Poster Session II - A272 EVOLUTION OF UC:CD INCIDENCE RATIOS ACROSS EPIDMIOLOGIC STAGES

Background Inflammatory bowel disease (IBD) progresses through distinct epidemiologic stages: Stage 1 (Emergence), low incidence/prevalence; stage 2 (Acceleration in Incidence), rapid increasing incidence; stage 3 (Compounding Prevalence), stabilizing incidence and increasing prevalence. As IBD emerges in a population, ulcerative colitis (UC) incidence typically exceeds Crohn’s disease (CD) incidence, but over time the UC:CD incidence ratio approximates 1. Aims To examine changes in UC:CD incidence ratios across epidemiologic stages. Methods Stage classifications were previously determined by machine-learning classifier applied to data from 522 population-based studies of 82 countries (1920–2024). Population-weighted mean incidence (excluding zeros) was used to calculate annual, country-specific UC:CD ratios. Ratios were analyzed using a generalized additive model for location, scale, and shape (GAMLSS), accounting for overdispersion and skewness, with pairwise Wilcoxon rank sum tests and Bonferroni correction for between-stage comparisons. Average annual percent changes (AAPC) with 95% confidence intervals (CI) were calculated for each country within each stage. AAPCs were pooled by meta-analysis within stages. Outliers were defined using the Tukey method and are presented separately. Results The GAMLSS model, adjusted for country, showed significant effects of epidemiologic stage on UC:CD incidence ratios (p<0.001), highlighting a progressive shift toward a 1:1 ratio (Figure 1). Significant differences were found between stages: Stage 1: Median: 3.62 (IQR: 3.9 [1.66–5.56]); Stage 2: Median: 1.8 (IQR: 2 [1.12–3.12]); Stage 3: Median: 1.56 (IQR: 1 [1.08–2.08]); all comparisons: p<0.001. Meta-analyses supported these results, decreasing in stages 1 and 2 before stabilizing in stage 3: Stage 1 (13 countries, 1954–2022) AAPC: −4.77 (95%CI: −8.59, −0.95); stage 2 (42 countries, 1952–2022) AAPC: −2.55 (95%CI: −4.04, −1.06); stage 3 (24 countries, 1970–2023) AAPC: −0.12 (95%CI: −0.66, 0.42). Outliers (n=81; 7.8%) were primarily observed in Asia and northern latitudes (Table 1) (eg, China, Faroe Islands). Conclusions The UC:CD incidence ratio significantly decreases during stage 1 and 2, then stabilizes in stage 3. Recognising stage-specific changes in the UC and CD incidence may clarify the epidemiologic evolution of IBD and guide research into environmental and diagnostic determinants. A272 Table 1: UC:CD Incidence Ratio Outliers Funding Agencies CIHRThe Leona M. and Harry B. Helmsley Charitable Trust; International Organization for the study Inflammatory Bowel Disease (IOIBD)