Rapid detection of Clostridium difficile in stool using the VIDASR C. difficile Toxin A II assay

A rapid laboratory diagnosis of Clostridium difficile-associated diarrhea (CDAD) is important in patient management and in the administration of appropriate therapeutic modalities. The VIDAS(R) C. difficile Toxin A II (CDA 2) assay (bioMerieux, Inc., Hazelwood, MO) was compared with the cell culture cytotoxicity assay (CCA) for the rapid detection of C. difficile in stool from patients in whom C. difficile infection was suspected. Thirty-eight consecutively collected CCA-positive stool specimens, and 33 CCA-negative stool specimens were tested by the CDA 2 assay. Where appropriate, discordant specimens were repeated and/or tested by isolation utilizing cycloserine-cefoxitin-fructose agar (CCFA). Among 12 discordant stool specimens, 7 were VIDAS(R)-/cytotoxicity+, 2 were VIDAS(R) equivocal (E)/cytotoxicity+, 2 were VIDAS(R) E/cytotoxicity-, and 1 was VIDAS(R)+/cytotoxicity-. One VIDAS(R) E/cytotoxicity+ lacked sufficient stool to be repeated. From the single VIDAS(R)+/cytotoxicity- specimen, C. sordelli was isolated. Specimens that were equivocal by VIDAS(R), were omitted from incorporation into this study's test parameters. The sensitivity, specificity, positive and negative predictive values for the CDA 2 assay were 80.6, 96.8, 96.7, and 81.1%, respectively. The specimens which yielded false negative VIDAS(R) results had low levels of toxin based on endpoint titrations using the cytotoxicity assay. Although the CDA 2 assay displayed a reduced sensitivity compared with the CCA, the automated assay is rapid (results promulgated within 2 h), with computer generated readings obviating visual interpretations. Recognition of the CDA 2 assay's limitations is important to addressing this test's clinical utility.