Fixed ratio or lower limit of normal for the FEV₁ /VC ratio: relation to symptoms and extended lung function tests

There is no general agreement on the spirometric definition of chronic obstructive pulmonary disease (COPD). The global initiative for obstructive lung disease recommends a fixed ratio between forced expiratory volume in one-second (FEV₁ ) and forced vital capacity (FVC) of <0·7 (FR) for the diagnosis of COPD. European Respiratory Society and American Thoracic Society favour the use of the fifth percentile of the age-related FEV₁ /FVC ratio (the lower limit of normal, LLN). The purpose of this study was to analyse extensive lung function tests in groups of subjects fulfilling none, either or both of the spirometric criteria for COPD. From a previous population-based study, 450 subjects were examined with spirometry, body pletysmography diffusing capacity for CO (D_L,CO ), Impulse Oscillometry System (IOS) and answered a questionnaire on respiratory symptoms and diseases. Seventy subjects fulfilled both spirometric COPD criteria (FR+LLN+), 62 subjects the fixed ratio criterion (FR+) only. Of the remaining 318 subjects, 236 were ever smokers (N-ES). Significant differences between all groups were seen for FEV₁ and D_L,CO . Significant differences between groups were also seen for residual volume (RV) and RV/total lung capacity. For IOS, variables and symptoms increasingly abnormal values were seen from never smokers to FR+LLN+. This study shows that subjects meeting both spirometric COPD criteria frequently have symptoms and findings at extended lung function tests compatible with the diagnosis. Also subjects meeting the fixed ratio criterion only tend to have more symptoms and lung function findings compatible with COPD than ever-smoking subjects with FEV₁ /VC > 0·7.