Toxin assays
The most common laboratory tests for detecting C. difficile are enzyme immunoassays (EIA) for detecting C. difficile toxins A and B. The immunoassays are simple to perform, provide rapid results, and are easily incorporated into the workflow of most laboratories. However, the sensitivity of these tests does not compare favorably to culture, cell cytotoxicity neutralization assay (CCNA), or molecular methods.
There are many test kits commercially available to detect C. difficile toxins. Depending on the assay, results are available in 15 minutes to 2 hours. Initially, toxin A was thought to be responsible for most of the effects of C. difficile disease, so most early kits only detected toxin A. With the realization that there are strains that produce aberrant or no toxin A (A-) that are known to produce infection, and more recently toxin B negative (B-) strains, it is now recommended to use kits detecting both toxins.
Glutamate Dehydrogenase (GDH) assays
Published studies have indicated that toxin immunoassays may not provide adequate sensitivity of detection. GDH assays initially attracted attention as a possible means of providing a rapid but more sensitive screening for C. difficile.
GDH is an enzyme produced by C. difficile. Enzyme immunoassays (EIAs) for the GDH antigen have been associated with a high negative predictive value. However, positive results are not necessarily associated with a toxin-producing strain. A second assay on GDH-positive samples is required to confirm the presence of a toxigenic strain. Initially, CCNA assays were recommended as the confirmatory method of choice; molecular methods (PCR for the toxin gene) were subsequently explored.
