Before giving antidiarrheal medications to a patient with diarrhea who recently completed antibiotic therapy, which test should be performed?

• A. Stool culture
• B. Clostridium difficile toxin stool assay
• C. Urinalysis
• D. Abdominal X-ray

Answer: (B)

When diarrhea follows antibiotic use, the strongest concern is Clostridioides difficile infection. Before giving antidiarrheal medications, we test the stool for C. difficile toxins to confirm active toxin production. A positive toxin test means the diarrhea is due to CDI, and using antimotility agents could worsen the infection, increase risk of ileus or toxic megacolon, and delay appropriate treatment. Stool culture isn’t ideal because it may detect colonization and isn’t rapid for clinical decision-making, urinalysis doesn’t diagnose CDI, and an abdominal X-ray isn’t a diagnostic test for CDI (though it can help assess complications if suspected). Thus, testing for C. difficile toxins in stool is the appropriate next step.