Geographic distributions of microscopic colitis and inflammatory bowel disease in the United States

Background: Crohn's disease (CD) and ulcerative colitis (UC) are characterized by similar geographic distributions. We used a large database of pathology reports to analyze the geographic distribution of microscopic colitis (MC) and compare it with those of UC and CD. Methods: A population of 671,176 individual patients with colonic biopsies was studied stratified by gender and state of residence. The occurrence of each diagnosis MC, UC, or CD, was expressed as proportional rate per 1000 colonoscopies with biopsies from each individual state. Results: UC and CD tended to be common in states in the Northeast or North Central regions of the U.S. and relatively rare among several southern states. MC appeared to follow a somewhat inverse pattern, as it was most common among some states from the Southwest (Colorado, New Mexico, Arizona, Nevada) and other states of southern latitude, such as Florida, Georgia, California, but relatively uncommon among states in the Northeast. The geographic distributions of UC and CD were significantly correlated with each other (R = 0.60 and P = 0.0004). No significant correlation was observed between MC and UC or CD. Conclusions: The differences in epidemiologic behavior point at a dissimilar set of risk factors that shape the occurrence of MC as opposed to UC or CD.