Lung volumes by computed tomography and plethysmography: are we measuring the same? CanCOLD study data.

Background: Measurement of lung volumes forms an integral part of pulmonary function testing. Lung volumes determined by computed tomography (CT) scans are well established, but the comparability to other methods like plethysmography in large cohorts remains in question. Methods: CanCOLD is a prospective longitudinal cohort study from Canada, including three matched groups of individuals with COPD I-II, smokers at risk and healthy controls. All participants underwent lung volume measurement by plethysmography and CT, using inspiratory and expiratory imaging. We compared total lung capacity (TLC) and residual volume (RV) in the different cohorts between plethysmography and CT. Results: Data from 1235 (518 females) individuals were analysed. Baseline characteristics were comparable in all three groups. Significant differences between CT and plethysmography could be observed in all groups, with consistently higher TLC and lower RV by plethysmography, respectively. Correlation was strong for TLC between the methods of measurement with a very stable bias of about 1.68 L for all groups, but the correlation was only low/moderate for RV. Variability of differences seemed to be higher for RV. No correction for supine position or dead airspace was used for CT-based measurements. Conclusion: Measurement of TLC and RV by plethysmography yields higher and lower values than by CT, respectively, so results of the different methods should not be used interchangeably.