What test is used to diagnose C. difficile infection?

• A. Stool culture
• B. Clostridium difficile toxin stool assay
• C. Blood test
• D. Urine antigen test

Answer: (B)

Diagnosing C. difficile infection hinges on detecting the toxin produced by the bacteria in the stool, since the illness results from toxin-mediated damage to the colon. A stool assay that tests for C. difficile toxins (typically toxins A and B) directly confirms the presence of the harmful toxin and correlates with active disease, making it the best choice.

Stool culture alone identifies the organism but does not indicate whether toxin is being produced or whether symptoms are due to active infection; many people can carry C. difficile without disease. Blood tests and urine antigen tests do not assess colonic toxin and are not reliable for diagnosing CDI.