424. What is the Clinical Significance of C. difficile Antigen Positive, 2-Step Toxin Negative Stools?
Session: Poster Session: Hospital-acquired and Transplant Infections
Friday, October 30, 2009: 12:00 AM
Room: Poster Hall A
Introduction: It is important to rapidly and accurately identify patients with C. difficile infection (CDI). Our current 2-step method allows rapid diagnosis of CDI by glutamate dehydrogenase antigen (Ag) plus toxin A/B immunochromotography assay (ICA) and detects additional cases by cell culture cytotoxicity neutralization assay (CCNA). Upon review of >800 Ag positive samples, we found that ~1/3 were negative for toxin by both ICA and CCNA. The goal of this study was to determine the significance of positive Ag only stools.
Methods: Stools were tested for Ag and toxin A/B by ICA using the Wampole Quik Chek® and Quik Check Tox A/B IITM tests. CCNA was performed on Ag positive, toxin ICA negative samples. Ag positive, 2-step toxin negative samples were tested by Prodesse ProGastroTM Cd PCR Assay for toxin B gene and culture with indirect CCNA (iCCNA). Results were correlated with clinical data obtained by retrospective chart review.
Results: Ag positive, 2-step toxin negative samples from 55 evaluable patients were analyzed. 24 samples were culture positive but toxin negative by both iCCNA and PCR (non-toxigenic strains). 10 samples were negative by both culture and PCR (false positive Ag tests). 21 were positive for toxin: 20 by PCR and 1 by iCCNA only. 12 toxin positive patients were treated for CDI, of which 8 (67%) were started on therapy prior to stool collection (mean 2.5 days). In 1 patient from which samples were sent on 2 consecutive days, the first was CCNA positive, whereas the second was CCNA negative after ~24hrs of therapy but remained positive by iCCNA and PCR. One additional untreated toxin-positive patient was treated for CDI within 7 days.
Conclusions: The majority of Ag positive, 2-step toxin negative samples represented either non-toxigenic strains (44%) or false positive Ag assays (18%). Among patients with toxin-producing strains who were treated for CDI, 2/3 received treatment prior to stool collection. Therefore, the sensitivity of the 2-step method is reduced when treatment is begun ≥ 1 day prior to stool collection, a common practice.
James Bondi, MS1, Amy Chang, MD1, Susan DiRenzo, MS1, Ignacio Echenique, MD1, Phyllis Flomenberg, MD, Donald Jungkind, PhD3, John Passick, MS1 and  A. Chang, None..
J. Passick, None..
S. DiRenzo, None..
J. Bondi, None..
I. Echenique, None..
D. Jungkind, None..
P. Flomenberg, None., (1)Thomas Jefferson University Hospital, Philadelphia, PA, (2)Thomas Jefferson University, Philadelphia, PA