Aim: The aim of this study was to determine preoperative clinical factors associated with subsequent diagnosis revision to Crohn's disease (CD) following total proctocolectomy with ileal pouch-anal anastomosis (IPAA) for ulcerative colitis (UC) or indeterminate colitis (IC) patients.

Method: Presumed UC and IC patients undergoing IPAA from a large single-institution prospective database with change of diagnosis to CD were identified and compared with patients without diagnosis change.

Results: A total of 2814 patients (47% male, median age 37 years) with presumed UC (85%) or IC (15%) underwent primary IPAA. At a median follow up of 9.6 years, 184 (7%) had the diagnosis revised to CD from histopathological examination of the colectomy specimen immediately in 97 (53%) or at a median interval of 36 months in 87 (47%). CD and UC/IC patients had had a similar operative technique, length of stay and 30-day morbidity. The postoperative CD diagnosis was associated with a preoperative diagnosis of IC (P < 0.0001) and perianal fistula (P = 0.002). Patients with a delayed diagnosis of CD were associated with a 3-stage procedure (P < 0.0001, OR = 2.8) (95% CI = 1.8-4.4), colonic stricture (P = 0.04, OR = 2.9 [95% CI = 1.1-7.4]), perianal fistula (P = 0.02, OR = 2.9 [95% CI = 1.2-7.2]), oral ulceration (P = 0.009, OR = 3.8 [95% CI = 1.2-9.6]) and younger age (P < 0.0001, OR = 0.048 [95% CI = 0.011-0.19]).

Conclusion: A few patients having IPAA for presumed UC/IC were subsequently diagnosed to have CD which was associated with perianal fistula and the diagnosis of postoperative preoperative IC. The delayed diagnosis of CD was associated with a three-stage procedure, colorectal stricture, anal fissure, mouth ulceration and younger age.