Variability of the computerized single‐breath nitrogen washout test in healthy adults. Results from a field survey in a French rural area

Abstract

Summary. Within‐subject (difference between paired tests, or coefficient of variation [c. var.] of three consecutive measurements) and between‐subjects (standard deviation of the mean group) variability were assessed for the variables derived from the single‐breath nitrogen (SBN 2 ) test in a group of healthy, asymptomatic subjects ( n = 289) aged 20–64 years, from a rural area in North‐East France. Duplicate measurements in 99 subjects showed excellent agreement between the two attempts (correlation coefficients between 0.94 and 0.98) for static lung volumes and the alveolar N 2 slope (PIII); closing volumes (CV) were more variable ( r = 0.77 for absolute value, 0.79 for % VC) while closing capacities (CC) were in intermediate position ( r = 0.86 for absolute value, and 0.88 for % TLC). The variability of three consecutive measures in 190 subjects showed the coefficients of variation to be low for static volumes and closing capacity (2.4–7.4%) and higher for PIII (15.1%) and CV or CV7VC (18.2 and 17.8%). The analysis of variance did not detect significant differences between the three sets of measurements, with the exception of a progressive increase of vital capacity (VC), and decrease in residual volume (RV) from test 1 to test 3, the total lung capacity being the same. Variability was uninfluenced by age or sex, except a higher c. var. for female VC as compared to males (2.8 vs. 2.2%, P = 0.02) and a higher variability of TLC (2.9 vs. 2.3%, P = 0.03) and VC (2.8 vs. 1.9%, P = 0.003) in people older than 40 years as opposed to those younger than 40 years.