtain TLC estimates during single breath diffusion capacity (DL(CO)) measurements. The aim of this study was to assess the accuracy of methane dilution TLC in normal subjects undergoing single breath diffusion capacity measurements using plethysmography as the gold standard comparison method.

Fifty non-smoking adults underwent lung function testing. Total lung volume was obtained by both plethysmography and methane dilution during a single breath DL(CO) measurement. Deming regression and the concordance correlation coefficient, r(ccc), were used to determine agreement between methods for TLC. Bias was the mean difference between methods and limits of agreement were the mean difference between methods +/- 1.96 (SD). All values are mean +/- SD unless otherwise stated.

Plethysmography and methane dilution TLC values were not significantly different. The r(ccc) was 0.87 (95% confidence interval (CI) 0.78-0.92). Deming regression revealed a slope of 0.93 (P = 0.17, H(o): beta = 1.0; 95% CI 0.84-1.03) and a y-intercept of 0.20 (P = 0.39, H(o): alpha = 0; 95% CI -0.27-0.70). The bias was 0.11 L favouring plethysmography. Limits of agreement varied as 0.11 +/- 0.92 L.

There is statistical agreement between methods suggesting the average TLC by methane could substitute for plethysmography in normals at the population level. At the individual level, a normal methane dilution value indicates a normal TLC whereas values below the normal range should be validated using plethysmography.