An investigation of European pathologists' approach to diagnose microscopic colitis.

Microscopic colitis (MC) comprising lymphocytic colitis (LC), collagenous colitis (CC) and the incomplete forms of microscopic colitis (MCi) are frequent causes of chronic watery diarrhea. The diagnosis is based on specific histological features in colonic biopsies. Especially regarding MCi, the histological features may be subtle. The PRO-MC collaboration was established in 2016 with the aims to systematically describe the disease course and to validate the diagnostic criteria of MC. In the present study, we analysed pathologists' initial approach to diagnose MC. Five pathologists with expertise in gastro-intestinal pathology reviewed the first 10 cases enrolled in the PRO-MC registry in six of the participating centres. Despite considerable differences in strategies in biopsy sampling, in choice of stains and in minimum number of biopsies and segments required for diagnosing MC, inter-observer agreement between the participating centres and expert pathologists as well as among the latter was substantial. Disagreed cases most often related to difficulties in distinguishing between MC subgroups. We recommend that pathologists as well as clinicians reach consensus in their diagnostic approach to MC, which is a prerequisite to compare MC cohorts internationally and to facilitate clinical MC trials and follow-up studies.