Progression to COPD in smokers with normal spirometry/low DLCO using different methods to determine normal levels

We thank Drs Quanjer and Miller for their commentaries to our recently published manuscript in the European Respiratory Journal [1]. Our manuscript describes a follow-up study of pulmonary function tests (PFTs) in two groups of healthy smokers with normal post-bronchodilator spirometry and total lung capacity (forced expiratory volume in 1s (FEV1), forced vital capacity (FVC) and total lung capacity (TLC) greater than or equal to 80% predicted and FEV1/FVC >0.7, as defined by the Global Initiative for Chronic Obstructive Lung Disease (GOLD) initiative) [2-5]. The smokers in one group had normal spirometry and normal diffusion capacity of the lung for CO (DLCO) defined as greater than or equal to 80% pred ("normal spirometry/normal DLCO", n=59) and the smokers in the other group had normal spirometry but low DLCO (