Which stool test is used to confirm C. difficile infection?

• A. Stool testing for C. difficile toxins/antigen
• B. Blood culture for C. difficile
• C. Stool culture for Campylobacter
• D. Urine antigen for C. difficile

Answer: (B)

Diagnosing C. difficile infection relies on detecting the toxin that causes disease, not just the presence of the organism. The illness results from toxin A and/or toxin B produced in the colon, so stool tests that look for these toxins or for C. difficile–specific antigens in stool are the way to confirm infection. Blood cultures won’t help because C. difficile is a gut bacterium and the infection is toxin-mediated in the intestinal tract, not bloodstream. A stool culture would identify Campylobacter if present, which is a different pathogen, and a urine test for C. difficile isn’t used clinically. In practice, many labs use a two-step approach that screens for the organism or its toxin components and then confirms toxin production (or toxin genes) to establish active infection.