Functional validation of the TOpClass classification system for perianal fistulising Crohn’s disease using real-world multicentre data: a 10-year retrospective observational study

Background and aims: Perianal fistulising Crohn’s disease (pfCD) is an aggressive, disabling condition with significant treatment challenges. The novel TOpClass classification system aims to better capture pfCD’s disease trajectory, improving prognostication and therapeutic decision-making compared with traditional anatomical classifications. This retrospective, multicentre study aimed to provide functional validation of TOpClass and evaluate its clinical applicability in managing pfCD. Methods: Data from eight expert centres included 112 patients with 10 years of follow-up. The study assessed transitions between TOpClass classifications in consecutive patients with Crohn’s disease (CD) and active perianal fistulas. Results: Most patients (72%) entered the study in Class 2a or 2b, with 56% improving and 24% worsening by study end. Among those in Class 1, 58% showed disease progression, highlighting pfCD’s refractory nature. Only 23% of the population achieved fistula healing. Subanalysis of the 16 patients starting in Class 2a (eligible for fistula closure) showed significant fistula healing (n=6) in those with less complex luminal disease and shorter CD duration (1.5 vs 17 years; p=0.02) and a non-penetrating, non-stricturing (Montreal B1) phenotype (100% vs 40%; p=0.03). The study highlighted treatment challenges, with 12% of patients requiring defunctioning surgery, 77% of whom had proctectomy. Of these, 55% progressed to Class 4, emphasising the need for careful preoperative and postoperative management. Conclusion: Our retrospective study demonstrates strong face validity of the TOpClass classification, supporting its clinical applicability. The findings and gaps will inform a prospective trial to confirm TOpClass’s utility and enhance our understanding of pfCD.