482 hospitals across 20 European countries were questioned about their methods and testing policy for Clostridium difficile infection. Furthermore, on one winter day and one summer day, each hospital sent all diarrhoeal samples to a national coordinating laboratory for standardized testing. 23% of samples positive for C. difficile infection according to the national laboratory were not diagnosed by the hospitals. The researchers estimate that ∼40,000 inpatients with C. difficile infection are potentially undiagnosed each year.