The most common laboratory tests for the detection of C. difficile are enzyme immunoassays (EIA) for the detection of C. difficile toxin A and toxin B. The immunoassays are simple to perform, provide rapid results, and are easily incorporated into the workflow of most laboratories. Sensitivities of these tests do NOT compare favorably to culture, cell cytotoxicity neutralization assay (CCNA), or molecular methods.
There are many test kits commercially available for detection of C. difficile toxins. Results are available in 15 minutes to 2 hours, depending on the assay. Initially, toxin A was thought to be the toxin responsible for the majority 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, by themselves, may not provide adequate sensitivity of detection. GDH assays initially attracted attention as a possible means to provide a rapid but more sensitive means for screening for C. difficile.
GDH is an enzyme produced by C. difficile. EIAs negative for the GDH antigen have been associated with high negative predictive values. 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 for this purpose.