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Introduction Airway dead space, as we measure it, is the volume between the airway opening and the numerous airway-alveolar interfaces of the lung. The multiple breath washout (MBW) derived lung clearance index (LCI) is influenced by both series (airway; VDaw) and parallel dead space volume (VD). The Fowler equal area method (VDawF) is the best-known derivation method for VDaw. The Langley (VDawL) and Cumming (VDawC) methods are derived from the cumulative expired gas vs 40–80% of expired volume using linear or quadratic regression. There are very few published data (Tang et al. BJA 2007; 98(6), 828–834) comparing the methods and no studies have included children.Aim To compare three different methods for deriving VDaw in health and disease.Methods Approvals were obtained for re-analysis of SF6 MBW mass spectrometer tests collected in 37 healthy adolescents (≥12 years) and 37 adolescents with Cystic Fibrosis (CF). Using the SF6 signal, VDawF, VDawL, and VDawC were derived and compared.Results Repeated ANOVA revealed significant differences between the methods in both health and CF (p<0.001). Pairwise comparisons showed a significant mean difference between VDawC and VDawF (-0.014L, p<0.0001), VDawC and VDawL (-0.011L, p<0.0001) and VDawF and VDawL (0.003L, p=0.012) in health. Similar results were also observed in CF but VDawF and VDawL did not differ. The difference between VDawL and VDawC (VDawL – VDawC) correlated with the normalised phase III slope of the first breath and the LCI, r = 0.75 ( figure 1) and 0.69 respectively (both p<0.001). The same relationships were not observed with VDawF – VDawC.Abstract P55 Figure 1Correlation plot of the difference between VDaw derived using the Langley method and VDaw derived using the Cumming method (VDawL - VdawC) vs the normalised phase III slope of the first breath (SnIII*VT). The blue circles are healthy adolescents, and the red are those with CFConclusions VDaw measured by expired gas concentration is a moving object as different results are produced depending on the method of its derivation. VDawC is smaller than VDawL and VDawF. MBW operators reliant on commercial MBW software, which reports VDawF and VDawL, should acknowledge that VDawL is sensitive to changes in the shape of the expirogram and thus, ventilation inhomogeneity.