Inflammatory bowel disease (IBD) is a disease with chronic inflammation in large bowel, which consists of Crohn's disease (CD) and Ulcerative colitis (UC). The incidence rate of IBD is the highest in Western countries, but the number of IBD cases is rising in Asia. Although IBD is believed to be associated with urbanization, such mechanism is not fully understood. Korea has a national health insurance system covering over 97% of the population. This study aims to find out the regional difference related to urbanization as well as the secular trends in IBD incidence by age and sex in Korea.

We extracted all claim data of IBD patients in National Health Insurance Database (NHID) from 2002 to 2015, which was defined as those who had main diagnostic code of Crohn's disease (CD) [K50] or Ulcerative colitis (UC) [K51] and have been prescribed relevant drugs simultaneously. We excluded patients who were diagnosed as IBD patients between 2002 and 2003 to select incident cases only. Annual age-standardized incidence rate was calculated as the number of incident IBD cases divided by the number of population who were registered in NHIS in the same year and stratified by age, sex, and region (metropolitan area or non-metropolitan area). Joinpoint regression was used for statistical analysis to compute annual percent changes (APC) in incidence rates.

A total of 55,613 IBD patients (17,178 for CD and 38,435 for UC) were identified from 2004 to 2015. The age-specific incidence rate of CD peaked at 10–29 years old in both men (9.04/100,000) and women (3.37/100,000) between 2004 and 2015. The incidence rate of CD was significantly increased in men aged 10–19 (APC=12.2%), 20–29 (5.8%), and 30–39 (2.3%) and women aged 10–19 (11.4%), and 20–29 (3.0%), whereas significant decreases were observed for men aged 50–59 (−2.5%), and 80+ years (−9.8%) and women aged 40–49 (−4.9%), 50–59 (−4.0%), and 60–69 years (−4.9%). The age-specific incidence rate of UC peaked in 60–69 years old in both men (12.87/100,000) and women (9.86/100,000) between 2004 and 2015. Significant increase in incidence rates of UC was observed in men aged 10–19 (6.8%), 20–29 (5.5%), and 30–39 (2.0%), and women aged 10–19 (9.7%), and 20–29 (3.7%), whereas significant decrease was observed for men aged 80+ years (−4.8%) and women aged 40–49 (−2.2%), 50–59 (−3.1%), 60–69 (−4.6%), and 70–79 (−4.8%) years old. The incidence rate of CD was 3.29/100,000 in metropolitan area and 2.96/100,000 in non-metropolitan area, which were both annually increasing (APC of CD in metropolitan area: 4.7%, non-metropolitan area: 4.0%). The incidence rate of UC was 7.15/100,000 in metropolitan area and 7.02/100,000 person-years in non-metropolitan area, which were increasing in both areas (APC of UC in metropolitan area: 1.6%, non-metropolitan area: 1.1%).

The incidence of both CD and UC in Korea was increased between 2004 and 2015. Metropolitan area showed higher incidence rate than non-metropolitan area. The most prominent increase of CD incidence was observed in 10–39 years old men and 10–29 years old women during the study period.