Incidence of Collagenous and Lymphocytic Colitis: A 5-Year Population-Based Study Fernando Ferna ´ndez-Ban ˜ ares, M.D., Antonio Salas, M.D., Montserrat Forne ´, M.D., Maria Esteve, M.D., Jorge Espino ´s, M.D., and Josep Maria Viver, M.D. Departments of Gastroenterology and Pathology, Hospital Universitari Mu ´tua de Terrassa, Barcelona, Catalonia, Spain Objective: The incidence of collagenous and lympho- cytic colitis is not well known. We sought to assess the incidence of collagenous and lymphocytic colitis in a well-defined population during a 5-yr study period. Methods: From January 1, 1993, to December 31, 1997, all new patients diagnosed with collagenous or lympho- cytic colitis living in the catchment area of the Hospital Mu ´ tua de Terrassa (Barcelona, Spain) were identified. Since 1993 all patients with chronic diarrhea were re- ferred for a diagnostic colonoscopy. Multiple biopsy sampling of the entire colon was performed when ap- pearance of the colonic mucosa was grossly normal. Results: Twenty-three cases of collagenous colitis and 37 of lymphocytic colitis were diagnosed. The female:male ratios were 4.75:1 and 2.7:1 for collagenous and lym- phocytic colitis, respectively. The mean age at onset of symptoms was 53.4  3.2 (range, 29 – 82) yr for collag- enous colitis, and 64.3  2.7 (range, 28 – 87) yr for lym- phocytic colitis (p  0.012). The mean annual incidence per 100,000 inhabitants based on the year of onset of symptoms was 1.1 (95% confidence interval [CI], 0.4 – 1.7) for collagenous colitis, and 3.1 (95% CI, 2.0 – 4.2) for lymphocytic colitis. A peak incidence was observed in older women in both diseases. A rate of microscopic colitis of 9.5 per 100 normal-looking colonoscopies per- formed in patients with chronic watery diarrhea was observed. Normal rectal biopsies were found in 43% and 8% of patients with collagenous and lymphocytic colitis, respectively. Conclusions: The incidence of lymphocytic colitis is three times higher than that of collagenous colitis. Microscopic colitis should be considered as a major possibility in the work-up of chronic diarrhea in older women. (Am J Gastroenterol 1999;94:418 – 423. © 1999 by Am. Coll. of Gastroenterology) INTRODUCTION Microscopic colitis is a term used to define those clini- copathological entities characterized by chronic watery di- arrhea, normal radiological and endoscopic appearances, and microscopic abnormalities (1, 2). Specific histopatho- logical appearances can be used to further classify collage- nous colitis (CC), lymphocytic colitis (LC), and other con- ditions (1, 2). CC differs from LC by the presence of a subepithelial collagen band (10 m) adjacent to the basal membrane. Both diseases disclose inflammatory changes in the lamina propria and superficial epithelial damage (1, 2). Although CC and LC are considered rare conditions, increasing awareness of these entities among pathologists and clinicians has resulted in more frequent diagnosis. How- ever, their incidence is not well known. In fact, there is only one population-based epidemiological study performed in Sweden reporting the incidence of CC (3), and to our knowl- edge there are no population-based data on LC. The aim of this study was to estimate the incidence rate of both CC and LC in a well-defined population during a 5-yr study period. MATERIALS AND METHODS Catchment area The Hospital Mu ´tua de Terrassa health area is of a mixed rural-urban type, with limited migration. From 1993 to 1995 the population increased by 2.2%, from 205,006 to 209,711 inhabitants. In 1996 there was a reorganization of the health area and the population was reduced to 189,768 inhabitants, a figure that increased to 189,801 in 1997. In the area there is only one hospital, where the only endoscopy unit and pathology department in the area are located. The hospital offers universal coverage for primary and specialist ser- vices, with an established system for referral from primary to secondary care. In the health area, there are seven gas- troenterologists working in primary health centers. All information concerning the population and the age and gender distribution was obtained from the town councils of all cities in the area. The age and gender distribution of the population in 1997, which was similar during the study period of 1993–1997, is described in Figure 1. Part of this study was presented at the AGA congress held in Wash- ington, D.C., in May 1997, and published as an abstract (Gastroenterol 1997;112:A15). Received May 4, 1998; accepted Sep. 18, 1998. THE AMERICAN JOURNAL OF GASTROENTEROLOGY Vol. 94, No. 2, 1999 Copyright © 1999 by Am. Coll. of Gastroenterology ISSN 0002-9270/99/$20.00 Published by Elsevier Science Inc. PII S0002-9270(98)00751-5 418