Age and body mass index affect fit of spirometry Global Lung Function Initiative references in schoolchildren

BACKGROUND: References from the Global Lung Function Initiative (GLI) are widely used to interpret children's spirometry results. We assessed fit for healthy schoolchildren. METHODS: LuftiBus in the School was a population-based cross-sectional study undertaken in 2013-2016 in the canton of Zurich, Switzerland. Parents and their children aged 6-17 years answered questionnaires about respiratory symptoms and lifestyle. Children underwent spirometry in a mobile lung function lab. We calculated GLI-based z-scores for forced expiratory volume in 1 s (FEV$_{1}$), forced vital capacity (FVC), FEV$_{1}$/FVC and forced expiratory flow for 25-75% of FVC (FEF$_{25-75}$) for healthy White participants. We defined appropriate fit to GLI references by mean values between +0.5 and -0.5 z-scores. We assessed whether fit varied by age, body mass index, height and sex using linear regression models. RESULTS: We analysed data from 2036 children with valid FEV$_{1}$ measurements, of whom 1762 also had valid FVC measurements. The median age was 12.2 years. Fit was appropriate for children aged 6-11 years for all indices. In adolescents aged 12-17 years, fit was appropriate for FEV$_{1}$/FVC z-scores (mean±sd -0.09±1.02), but not for FEV$_{1}$ (-0.62±0.98), FVC (-0.60±0.98) and FEF$_{25-75}$ (-0.54±1.02). Mean FEV$_{1}$, FVC and FEF$_{25-75}$ z-scores fitted better in children considered overweight (-0.25, -0.13 and -0.38, respectively) than normal weight (-0.55, -0.50 and -0.55, respectively; p-trend <0.001, 0.014 and <0.001, respectively). FEV$_{1}$, FVC and FEF$_{25-75}$ z-scores depended on both age and height (p-interaction 0.033, 0.019 and