How is IBDU defined and how does it differ from indeterminate colitis

What is known

- The term 'indeterminate colitis' was first proposed by Kent in 1970 for colectomy specimens with overlapping features or insufficient evidence for a final diagnosis. ¹ - In 2005 the Montreal/World Congress working party introduced 'IBD unclassified' for pre-surgical cases where clinical, endoscopic, and biopsy findings indicate IBD but no further diagnosis is possible, reserving 'indeterminate colitis' for cases with a surgical specimen. ^1,2 - European guidelines advocate using 'IBDU' and restricting 'indeterminate colitis' to cases with complete histological analysis of surgical specimens. ³ - IBDU represents a true overlap of phenotypes between UC and Crohn's rather than a misclassification; IC is not a distinct disease entity and has no diagnostic criteria, serving as an interim diagnosis until reclassification. ^4,5 - Confusing IC and IBDU matters clinically because how a patient fares after colectomy differs markedly between the two, especially regarding pouch surgery. ⁶

What is unknown / caveats

- Some studies and a meta-analysis use the term 'indeterminate colitis' interchangeably with IBDU, and other labels ('uncertain colitis', 'possible IBD') are used with varying definitions - Uniform, universally applied clinical definitions for cases without a definitive IBD diagnosis do not exist - Reported frequency varies widely (roughly 1-20%) due to inconsistent criteria and terminology - Terminology is applied inconsistently across the literature, so the IBDU/IC distinction is not uniformly observed.

## References

1. [No authors listed]. Clinical features of inflammatory bowel disease unclassified: a case-control study. BMC Gastroenterol. 2024;24(1):105. PMID: 38481157.

2. Maunoury V, Savoye G, Bourreille A, Bouhnik Y, Jarry M, Sacher-Huvelin S, Ben Soussan E, Lerebours E, Galmiche JP, Colombel JF. Value of wireless capsule endoscopy in patients with indeterminate colitis (inflammatory bowel disease type unclassified). Inflamm Bowel Dis. 2007;13(2):152-5. PMID: 17206697.

3. [No authors listed]. Disease course of inflammatory bowel disease unclassified in a European population-based inception cohort: An Epi-IBD study. J Gastroenterol Hepatol. 2019;34(6):996-1003. PMID: 30562421.

4. Lee SH, Shin M, Kim SH, Kim SP, Yoon HJ, Park Y, Koh J, Oh SH, Ko JS, Moon JS, Kim KM. Prevalence of Inflammatory Bowel Disease Unclassified, as Estimated Using the Revised Porto Criteria, among Korean Pediatric Patients with Inflammatory Bowel Disease. Pediatr Gastroenterol Hepatol Nutr. 2024;27(4):206-214. PMID: 39035400.

5. Odze RD. A contemporary and critical appraisal of 'indeterminate colitis'. Mod Pathol. 2015;28 Suppl 1:S30-46. PMID: 25560598.

6. Martland GT, Shepherd NA. Indeterminate colitis: definition, diagnosis, implications and a plea for nosological sanity. Histopathology. 2007;50(1):83-96. PMID: 17204023.

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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._