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The page below is a sample from the LabCE course Clostridium difficile Infection (CDI): Overview, Laboratory Tests and Updated Guidelines.. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Diagnosis of CDI: Laboratory Methods

A number of laboratory stool tests are available alone or in combination to help diagnose CDI. These laboratory-based tests typically examine a stool sample for certain toxins and/or a specific enzyme antigen produced by the C difficile bacteria. Several types of laboratory methods can be used including selective anaerobic culture, cell culture cytotoxicity assay, enzyme immunoassay (EIA), and the nucleic acid amplification test (NAAT) or polymerase chain reaction (PCR).

The following is a summary of the various stool-based laboratory methods used to help diagnose CDI:
Selective anaerobic culture: For clinical diagnosis of CDI, the selective anaerobic culture is seldom used. Culture on selective medium alone cannot distinguish toxin-producing strains from non-toxin producing strains. Selective anaerobic culture usually requires the use of a second test to detect toxin production and can be generally too slow and labor intensive for routine clinical use. Rather, selective anaerobic culture is useful for epidemiologic studies.
Cell culture cytotoxicity assay: This assay analyzes the effects of the C diff toxin on human cells grown in a culture. It has been used in the past as a gold standard test for the diagnosis of CDI. The cell culture cytotoxicity assay is performed by adding a prepared stool sample, which has been diluted, buffered and filtered, to a monolayer of cultured cells. If C diff toxin is present, it exerts a cytotoxic effect on the cells. The cytotoxicity is demonstrated by the use of specific antiserum. .
The cytotoxicity cell test is usually sensitive, but tends to be less widely available, more cumbersome to perform and typically requires 24 to 48 hours for test results. Some laboratories may use another C diff test (such as the EIA) in combination with the cell cytotoxicity assays to ensure accurate results.