Clostridium difficile infection laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Laboratory Findings

Cytotoxicity assay

C. difficile toxin detection as cytopathic effect in cell culture, and neutralized with specific anti-sera is the practical gold standard for studies investigating new CDAD diagnostic techniques. Toxigenic culture, in which organisms are cultured on selective medium and tested for toxin production remains the gold standard and is the most sensitive and specific test, although it is slow and labour-intensive.

Enzyme-linked immunoabsorbant assay (ELISA) for toxin

Assessment of the A and B toxins by enzyme-linked immunoabsorbant assay (ELISA) for toxin A or B (or both) has:

• sensitivity 63-99%
• specificity 93-100%

At a prevalence of 15%, this leads to:

• positive predictive value 73%
• negative predictive value 96%

Experts recommend sending as many as three samples to rule-out disease if initial tests are negative. C. difficile toxin should clear from the stool of previously infected patients if treatment is effective.

Unfortunately, many hospitals only test for the prevalent toxin A. Strains that express only the B toxin are now present in many hospitals and ordering both toxins should occur. Not testing for both may contribute to a delay in obtaining laboratory results, which is often the cause of prolonged illness and poor outcomes.

Other stool tests

Stool leukocyte measurements and stool lactoferrin levels have also been proposed as diagnostic tests, but may have limited diagnostic accuracy.