Background: Diarrhea predominant irritable bowel syndrome (IBS-D) is a common functional disorder of the gastrointestinal tract shares similar symptoms and endoscopic findings with microscopic colitis (MC) with a major difference in treatment. IBS-D is diagnosed using criteria based on clinical symptoms, while histological evidence is mandatory for MC diagnosis. There is a continuous debate about the value of endoscopically normal colonic mucosa biopsies in IBS-D patient investigation.
Objective: To determine the prevalence of MC in patients meeting Rome IV criteria of IBS-D.
Patients and methods: A cross-sectional survey was performed in the Department of Hepatogastroenterology and Infectious Diseases, Al-Azhar University Hospitals. One hundred patients attended the outpatient clinics from January 2020 to April 2020 fulfilling Rome IV criteria for diagnosis of IBS-D was prospectively enrolled. Patients aged from 18 to 65 years old.  Patients with other causes of chronic diarrhea were excluded. The studied patients were subjected to full history taking, extensive clinical examination, thorough laboratory investigations and complete colonoscopy with multiple biopsies were taken from endoscopically normal looking mucosa at different parts of the colon, and processed for histopathological examination.
Results: The mean age of participants was 40.7 ± 11.01 years. Fifty three cases were males (53%) and 47 were females (47%). Twenty out of 100 patients were diagnosed as MC, 18 (90%) of them were lymphocytic colitis (LC) and 2 patients (10%) were collagenous colitis (CC). The mean age in MC patients was 50.8 ± 7.3 years. Nocturnal diarrhea and weight loss were only seen in cases diagnosed as MC (15 out of 20 patients 75%) and 15 out of 20 patients (75%) respectively. There was a significant difference between normal biopsy & MC patients as regard inflammatory markers C-reactive protein(CRP) and  Erythrocyte sedimentation rate (ESR), but their levels were still within normal range.
Conclusion: MC is not an uncommon disease that is often diagnosed in elderly people. It can be easily confused with IBS-D and histopathological examination of colonoscpic biopsies obtained from the normally appearing mucosa is the only way for differentiation. In cases with IBS-D, nocturnal diarrhea and weight loss increase the possibility of MC and require colonoscopic biopsy evaluation. In patients with IBS-D criteria mild increase in inflammatory markers (CRP and ESR) may occur, but these markers are non-specific and don't direct us for MC diagnosis.