The first step is an immunoassay to simultaneously assess for toxin and GDH presence. If toxin (either A or B or both) and GDH are present, the specimen is considered positive for toxigenic C difficile infection. If both toxin and GDH are absent, then the specimen is considered negative (Figure 1).
Specimens with discordant results (ie, GDH-positive but toxin-negative or GDH-negative but toxin-positive) proceed to the second step: reflex (at additional charge and additional CPT code) to a PCR C difficile gene detection test. According to our validation studies, discordant results occur in about 6% of cases.
If the gene is detected, results will include genotype information that indicates if the hypervirulent strain (ribotype O27/NAP1/toxinotype III) is present. Table 1 details how the results will be reported and how to interpret them.