1 in 3 with MRSA not nasally colonised

Link: Prediction of methicillin-resistant Staphylococcus aureus involvement in disease sites by concomitant nasal sampling -- Robicsek et al., 10.1128/JCM.01746-07 -- Journal of Clinical Microbiology.

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.