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Multiple breath washout (MBW) is an inert gas washout test that measures ventilation distribution inhomogeneity. Convection dependent ‘Scond’ and diffusion-convection dependent ‘Sacin’ inhomogeneity indices are derived from the progression of concentration normalised phase III slopes (SnIII) in MBW and are used to reflect conductive and acinar airway impairment. There is limited evidence on the influence of alternative methods for deriving Scond from MBW tests in children across a wide age range.Aim To compare different protocols for deriving Scond in children.Methods Mass spectrometer (AMIS 2000) SF6-MBWs in healthy children and children with Cystic Fibrosis (CF) were reanalysed using in-house LabView® software. Children with advanced lung disease (lung clearance index ≥10) were excluded. Scond was derived as the linear increase in the SnIII between 1.5 and 6 turnovers. The SnIII were modified for each breath by eye. Three protocols were used for deriving Scond: A) all breaths with acceptable SnIII pooled across 3 runs as suggested by the ERS/ATS consensus statement (Robinson, et al ERJ 2013), (B) all breaths with acceptable SnIII, mean of 3 runs, similar to commercial software (ExhalyzerD® Eco Medics AG) and C) all breaths irrespective of SnIII quality, mean of 3 runs. ULN was defined as +1.96SD.Results MBW tests in 100 children (3–17 yrs, 50 with CF) were reanalysed. Scond was similar using protocol A and B in health (mean±SD 0.020±0.010 vs 0.021±0.010) and in CF (0.059±0.030 vs 0.059±0.030). Bland-altman analysis found a low bias (0.0003) and narrow limits of agreement (LOA), -0.0024 to 0.0029) between A and B. Scond was higher using B than C in CF (0.059±0.030 vs 0.053±0.034, p<0.05), but not in health (0.020±0.010 vs 0.018±0.016). Wide LOA were seen between B and C, -0.024 to 0.031. Proportion of children with CF and abnormal Scond was higher with B than C, 70% vs 42%. Intra-test repeatability was lower for B than C, 0.009±0.005 vs 0.023±0.023, p<0.05 in health. A similar result was found for CF. ( Figure 1)Abstract P54 Figure 1Six plots of SnIII vs turnover (TO), corresponding to three trials of a MBW (trial 1 in green (a), trial 2 in blue (b), trial 3 in orange (c)) and two different protocols (B and C) used for deriving Scond. The three plots on the top row are using Protocol B and the three on the bottom are using Protocol C; the latter includes yellow markers/data points to represent the inclusion of poor quality SnIII/breathsConclusion Different protocols for deriving Scond produce different results from the same MBW. The process of rejecting poor quality SnIII is essential for producing reliable Scond in children.