What is the role of histopathology in diagnosing indeterminate colitis at colectomy

What is known

- Pathologists diagnosed indeterminate colitis on colectomy specimens when features of both UC and CD were present and Crohn's could not be definitively supported clinically or histologically, e.g., unusual disease distribution or inflammation extending into submucosa/muscularis. ¹ - Pathology definitions describe indeterminate colitis as colectomy specimens showing overlapping features of CD and UC, or where a clear pathologic distinction is impossible despite an adequate resection specimen. ² - In one cohort the diagnosis of indeterminate colitis was made only on review of the resected colectomy specimen, when macroscopic and microscopic features of UC and CD overlapped. ³ - Histopathologic examination of colectomy/rectal resection specimens can incidentally reveal Crohn's disease even when preoperative imaging and endoscopy indicated UC, supporting staged surgery with pathology review before IPAA. ⁴

What is unknown / caveats

- Indeterminate colitis is usually a provisional/temporary diagnosis that is later reclassified as UC or CD - Reclassification proportions differ across studies (e.g., 54% to UC vs 40% to CD in different series) - The excerpts note the diagnosis remains provisional and no reliable histologic criteria fully resolve the indeterminate category.

## References

1. Rauh SM, Schoetz DJ J, Roberts PL, Murray JJ, Coller JA, Veidenheimer MC. Pouchitis--is it a wastebasket diagnosis?. Dis Colon Rectum. 1991;34(8):685-9. PMID: 1649737.

2. Geboes K, Colombel JF, Greenstein A, Jewell DP, Sandborn WJ, Vatn MH, Warren B, Riddell RH. Indeterminate colitis: a review of the concept--what's in a name?. Inflamm Bowel Dis. 2008;14(6):850-7. PMID: 18213696.

3. Tan KK, Ravindran P, Young CJ, Solomon MJ. The extent of inflammation is a predictor for pouch-related complications in ileal pouches in patients with ulcerative or indeterminate colitis. Colorectal Dis. 2014;16(8):620-5. PMID: 24635999.

4. Connelly TM, Lincango E, Holubar SD. Crohn's of the Pouch: Now What?. Clin Colon Rectal Surg. 2022;35(6):475-486. PMID: 36591396.

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_Draft, generated by the IBDology RAG and not yet clinician-reviewed. Answers are grounded in the retrieved literature listed above; a high faithfulness score means the answer matches its sources, not that the sources are correct._