JCM Accepts, published online ahead of print on 21 October 2009

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J. Clin. Microbiol. doi:10.1128/JCM.01332-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The clinical application of real-time PCR to screening critically ill and emergency surgical patients for methicillin-resistant Staphylococcus aureus: a quantitative analytical study.

M. Trent Herdman, Duncan Wyncoll, Eugene Halligan, Penelope R. Cliff, Gary French, and Jonathan D. Edgeworth^*

Directorate of Infection, St. Thomas' Hospital, Guy's & St. Thomas' NHS Foundation Trust, London; Department of Critical Care, St. Thomas' Hospital, Guy's & St. Thomas' NHS Foundation Trust, London; Infection and Immunology Delivery Unit, Guy's and St Thomas' NHS Foundation Trust, London; Department of Infectious Diseases, King's College London School of Medicine

The clinical utility of real-time PCR screening assays for methicillin-resistant Staphylococcus aureus (MRSA) colonization is constrained by the predictive values of their results: as MRSA prevalence falls, the assay's positive predictive value drops, and a rising proportion of positive PCR assays will not be confirmed by culture. We provide a quantitative analysis of universal PCR screening of critical care and emergency surgical patients using the BD GeneOhm MRSA PCR system, involving 3,294 assays over six months. 248 PCR assays (7.7%) were positive; however, 88 failed to be confirmed by culture, giving a PPV of 65%. Multivariate analysis was performed to compare PCR-positive culture-positive (P+C+) and PCR-positive culture-negative (P+C-) assays. P+C- results were positively associated with a history of methicillin-sensitive Staphylococcus aureus infection or colonization [OR (95%CI): 3.15 (1.32-7.54)], and high Ct values, indicative of a low concentration of target DNA [OR: 1.19 per cycle (1.11-1.26)]. P+C- results were negatively associated with a history of MRSA infection or colonization [OR: 0.19 (0.09-0.42)] and male sex [OR: 0.40 (0.20-0.81)]. P+C+ patients were significantly more likely to have subsequent positive MRSA culture assays and microbiological evidence of clinical MRSA infection. The risk of subsequent MRSA infection in P+C- patients was not significantly different from that in case-matched PCR-negative controls. We conclude that, given the low PPV and poor correlation between a PCR-positive assay and clinical outcome, it would be prudent to await culture confirmation before altering infection control measures on the basis of a positive PCR result.