Should laboratories test for toxin A-negative, toxin B-positive Clostridium difficile?

We evaluated 300 stool specimens from patients suspected of having Clostridium difficile-associated diarrhea to determine the prevalence of toxin A-negative, toxin B-positive C difficile isolates. Each specimen was cultured anaerobically on selective media using the heat-shock method and screened for C difficile toxins by two enzyme immunoassays-one that uses an anti-toxin A monoclonal antibody and one that uses a mixture of an anti-toxin A and an anti-toxin B monoclonal. The C difficile strains isolated from specimens were retested for production of toxin B by a tissue culture assay and for production of toxin A by a second toxin A immunoassay. Thirty-two specimens were positive and 234 specimens were negative for all assays. Thirty-four specimens had discrepant results. After discrepancy analysis, all of the isolated difficile strains were determined to be either nontoxigenic (producing neither of the toxins) or toxigenic (producing both toxins). These data indicate that the prevalence of toxin A-negative, toxin B-positive strains of C difficile is not of concern in routine diagnostic testing for C difficile toxins in our study population.