Background: COPD is a chronic disease traditionally associated with increased symptoms as lung function deteriorates. Follow-up times in previous cohort studies were limited to a few years. Interestingly, newer longer observational studies show a more comprehensive picture on disease progression over time. Therefore, the question on the relevancy of the follow-up time in cohort studies remains open. Methods: The ON-SINT study is an observational, retrospective, nationwide, real-life cohort study, in which patients diagnosed with COPD were recruited between December 2011 and April 2013 by primary care (PC) and secondary care (SC) physicians. Patients were evaluated at the inclusion visit and at the initial visit when the diagnosis of COPD was first established. Distribution of lung function decline over the years was studied comparing those cases with longer follow-up times, with the median of the distribution as the cutoff point. Results: The sample included 1214 patients of which 857 (70.6%) were recruited by PC and 357 (29.4%) by SC physicians. Median follow-up time was 6.26 years. Mean annual change in the complete cohort were -4.5 (222) ml year-1 for FVC and 5.5 (134) ml year-1 for FEV1. We confirm the variable distribution of FEV1 decline and found that longer follow-up periods reduce this variability. Of note, FEV1 decline was different between groups (shorter: 19.7 [180.4] vs longer: -9.7 [46.9]; p = 0.018). Further, our data revealed differences in the clinical presentation according to follow-up times, with special emphasis on dyspnea (OR: 1.035; 95%CI: 1.014-1.056), exacerbations (OR 1.172; 95%CI 1.045-1.315) and CAT scores (OR 1.047; 95%CI 1.019-1.075) being associated with longer follow-up times. Conclusions: This study describes the impact of follow-up periods on lung function variability, and reveals differences in clinical presentation according to follow-up times, with special emphasis on dyspnea, exacerbations and CAT scores.
Keywords: COPD; FEV1 decline; clinical presentation; cohort; progression.