Classification of COPD into different GOLD stages is based on forced expiratory volume in 1？s (FEV1) and forced vital capacity (FVC) but has shown to be of limited value. The aim of the study was to relate spirometry values to more advanced measures of lung function in COPD patients compared to healthy smokers. The lung function of 65 COPD patients and 34 healthy smokers was investigated using flow-volume spirometry, body plethysmography, single breath helium dilution with CO-diffusion, and impulse oscillometry. All lung function parameters, measured by body plethysmography, CO-diffusion, and impulse oscillometry, were increasingly affected through increasing GOLD stage but did not correlate with FEV1 within any GOLD stage. In contrast, they correlated fairly well with FVC%p, FEV1/FVC, and inspiratory capacity. Residual volume (RV) measured by body plethysmography increased through GOLD stages, while RV measured by helium dilution decreased. The difference between these RV provided valuable additional information and correlated with most other lung function parameters measured by body plethysmography and CO-diffusion. Airway resistance measured by body plethysmography and impulse oscillometry correlated within COPD stages. Different lung function parameters are of importance in COPD, and a thorough patient characterization is important to understand the disease. 1. Introduction Spirometry and body plethysmography are the most commonly used methods to diagnose, characterize, and assess chronic pulmonary obstructive disease (COPD). The global initiative of obstructive lung diseases (GOLD) classification of COPD [1] is acknowledged around the globe and is recommended both by the American Thoracic Society and the European Respiratory Society. It has long been based on spirometry and health status alone. However, a new version from 2011 proposes the importance of considering exacerbation frequency and assessing the severity of breathlessness, using the modified Medical Research Council Questionnaire (mMRC), in the classification of COPD. For practical purposes, flow-volume spirometry is used to characterize lung function in COPD patients. It is easily used, and the measurements derive reproducible data. Forced expiratory volume in 1？s (FEV1) is most commonly used but is of limited value in relation to functional ability and quality of life when used alone [2, 3]. On the other hand, spirometry also provides data of forced vital capacity (FVC) and inspiratory capacity (IC) which are the tools of choice for most population surveys. It has long been known that
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