Bronchiectasis is a chronic respiratory disease characterised by abnormal dilatation of bronchi, which presents clinically with cough, sputum production and recurrent infection [1]. Although bronchiectasis had been regarded as an "orphan" disease [2], recent studies have shown that the prevalence of bronchiectasis is increasing and this disease causes a significant burden on public health, including increased healthcare costs, hospital admission and mortality [3][4][5].Data on the prevalence of bronchiectasis and bronchiectasis-related comorbidities are relevant, since comorbidities are important factors for predicting the risk of mortality in patients with bronchiectasis [6]. However, epidemiological data on the prevalence of bronchiectasis remain limited, especially in Asian populations. Furthermore, only a few studies have included a comprehensive evaluation of the prevalence of bronchiectasis-related comorbidities among Asians [1,7]. Thus, in the present study, the overall prevalence of bronchiectasis and associated comorbidities were investigated using a representative sample of national health insurance claims data in South Korea.To identify patients with bronchiectasis and investigate their comorbidities, data were used from the 2012-2017 Health Insurance Review and Assessment Service, National Patient Sample (HIRA-NPS), which is nationally representative and open to the public for research purposes [8]. The HIRA-NPS data are cross-sectional and composed of health insurance claim records during the year. The database includes approximately 1 400 000 individuals each year drawn by 3% stratified random sampling by age and sex from the entire population who had claims records during the year. It also provides information on healthcare costs, composed of payer's amounts and patient's out-of-pocket costs. South Korea has a government-run mandatory national health security system; 97% of the population is enrolled in the National Health Insurance and 3% in Medical Aid programmes [9].

There are limited data regarding whether mortality is higher in patients with non cystic fibrosis bronchiectasis (bronchiectasis) than in those without bronchiectasis. Using 2005–2015 data from the Korean National Health Insurance Service, we evaluated hazard ratio (HR) for all-cause mortality in the bronchiectasis cohort relative to the matched cohort. The effect of comorbidities over the study period on the relative mortality was also assessed. All-cause mortality was significantly higher in the bronchiectasis cohort than in the matched cohort (2505/100,000 vs 2142/100,000 person-years, respectively; P < 0.001). Mortality risk was 1.15-fold greater in the bronchiectasis cohort than in the matched cohort (95% confidence interval [CI] 1.09–1.22); mortality was greatest among elderly patients (HR = 1.17, 95% CI 1.10–1.25) and men (HR = 1.19, 95% CI 1.10–1.29). Comorbidities over the study period significantly increased the risk of death in the bronchiectasis cohort relative to the matched cohort: asthma (adjusted HR = 1.20, 95% CI 1.11–1.30), chronic obstructive pulmonary disease (adjusted HR = 1.24, 95% CI 1.15–1.34), pneumonia (adjusted HR = 1.50, 95% CI 1.39–1.63), lung cancer (adjusted HR = 1.85, 95% CI 1.61–2.12), and cardiovascular disease (adjusted HR = 1.34, 95% CI 1.23–1.45). In contrast, there were no significant differences in the risk of death in patients without bronchiectasis-related comorbidities and the matched cohort, except in the case of non-tuberculous mycobacterial infection. In conclusion, all-cause mortality was higher in patients with bronchiectasis cohort than those without bronchiectasis, especially in elderly patients and men. Comorbidities over the study period played a major role in increasing mortality in patients with bronchiectasis relative to those without bronchiectasis.