P876 Disease course of Inflammatory Bowel Disease Unclassified in adult patients : a population-based study (1988-2014)

Abstract Background Inflammatory bowel disease unclassified (IBDU) represents 1 to 20 % of inflammatory bowel disease (IBD). There are few data on the natural history of IBDU. The aim of this study was to evaluate the disease course of IBDU in a population-based cohort. Methods All patients diagnosed with IBDU between 1988 and 2014, aged > 17 years at diagnosis, and prospectively enrolled in a population-based registry, were followed until diagnostic reclassification as Crohn’s disease (CD) or ulcerative colitis (UC), or date of loss of follow-up. Clinical presentations at diagnosis of IBDU, CD or UC were compared. The probability of diagnostic reclassification of IBDU during follow-up was studied by the Kaplan-Meier method, and its variation factors by Cox models. Results A total of 480 patients with an initial diagnosis of IBDU were included, representing 3.6% of incident IBD cases in the registry. The probability of IBDU diagnosis within IBD significantly decrease over the study period (6.2% from 1988 to 1996, 3.3% from 1997 to 2005 and 2.1% from 2006 to 2014, p<0.0001). Small bowel exploration and gastroscopy rates at diagnosis were significantly lower in IBDU than in UC and CD. Of these 480 patients, 388 were followed up, with a median duration of 2.4 years (IQR: 0.5-9.5). The cumulative probability of reclassification of IBDU was 17% at 1 year, 41% at 5 years, and 54% at 10 years (median time since initial diagnosis: 8.0 years (IQR: 6.3-12.0)); the probability of reclassification to UC was 10% at 1 year, 22% at 5 years, and 30% at 10 years, and to CD was 5%, 13%, and 18%, respectively. The median time to diagnostic reclassification decreased over the study period (14.3 years (IQR: 9.0-17.2) in 1988-1996, 5.1 years (3.1-9.4) in 1997-2005, and 5.6 years (3.4-8.3) in 2006-2014, p<0.0001), in parallel with an increased rate of reclassification during the most recent periods of diagnosis (HR: 1.76 in 1997-2005 and 1.67 in 2006-2014). Presence of rectal bleeding at diagnosis and 1st degree family history of IBD were respectively associated with risk of reclassification to UC (HR: 1.65, p=0.044) and CD (HR: 2.72, p=0.0134). Exposure to anti-TNF increased significantly after diagnostic reclassification (HR: 4.05, p<0.0001). During follow-up, 13.4 % of patients with IBDU required surgery. Conclusion In this population-based study, almost half of the patients with an initial diagnosis of IBDU were reclassified during follow-up, after a median time from diagnosis of 8 years. The rate of IBDU and the delay to diagnostic reclassification decreased over the study period, presumably due to an improvement in primary diagnostic evaluation.