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J. Clin. Microbiol. doi:10.1128/JCM.01746-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Prediction of methicillin-resistant Staphylococcus aureus involvement in disease sites by concomitant nasal sampling

Northwestern University's Feinberg School of Medicine, Departments of Medicine, and Pathology; Evanston Northwestern Healthcare, Division of Infectious Diseases and Division of Microbiology, Department of Pharmacy, and Center on Outcomes, Research, and Education

^* To whom correspondence should be addressed. Email: arobicsek{at}enh.org.

	Abstract

Background. Nasal colonization with methicillin-resistant Staphylococcus aureus (MRSA) is believed to precede disease. It is therefore reasonable to expect that testing for nasal MRSA colonization could provide guidance in choice of empiric therapy for infections.

Methods. Retrospective review of 5,779 nasal MRSA tests obtained within 24 hours before or after a clinical culture growing any organism.

Results. A positive nasal MRSA test strongly predicted MRSA involvement in a clinical site (relative risk 12.9, 95% CI, 10.4, 16.1). Nasal MRSA colonization also strongly predicted antimicrobial resistance in other organisms. A negative nasal test was less useful; only 217 (67.2%, 95% CI 61.8, 72.3) of 323 patients with clinical cultures involving MRSA had detectable concomitant nasal MRSA colonization. Patients with clindamycin-susceptible MRSA were less likely (59%) to have nasal colonization than those with clindamycin-resistant MRSA (71%, P = 0.042).

Conclusions. Patients nasally colonized with MRSA were substantially more likely to have antibiotic resistant flora in clinical isolates and this should be considered when initiating therapy. However, nearly a third of MRSA-infected patients were not nasally colonized, suggesting that nasal colonization need not precede disease and that a negative test for nasal colonization would not rule out MRSA disease in settings of moderate or high prevalence.

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