Assessment of hyperinflation: comparison of Global Lung Function Initiative and European Community for Steel and Coal lung volume reference equations

Background Assessment of static hyperinflation severity is crucial to identify COPD patients eligible for lung volume reduction. The current recommendation of residual volume ≥175% predicted may need to be reconsidered owing to potential differences between the Global Lung Function Initiative (GLI) and the European Community for Steel and Coal (ECSC) reference equations for residual volume and concerns about using percentage of predicted. Methods We compared the residual volume reference values derived from the GLI and ECSC equations using mathematically simulated data and used a receiver operating characteristic curve to establish a new GLI-derived z-score cut-off for residual volume using body plethysmography data from patients with severe COPD. Results The GLI reference equation for residual volume consistently yields a lower predicted residual volume for individuals with an average or below-average height (females ≤163 cm and males ≤177 cm). Our clinical cohort consisted of 1011 patients with COPD (graded using the Global Initiative for Chronic Obstructive Lung Disease (GOLD) criteria as 38% GOLD 3 and 59% GOLD 4). In this cohort, a GLI-derived residual volume z-score of ≥2.9 could accurately replace the ECSC-derived 175% predicted cut-off and a z-score of ≥3.5 was established for the 200% predicted cut-off. Conclusion There are substantial differences in predicted residual volume values between the GLI and ECSC equations, with the GLI generally yielding a lower predicted residual volume in the majority of individuals. A GLI-derived residual volume z-score of ≥2.9 could be used to replace the currently used cut-off of ≥175% predicted to identify potential lung volume reduction candidates.