A stool sample from a patient suspected of Clostridium difficile infection appears firm and formed. What should you do?

• A. Reject the specimen for Clostridium difficile testing; stool must take the shape of the container
• B. Proceed with testing on the formed stool
• C. Request a repeat liquid stool sample
• D. Test for another pathogen instead

Answer: (A)

Testing for Clostridioides difficile relies on a liquid or unformed stool specimen to produce accurate results. Formed stool can distribute toxin unevenly and often contains too little toxin to be detected, increasing the risk of a false-negative result. Because of this, a formed stool specimen should not be used for CDI testing; you should reject it and request a repeat sample when the patient has loose, watery stool. Trying to test a formed specimen won’t reliably diagnose CDI, and testing for another pathogen would not address the suspected CDI diagnosis.