About the PhD thesis

This PhD thesis comprises three studies on inflammatory bowel dis-ease (IBD), utilizing Danish nationwide and regional register data. Due to the varied severity and manifestations of IBD, long-term, pop-ulation-based follow-up studies are essential for advancing our un-derstanding of the disease. While Danish registries offer a valuable resource, it is important to consider both the strengths and limitations of using administrative data in research.

Study I examined the validity of IBD diagnosis codes in the Danish National Patient Registry (NPR).

Study II investigated the long-term prognosis of patients with mild IBD.

Study III explored the association between IBD and obstructive lung diseases (OLD), assessing the temporal risk of developing these conditions.

Study I validated IBD diagnosis codes in the NPR, identifying that two registrations provided optimal criteria for case identification. Study II, combining regional and national data, revealed that some patients with mild IBD exhibit a non-progressive course over long-term follow-up. Study III demonstrated an increased risk of OLD both before and after IBD diagnosis.

Together, these findings improve the accuracy of IBD case identifica-tion, enhance understanding of long-term outcomes in mild IBD, and highlight the need for awareness of respiratory diseases in IBD, in-forming both clinical practice and future research.