Pathology of indeterminate colitis.

The term indeterminate colitis (IC) is an interim, or preliminary, descriptive term used by pathologists for cases of inflammatory bowel disease (IBD) in which a definite diagnosis of ulcerative colitis (UC) or Crohn's disease (CD) cannot be established based on the information available at the time of surgical sign-out. Most cases are due to fulminant ulcerative colitis, a condition in which the classic pathologic features of UC are often obscured and may overlap with CD. For instance, fulminant UC may show early superficial fissuring ulceration, transmural lymphoid aggregates and relative rectal sparing, simulating CD. Other common causes for establishing a diagnosis of IC include confusion of backwash ileitis in UC for terminal ileal involvement in CD, failure to accept hard criteria, such as granulomas, or segmental disease, as representative of CD, and failure to recognize unusual variants of UC that can cause CD-like patchiness of disease. Also, a diagnosis of IC should be avoided on biopsy studies and should not be established until all available clinical, endoscopic, radiologic and pathologic information are available for review. Some cases of IC may represent other forms of colitis as well, such as chronic ischemic or infectious colitis. The natural history of IC more closely resembles that of UC than CD. In fact, most cases of IC represent UC upon long-term clinical follow-up. Although, in some instances, serologic testing for ANCA or ASCA may be helpful in separating UC from CD in patients with IC, there is much overlap in the results of these assays for cases in which CD involves the colon in a UC-like pattern. Approximately 20% of IC patients develop severe pouch complications, which is intermediate in frequency between that seen in UC or CD. The risk of pouch complications, such as perianal fistulas or abscesses, and the risk of pouch breakdown is, overall, quite similar between IC and UC patients, supporting the notion that most patients with IC probably have UC and can safely undergo an ileal pouch-anal anastomosis procedure and have a reasonably good chance of having a good outcome.