Kim G. Nielsen

Introduction Multiple breath washout (MBW) is used for early detection of cystic fibrosis (CF) lung disease, with SF 6 MBW commonly viewed as the reference method. The use of N 2 MBW in infants and toddlers has been questioned for technical and physiological reasons, but a new correction of the N 2 signal has minimized the technical part. The present study aimed to assess the remaining differences and the contributing mechanisms for the differences between SF 6 and N 2 MBW,corrected—such as tidal volume reduction during N 2 washout with pure O 2 . Method This was a longitudinal multicenter cohort study. SF 6 MBW and N 2 MBW were performed prospectively at three CF centers in the same visits on 154 test occasions across 62 children with CF (mean age: 22.7 months). Offline analysis using identical algorithms to the commercially available program provided outcomes of N 2,original and N 2,corrected for comparison with SF 6 MBW. Results Mean functional residual capacity, FRC N2,corrected was 14.3% lower than FRC N2, original , and 1.0% different from FRC SF6 . Lung clearance index, LCI N2,corrected was 25.2% lower than LCI N2,original , and 7.3% higher than LCI SF6 . Mean (SD) tidal volume decreased significantly during N 2 MBW corrected , compared to SF 6 MBW (−13.1 ml [−30.7; 4.6], p < 0.0001, equal to −12.0% [−25.7; 1.73]), but this tidal volume reduction did not correlate to the differences between LCI N2,corrected and LCI SF6 . The absolute differences in LCI increased significantly with higher LCI SF6 (0.63/LCI SF6 ) and (0.23/LCI SF6 ), respectively, for N 2,original and N 2,corrected , but the relative differences were stable across disease severity for N 2,corrected , but not for N 2,original . Conclusion Only minor residual differences between FRC N2,corrected and FRC SF6 remained to show that the two methods measure gas volumes very similar in this age range. Small differences in LCI were found. Tidal volume reduction during N 2 MBW did not affect differences. The corrected N 2 MBW can now be used with confidence in young children with CF, although not interchangeably with SF 6 .