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

References from the Global Lung Function Initiative (GLI) are widely used to interpret children's spirometry results. We assessed fit for healthy schoolchildren.

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 (FEV₁), forced vital capacity (FVC), FEV₁/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.

We analysed data from 2036 children with valid FEV₁ 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 FEV₁/FVC z-scores (mean±sd -0.09±1.02), but not for FEV₁ (-0.62±0.98), FVC (-0.60±0.98) and FEF_25-75 (-0.54±1.02). Mean FEV₁, 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). FEV₁, FVC and FEF_25-75 z-scores depended on both age and height (p-interaction 0.033, 0.019 and <0.001, respectively).

GLI-based FEV₁, FVC, and FEF_25-75 z-scores do not fit White Swiss adolescents well. This should be considered when using reference equations for clinical decision-making, research and international comparison.