Objectives Aspirin may be involved in microscopic colitis (MC) development, but there are no data on colon histology in asymptomatic aspirin users. We prospectively assessed colonic and rectal mucosa from aspirin users, searching for MC features.

Methods From colonoscopy screenees, two biopsy samples were taken from each of three locations: ascending colon, transverse colon and rectum. A pathologist measured chronicity of inflammation and activity indicators, epithelial cell height and subepithelial collagen layer width. Intraepithelial lymphocytes (IELs), intralaminal eosinophils and apoptotic cells/100 crypts were counted. Panel data models were used to analyse associations between aspirin use, biopsy location and microscopic parameters.

Results Of 100 screenees (age: 40–65 years), 42 were current aspirin users. Median duration of aspirin usage was 48 months (range: 36–60) with dosage ranging from 75–325 mg/day. We observed reduced epithelium polymorphs in subjects who used aspirin for <48 months versus non-users (p=0.008). Paneth cell metaplasia was significantly less frequent in aspirin users versus non-users (p=0.006). Inflammatory cells in lamina propria (eosinophils) and epithelium (IELs) were most abundant in the ascending colon and decreased distally (ascending colon vs transverse colon and transverse colon vs rectum). Cryptitis was more frequent in the ascending colon vs the rectum.

Conclusions We observed no specific MC features in asymptomatic chronic low-dose aspirin users. We found subtle physiological and histopathological differences between the bowel segments.