Friday, March 19, 2010
Grand Hall (Hyatt Regency Atlanta)
Background: Clostridium difficile
infection is associated with diarrhea and pseudomembraneous colitis which are significant causes of patient morbidity and mortality. Rapid and effective identification of C. difficile
is essential in order to mitigate spread of the organism among patients and to institute timely therapy. Screening tests currently rely on enzyme immunoassays (EIA) that detect either C. difficile
glutamate dehydrogenase antigen or toxins A (enterotoxin) and B (cytotoxin). The former test does not discriminate between toxigeninc and nontoxigenic strains which makes it highly sensitive but not very specific. As such, this test is usually used first and positive samples are further evaluated by EIA for toxin or by cytotoxic assay.
Objective: The purpose of this study was to compare three C. difficile EIA testing methodologies: 1) C DIFF QUIK CHEK® antigen (QC-Ag), 2) C DIFF QUIK CHEK® toxin (QC- TOX AB) and 3) Meridian ImmunoCard Toxins AB® (Mer-tox AB).
Methods: Clinical diarrheal stool samples were simultaneously tested using the QC-Ag and Mer-tox AB. All samples found to be positive using the antigen assay were subsequently tested using QC-TOX AB. Samples with discordant results by the three EIAs were additionally evaluated by cytotoxic assay. True positives were defined as samples testing positive by either: 1) Cytotoxic assay, one toxin EIA test and the QC-Ag, or 2) Both QC-TOX AB and Mer-tox AB. True negatives were interpreted to be samples testing negative by: 1) QC-Ag and Mer-tox AB, 2) both EIA tests or 3) Cytotoxic assay and one of the EIA tests. Discrepant results were resolved by culture and toxogenicity testing.
Results: Of 603 samples, 550 were negative while 53 (9%) were considered diagnostic for C. difficile. Sensitivities for QC-Ag, QC-TOX AB and Mer-tox AB were 100%, 74.5% and 98.1%, respectively. Specificities for each test in the same order were 88.5%, 96.8% and 99.8%. Positive predictive values for QC-TOX AB and Mer-tox AB were 95.3% and 98.1% while their negative predictive values were 81.3% and 99.8%, respectively.
Conclusions: The antigen test demonstrated excellent sensitivity at the expense of specificity. Compared to the QC-TOX AB, the Mer-tox AB assay displayed exceptional testing characteristics. These qualities could permit the Mer-tox AB system to be used as a fast and highly effective stand-alone assay for the identification C. difficile.