Bar represents median, comparison by Mann-Whitney test. *p<0.01. S66 THE GLI SPIROMETRY REFERENCE EQUATIONS INFLUENCE THE APPARENT RATE OF DECLINE IN FEV1 AMONG CHILDREN AND ADOLESCENTS WITH CYSTIC

Background In patients with cystic fibrosis (CF), interpretation of cross-sectional FEV1 data is greatly influenced by choice of spirometry reference equation, particularly during childhood (Stanojevic; J Cyst Fibros 2014). We hypothesised that availabil-ity of the Global Lung Function Initiative (GLI) spirometry refer-ence equations (Quanjer; ERJ 2012) will also affect the apparent rate of decline in lung function over time, thereby potentially altering our understanding of disease progression in CF. Methods Data were extracted from two patient registries: the UK CF Registry (n = 6043 subjects; 20,013 test occasions over a period of 5 years) and the Toronto CF database (n = 1023 subjects; 27,868 test occasions over a period of 23 years). Spiro-metric outcomes were interpreted using%predicted FEV1 calcu-lated from GLI, Knudson (as currently used by the UK CF Registry), and Wang-Hankinson (as used by the US CF Founda