Two spirometry criteria have been proposed for early chronic obstructive pulmonary disease (COPD) in young smokers: 1) forced expiratory volume in 1 s (FEV₁)/forced vital capacity (FVC) < the lower limit of normal (LLN), and 2) FEV₁ decline ≥60 ml/year. These criteria have yet to be validated. This study explored clinical factors associated with these two spirometry criteria.

This retrospective study analysed medical check-up data from 13,010 consecutive subjects aged <50 years who underwent current and 3 previous spirometry tests in Japan. Current ≥10 pack-year smokers were the main focus of analysis; those meeting one or more spirometry criteria were diagnosed with early COPD. Early COPD was categorized into three subtypes: FEV₁/FVC < LLN and FEV₁ decline <60 ml/year (type 1), FEV₁/FVC ≥ LLN and FEV₁ decline ≥60 ml/year (type 2), and FEV₁/FVC < LLN and FEV₁ decline ≥60 ml/year (type 3).

Of the 1579 current ≥ 10 pack-year smokers, 488 (30.9%) met the early COPD criteria. Multivariate multinomial logistic models adjusted for age, sex, height, body mass index (BMI) and smoking history indicated that past BMI increase and low exercise were associated with higher type 2 early COPD incidence (odds ratio (OR) [95% confidence interval (CI)] = 4.30 [3.10, 6.04], and 0.80 [0.69, 0.93], respectively) but not with higher type 1 incidence. A history of asthma was associated with higher type 3 incidence (OR [95% CI] = 1.98 [1.18, 3.07]).

The 3 types of spirometry-based early COPD have different clinical factors. Their trajectories should be explored in longitudinal studies.