Abstract
Fecal microbiota transplantation (FMT) is a recommended therapy for recurrent Clostridioides difficile infection and is being investigated as a potential therapy for dozens of other indications, notably inflammatory bowel disease. The immense variability in human stool, combined with anecdotal reports from FMT studies, have suggested the existence of "donor effects", in which stool from some FMT donors is more efficacious than stool from other donors. In this study, simulated clinical trials were used to estimate the number of patients that would be required to detect donor effects under a variety of study designs. In most cases, reliable detection of donor effects required more than 100 patients treated with FMT. These results suggest that previous reports of donor effects need to be verified with results from large clinical trials and that patient biomarkers may be the most promising route to robustly identifying donor effects.