Epidemiology of microscopic colitis.

We read the interesting paper by Pardi et al on the epidemiology of microscopic colitis in Olmsted County, Minnesota, USA (Gut 2007; 56: 504–8). They concluded that the incidence of microscopic colitis increased significantly over time, and by the end of the study, the incidence and prevalence were significantly higher than reported previously. In fact, compared with other epidemiological studies, in the last years of the study period they describe the highest incidence figures of microscopic colitis ever reported. However, is difficult to compare the crude incidence rates of several studies if we do not know the age structure of the population pyramid of each geographical area. As microscopic colitis is associated with older age, a higher incidence will be expected among aging populations. A solution for this is to report the standardised incidence rates; however, they are not reported in most of the published epidemiological studies. Another possibility is to describe the population pyramid of each geographical area. We think that the paper of Pardi et al would have been more understandable if they had described the population pyramid of Olmsted County. In our epidemiological study on the incidence of microscopic colitis in Terrassa, Spain, we reported the population pyramid. Thus, Pardi et al should assess if there are any differences in the age structure and sex structure of the population pyramids between Olmsted County and Terrassa. In this sense, if population pyramids were similar, they could conclude that there is a true difference in the incidence of microscopic colitis.