Validation of ulcerative colitis and Crohn's disease and their phenotypes in the Danish National Patient Registry using a population-based cohort.

Introduction The Danish National Patient Registry (DNPR) has been the source of several epidemiological studies of inflammatory bowel disease (IBD). However, the validation dates back to 1996 and lacks outpatient records and disease classification. The aim of this study was to update the validation and assess the validity and reliability of using the registry in disease classification. Methods Validation of the registry was done using a population-based inception cohort of IBD patients from 2003 to 2011 consisting of 513 patients. Specificity and sensitivity were calculated for the diagnoses of Crohn's disease (CD) and ulcerative colitis (UC), age at diagnosis and disease classification according to the Montreal Classification at both time of diagnosis and end of follow-up. Results The registry showed high validity and reliability in identifying CD and UC patients concerning correct age classification and identifying perianal disease. The registry showed inconsistent, unreliable results in further disease classification. Conclusions The DNPR has good validity and reliability in identifying patients with CD and UC, and defining the age of patients at diagnosis. However, categorising IBD patients according to the Montreal Classification should not be carried out using DNPR data in their current form, except when identifying CD patients with perianal disease.