A 65 year old woman presents with a six week history of diarrhoea, emptying her bowels between four and six times per day with the passage of watery stool. There is no associated abdominal pain, rectal bleeding or weight loss. She has a medical history of gastro-oesophageal reflux disease, depression and osteoarthritis and her medications include pantoprazole, sertraline and meloxicam.
Screening blood tests are normal and stool microscopy is negative for any pathogens. She is referred for colonoscopy. The colon appears normal macroscopically but colonic biopsies reveal an intraepithelial lymphocytic infiltrate and a thickened subepithelial collagen band consistent with collagenous colitis.