Outcomes and Reclassification Patterns in Inflammatory Bowel Disease Unclassified: A Multicenter Study Matched With Ulcerative Colitis From Spanish ENEIDA Registry.

BACKGROUND AND AIM: Around 10% of patients with inflammatory bowel disease do not fulfill the diagnostic criteria for Crohn's disease (CD) or ulcerative colitis (UC). These cases are labelled as inflammatory bowel disease unclassified (IBDU). The aim of the study was to compare therapeutic requirements in patients with IBDU and UC and to identify predictive factors of reclassification. METHODS: This was a multicenter retrospective cohort study based on the ENEIDA registry. Patients diagnosed with IBDU or indeterminate colitis between January 2010 and December 2016 were included. All cases were matched by sex, age at diagnosis, and disease extent with at least two UC patients from the same hospital. The Kaplan-Meier method was performed to assess diagnostic changes during follow-up. Fine-Gray competing-risks regression analysis identified predictive factors of reclassification. RESULTS: Two hundred and nine IBDU patients and 465 UC patients were included. After a median follow-up of 90 months, no significant differences were observed in the use of immunomodulator (37.3% vs. 37%, p = 0.934) and advanced therapies (29.2% vs. 23.9%, p = 0.143) between IBDU and UC. No statistical differences were found in the surgical rates (5.7% vs. 4.7%, p = 0.579). Overall, 59 IBDU patients (28.2%) were reclassified (30 UC and 29 CD). Baseline extensive colitis was associated with a lower probability of subsequent reclassification to UC (SHR: 0.33; 95% CI: 0.12-0.91). No independent predictive factors for reclassification to CD were identified. CONCLUSIONS: Therapeutic management of IBDU and UC patients tended to be similar. Two-thirds of patients remained classified as IBDU.