Evaluation of the Global Lung Function Initiative reference equations in Belgian adults

Over the past decade, the Global Lung Function Initiative (GLI) Network has published all-age reference equations on spirometry, diffusing capacity of the lung for carbon monoxide (D _LCO) and lung volumes.

We evaluated the appropriateness of these equations in an adult Caucasian population. Retrospective lung function data on subjects who performed tests prior to a diagnostic sleep investigation were analysed. From the medical records, lung healthy, lifetime nonsmoking, nonobese subjects were selected, resulting in a population of 1311 subjects (68% male; age range 18-88 years).

Multiple linear regression analysis revealed that lung function z-scores did not differ between subjects with and without sleep apnoea but did depend on height and age. The average forced expiratory volume in 1 s (FEV₁)/forced vital capacity (FVC) z-score was 0 but exhibited an inverse association with height in both sexes (p<0.01). Values of FEV₁ and FVC in both sexes were larger than predicted (mean±sd z-score +0.30±0.96 or 104±13% pred; p<0.01). Overall, static lung volumes and D _LCO were adequately predicted. However, D _LCO z-scores were inversely associated with height in males and age in females (p<0.01). For all lung function indices, the observed scatter was reduced compared with the prediction. Therefore, for all indices <5% of the data were below the GLI-proposed lower limit of normal (LLN) threshold.

GLI reference equations provide an adequate fit in Belgian adults. However, the GLI-proposed LLN is too low for our Antwerp population, resulting in underdiagnosis of disease. Furthermore, airway obstruction and diffusion disorders might be misclassified due to height and age associations.