AbstractBackgroundHistological disease severity assessment in ulcerative colitis [UC] has become a mainstay in the definition of clinical endpoints [‘histological remission’] in clinical trials of UC. Several scores have been established in the microscopic assessment of disease activity, but the Nancy index [NI] stands out as being the histological index with the fewest scoring items. To what extent histological assessment using the NI is affected by interobserver reliability in a real-word setting is poorly understood. We therefore performed a single-centre retrospective analysis of NI assessment in patients with UC.MethodsWe retrospectively evaluated the NI in two independent cohorts [total: 1085 biopsies, 547 UC patients] of clinically diagnosed UC patients, who underwent colonoscopy between 2007 and 2020. Cohort #1 consisted of 637 biopsies from 312 patients, while Cohort #2 consisted of 448 biopsies from 235 patients. Two blinded pathologists with different levels of expertise scored all biopsies from each cohort. A consensus conference was held for cases with discrepant scoring results. Finally, an overall consensus scoring was obtained from both cohorts.ResultsThe interobserver agreement of the NI was substantial after the assessment of 1085 biopsy samples (κ = 0.796 [95% confidence interval, CI: 0.771–0.820]). An improvement of the interobserver agreement was found with increasing numbers of samples evaluated by both observers (Cohort #1: κ = 0.772 [95% CI: 0.739–0.805]; Cohort #2: κ = 0.829 [95% CI: 0.793–0.864]). Interobserver discordance was highest in NI grade 1 [observer 1: n = 128; observer 2: n = 236]. Interobserver discordance was lowest in NI grades 0 [observer 1: n = 504; observer 2: n = 479] and 3 [observer 1: n = 71; observer 2: n = 66].ConclusionThe NI is an easy-to-use index with high interobserver reliability for assessment of the histological disease activity of UC patients in a real-world setting. While NI grades 0 and 3 had a high level of agreement between observers, NI grade 1 had a poorer level of agreement. This highlights the clinical need to specify histological characteristics leading to NI grade 1.