Diversion-Associated Rectal Pathology in Patients With and Without Inflammatory Bowel Disease: A Rectum-Exclusive,...

Diversion colitis is an established clinicopathologic complication of fecal stream diversion and may mimic inflammatory bowel disease (IBD) histologically, creating diagnostic and management challenges in the diverted rectum or rectosigmoid colon. We aim to clarify the histologic spectrum of diversion-associated pathology in patients with and without IBD, emphasizing de novo diversion colitis in non-IBD patients and interpretation of chronic colitis patterns in established IBD. We conducted a blinded, rectum‑exclusive histologic comparison of 63 diverted rectal resections (17 non‑IBD, 46 IBD), using a 5‑domain semiquantitative scheme for lymphoid follicles, activity, chronicity, mucosal atrophy, and surface integrity. Reproducibility was assessed by a blinded rereview after a 6-month washout. Ulcerative colitis versus Crohn's disease subgroups were compared by diversion interval-adjusted proportional-odds models with false-discovery rate correction. Available prediversion slides from the IBD group (31/46) were also reviewed for distal margin involvement. Diversion colitis occurred in 3/17 (18%) non‑IBD diverted rectum specimens, while most non‑IBD cases showed diversion change only. In contrast, 43/46 (93%) IBD cases exhibited a chronic colitis pattern, with or without activity, and also higher grades across all 5 parameters than non‑IBD cases. Higher rate of moderate-to-marked chronic active colitis in IBD, together with frequent prediversion distal margin involvement, favors persistent IBD as a likely interpretation in most cases, although severe diversion colitis cannot be definitively excluded by histology alone. Ulcerative colitis cases presented more severe histologic changes of chronic active colitis than those with Crohn's disease. These findings support a context-aware spectrum and practical reporting terminology after clinicopathologic correlation: (i) diversion change, (ii) diversion colitis, and (iii) IBD with superimposed diversion changes.