Increasing incidence and prevalence of Microscopic Colitis in Sweden: A nationwide population-based cohort study.

BACKGROUND: There are few population-based studies on the incidence and prevalence of microscopic colitis (MC). OBJECTIVE: To assess incidence and prevalence of MC in Sweden. DESIGN: Nationwide population-based cohort study including all incident cases of biopsy-confirmed MC and all biopsied cases with normal mucosa from 1995 to 2021. Incidence rates (IRs) were age-standardized to the 2021 Swedish population. Age-specific IRs were plotted by sex. Poisson regression estimated trends and female-to-male incidence rate ratios (IRRs). A combined model of MC and normal mucosa evaluated whether changes in MC incidence exceeded background biopsy trends. Point prevalence on 31 December 2021 was derived by dividing MC cases with population denominators. Lifetime risk was computed using a competing-risk cumulative incidence estimator. RESULTS: We identified 22,519 incident MC cases (71% women) from 1995 to 2021. The mean age-standardized IR across the study period was 8.8 (95% confidence interval(CI)=7.1-10.5) cases per 100,000 person-years and rose steeply from 1995 until 2007 (+17% per year, 95% CI=1.15-1.19), then increased more modestly (+3%, 1.02-1.04). The incidence of MC increased faster than that of normal mucosa, the mean difference was 4.33% per year (95% CI 3.19-5.48). The prevalence of MC was 170 per 100,000 inhabitants in 2021. Lifetime risk was 1 in 54 for women and 1 in 133 for men. CONCLUSION: In Sweden, incidence and prevalence of MC continued to rise through 2021. The steeper slope of MC incidence in relation to normal mucosa indicates either a true rise in disease occurrence or an ongoing diagnosis of prevalent cases related to an increased awareness of MC.