Heterogeneity in Histological Evaluation of Microscopic Colitis in Randomized Clinical Trials: An Umbrella Review

BACKGROUND: The diagnosis of microscopic colitis (MC) is based on endoscopic biopsy with histological assessment. Histological outcomes (remission, response or improvement) are important treatment targets in clinical trials. Although a substantial body of research on MC has been published in recent years, no standardized criteria currently exist for its histological outcomes. We sought to review and summarize the histological evaluation of MC in published systematic reviews (SRs) assessing the efficacy of interventions and to examine the heterogeneity in histological evaluation among the randomized controlled trials (RCTs) included in those SRs. METHODS: We conducted an umbrella review (ie, an overview of systematic reviews) of published SRs. A literature search of the Cochrane Database of Systematic Reviews, MEDLINE, and Embase was performed up to May 2025. Definitions of histological evaluation and monitoring following interventions were extracted and summarized from the published SRs and the RCTs included within them. RESULTS: Fourteen SRs with meta-analyses that focused on interventions were included. Nineteen RCTs were included in these SRs. Of them, 12 fully published RCTs reported histological outcome data and met our inclusion criteria. The definitions for histological outcomes varied between RCTs but were generally based on reduction in lamina propria cellularity, intraepithelial lymphocytes, or collagen band thickness. CONCLUSIONS: This umbrella review highlights the heterogeneity in the definitions of histological outcomes in MC RCTs. The summarized evidence will support ongoing efforts to develop consensus definitions for histological outcomes in order to facilitate clinical trials of medical therapies for MC.