Identifying patients with inflammatory bowel disease in the Danish National Patient Register


INTRODUCTION: The Danish National Patient Register (NPR) is an indispensable source of data for population-based studies of inflammatory bowel disease (IBD). Current case-validation algorithms are at risk of overestimating the occurrence of IBD in Denmark. We aimed to develop a new algorithm for validating IBD patients in the Danish NPR and compared it with the algorithm currently used.

METHODS: We used the Danish NPR to identify all IBD patients between 1973 and 2018. In addition, we compared the traditional two-registration validation method with a newly developed ten-step method. Data were provided by Statistics Denmark.

RESULTS: In total, 69,908 IBD patients (Crohn's disease (CD): 23,500 (33.6%); ulcerative colitis (UC): 38,728 (55.4%); IBD unclassified (IBDU): 7,680 (11.0%)) and 84,872 (UC: 51,304 (60.4%), CD: 20,637 (24.3%), IBDU: 9,931 (11.7%)) were identified using the new and the traditional algorithm, respectively, yielding 21.4% more patients. The sensitivity of each algorithm was 98%; however, the new algorithm demonstrated a superior positive predictive value (PPV) (69% (95% confidence interval (CI): 66-72%) versus 57% (95% CI: 54-59%), p less-than 0.05). The overall incidence rate in 2017 was 44.36 (95% CI: 42.66-46.11) versus 53.41 (95% CI: 51.54-55.33, p less-than 0.0001) for the new and the traditional method, respectively.

CONCLUSION: We developed a new and more refined algorithm for validating IBD patients in the Danish NPR. The algorithm will ensure that new studies based upon one of the world's most comprehensive registers will be of an even higher quality. We recommend that all future studies of IBD in Denmark use the new algorithm.

FUNDING: none.


Original languageEnglish
JournalDanish Medical Journal
Issue number4
Publication statusPublished - 27 Mar 2023


  • Humans
  • Registries
  • Inflammatory Bowel Diseases/diagnosis
  • Colitis, Ulcerative/diagnosis
  • Crohn Disease/diagnosis
  • Denmark/epidemiology


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