Clinical comparison fo the molecular-based BD GeneOhm Cdiff assay to the cytotoxicity tissue culture assay for the direct detection of toxin B gene from toxigenic Clostridium difficile strains in faecal specimens

Abstract number: P1756

Fuller D., Buckner R., Newcomer K., Davis E., Davis T., Lineback P., Kolb G., Allen S., Blue D.

Objectives: The primary objective of this prospective evaluation was to demonstrate the use of the BD GeneOhm™ Cdiff (BD Diagnostics, San Diego, CA) real-time Polymerase Chain Reaction (PCR) assay as a diagnostic test for the detection of toxigenic Clostridium difficile (Cdiff) strains from faecal specimens. The performance of PCR was compared to a cytotoxicity reference standard (TechLab^®, Blacksburg, VA). The BD GeneOhm™ Cdiff assay is a qualitative in vitro diagnostic test performed on the Cepheid SmartCycler^® (Cepheid, Sunnyvale, CA), a random-access real-time PCR instrument. The assay utilizes PCR for the amplification of the toxin B gene (tcdB) and fluorogenic target-specific hybridisation probes for the detection of the amplified DNA. The amplification, detection and interpretation of the signals are done automatically by the SmartCycler^® software.

Methods: Liquid to soft stools received in the clinical laboratory for Cdiff testing were included in the evaluation. Briefly, stools were tested with the TechLab Cdiff chek™-60 enzyme immunoassay (EIA) for detection of the "common antigen", glutamate dehydrogenase (GDH), and positive results were confirmed with the Tox A/B assay. If the Tox A/B was negative, a cytotoxicity neutralisation assay was also performed. Concurrently, the BD GeneOhm™ PCR assay was also performed on each stool specimen. Additional test methods (culture, cytotoxin/neutralisation testing) were also performed on discordant specimens to aid in resolving discrepancies. Each stool was collected, processed, and tested according to the institution's standard of care and each assay was performed according to manufacturer's package insert.

Results: Of the 298 specimens included in this study, 247 (83%) tested negative with both PCR and cytotoxicity while 29 (10%) tested positive with both assays yielding 90.6% sensitivity and 92.9% specificity. After resolution of discordant results, the sensitivity and specificity was 93.6% and 98.4% respectively with a prevalence of nearly 15%.

Conclusions: The diagnosis of toxigenic C. difficile is usually done by a combination of cytotoxicity assay, culture and EIA, all of which are either labor intensive and time-consuming or lack sensitivity or specificity. The BD GeneOhm™ Cdiff assay (performed directly on stool specimens) offers sensitivity and specificity that is comparable to the cytotoxicity references standard and produces results in about one hour.