Objective: Although the epidemiology of microscopic colitis has been described in Europe, no such data exist from North America. We studied the incidence, prevalence and temporal trends of microscopic colitis in a geographically defined US population. Design and setting: In this population based cohort study, residents of Olmsted County, Minnesota, with a new diagnosis of microscopic colitis, and all who had colon biopsies for evaluation of diarrhoea, between 1 January 1985 and 31 December 2001 were identified. Biopsies were reviewed for confirmation (cases) and to identify missed cases (diarrhoea biopsies). Main outcome measures: Incidence rates, age and sex adjusted to the 2000 US white population. Poisson regression assessed the association of calendar period, age and sex with incidence. Results: We identified 130 incident cases for an overall rate of 8.6 cases per 100 000 person-years. There was a significant secular trend, with incidence increasing from 1.1 per 100 000 early in the study to 19.6 per 100 000 by the end (p<0.001). Rates increased with age (p<0.001). By subtype, the incidence was 3.1 per 100 000 for collagenous colitis and 5.5 per 100 000 for lymphocytic colitis. Collagenous colitis was associated with female sex (p<0.001 ) but lymphocytic colitis was not. Prevalence (per 100 000 persons) on 31 December 2001 was 103.0 (39.3 for collagenous colitis and 63.7 for lymphocytic colitis). Conclusions: The incidence of microscopic colitis has increased significantly over time, and by the end of the study, the incidence and prevalence were significantly higher than reported previously. Microscopic colitis is associated with older age, and collagenous colitis is associated with female sex.
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