Inflammatory bowel disease and health registry data: Estimates of incidence, prevalence and regional treatment variation

The aim of this thesis is to increase the understanding of how we can use health registry data in inflammatory bowel disease (IBD) research and provide knowledge on the epidemiology and treatment of IBD in Norway. IBD includes two major disorders, Crohn’s disease (CD) and ulcerative colitis (UC). For IBD patients, there are several accepted treatment options available, including both different medical therapies and surgery. The thesis consists of three articles. Article I studies the regional variation of surgery and advanced medical therapy among IBD patients in Norway’s four health regions. The study found that the cumulative incidence of advanced medical therapy and surgery varied across the health regions. There is conflicting evidence on the development of IBD incidence over time and geographical areas. Since IBD is a chronic disease with low mortality, the prevalence will likely increase over time due to an aging population. Article II provides estimates for IBD incidence and prevalence in Norway and assess the impact of different case definitions on these estimates. The results suggest that Norway has among the highest incidence and prevalence of IBD in the world. Article III introduces a novel method to estimate the incidence of CD using hospital data. Based on the results from article II & III we provide recommendations for how an incident patient should be defined when utilizing registry data, and different ways to determine incidence and prevalence dependent on the data available. The findings from this thesis enable future register-based research on IBD to be based on more informed methodological choices and increased awareness of the strength and limitations of two of Norway’s largest health registries; the Norwegian Patient Registry and the Norwegian Prescription Database