Prognostic value of long-term trajectory linearity in rapid lung function decline

Background Rapid lung function decline has been identified as a risk factor for poor prognosis. However, among rapid decliners, the prognostic significance of the linearity of lung function trajectory remains unclear. Methods We used the Ansan–Ansung cohort, a population-based prospective cohort study. We analysed trajectory patterns using consecutive lung function measurements over 12 years. Rapid lung function decline was defined as the annual forced expiratory volume in 1 s decline of >60 mL·year −1 . Linearity was quantified as the absolute difference between the areas above and below the straight line connecting the first and last lung function measurements (absolute deviation area). Trajectories were classified as nonlinear when this value exceeded the highest quartile and linear when it was below. The outcome was all-cause mortality, and the risk was assessed using multivariable Cox proportional hazards regression analysis. Results 4809 individuals were followed-up for a median of 8.8 years, and 278 (5.8%) individuals died. Rapid decliners with linear trajectory had an increased risk of mortality compared with non-rapid decliners with linear trajectory (adjusted hazard ratio (aHR) 1.74, 95% CI 1.22–2.46). However, among individuals with nonlinear trajectory, the mortality risk was not significantly increased, even among rapid decliners (aHR 0.99, 95% CI 0.59–1.68). Conclusion Among rapid decliners, linear trajectory was associated with a higher mortality risk. Trajectory linearity may therefore be a useful prognostic marker in this population.