Interpretation of Clostridium difficile testing

Clostridium difficile is the cause of pseudomembranous colitis. Not all clostridium difficile is toxin producing and therefore the mere presence of clostridium difficile in a stool sample does not confirm the diagnosis.

Therefore the recommended two stop testing involves first a test for the organism, GDH (glutamate dehydrogenase) and then presence of the toxin, by EIA (enzyme immunoassay).

non diarrhea samples should not be tested

Direct testing of stool samples for toxin A or toxin B has a 31% sensitivity and therefore should be done with the GDH. Prior to introduction of GDH three samples were needed to exclude clostridium difficile

GDH may be present in both toxigenic and non-toxigenic clostridium difficile


GDH+ and toxin+ = positive *clostridium difficile*
GDH- and toxin - = negative for *clostridium difficile*

Specimens that are discordant GDH+ and toxin- OR GDH- and toxin+ are further tested by PCR assay. This may occur ~6% of the time.

PCR positive = likely *clostridium difficile *
PCR negative = unlikely *clostridium difficile *



  1. Quest diagnostics: testing for clostridium difficile