OBJECTIVE: The significantly higher incidence rates of microscopic colitis (MC) in Denmark compared to Sweden remains unexplained.

METHODS: Consecutive patients with newly diagnosed MC in the neighbouring regions of Skåne in 2011-2015 and Zealand in 2010-2016 were prospectively identified. Data on large bowel endoscopies and biopsies rates were retrieved. Information on putative factors were obtained from registers and literature. Interobserver agreement between pathologists from both regions on 40 blinded hematoxylin and eosin (H&E)-stained colon biopsies (collagenous colitis (CC), lymphocytic colitis (LC), non-specific inflammation and normal) was evaluated using kappa statistics.

RESULTS: The mean annual incidence per 105 inhabitants in Skåne and Zealand 2010-2015 was 5.9 (95% CI 4.6-7.3) versus 16.4 (95% confidence intervals (95% CI) 13.6-19.2) for CC and 2.7 (95% CI 1.0-4.3) versus 11.1 (95% CI 8.8-13.4) for LC, respectively. Number of endoscopies with biopsy per 1000 and the rate of MC per endoscopy with biopsy was higher in Zealand (34-52/1000) than in Skåne (12-21/1000). The kappa value for overall agreement between pathologists was good (0.72; 95% CI 0.64-0.79). Prescription of proton pump inhibitors and selective serotonin reuptake inhibitors was higher in Skåne in the relevant age groups and prescription of non-steroidal anti-inflammatory drugs and smoking rate higher in Zealand. Alcohol consumption was higher in Denmark than in Sweden.

CONCLUSION: The incidence of MC and number of cases per colonic biopsy was higher in Zealand and could not be readily explained by endoscopy or biopsy rates, differences in histological assessment or putative risk factors.