BACKGROUND: Asymptomatic colonization with methicillin-resistant Staphylococcus 
aureus (MRSA) has been described as a risk factor for subsequent MRSA infection. 
MRSA is an important nosocomial pathogen but has currently been reported in 
patients without typical risk factors for nosocomial acquisition. This study was 
designed to evaluate the impact of asymptomatic nares MRSA colonization on the 
development of subsequent MRSA infection. The incidence of MRSA infection was 
examined in patients with and patients without MRSA or methicillin-susceptible 
S. aureus (MSSA) colonization at admission to the hospital and in those who 
developed colonization during hospitalization.
METHODS: Patients admitted to 5 representative hospital units were prospectively 
evaluated. Nares samples were obtained for culture at admission and during 
hospitalization. Laboratory culture results were monitored to identify all MRSA 
infections that occurred during the study period and 1 year thereafter.
RESULTS: Of the 758 patients who had cultures of nares samples performed at 
admission, 3.4% were colonized with MRSA, and 21% were colonized with MSSA. A 
total of 19% of patients with MRSA colonization at admission and 25% who 
acquired MRSA colonization during hospitalization developed infection with MRSA, 
compared with 1.5% and 2.0% of patients colonized with MSSA (P<.01) and 
uncolonized (P<.01), respectively, at admission. MRSA colonization at admission 
increased the risk of subsequent MRSA infection, compared with MSSA colonization 
(relative risk [RR], 13; 95% confidence interval [CI], 2.7-64) or no 
staphylococcal colonization (RR, 9.5; 95% CI, 3.6-25) at admission. Acquisition 
of MRSA colonization also increased the risk for subsequent MRSA infection, 
compared with no acquisition (RR, 12; 95% CI, 4.0-38).
CONCLUSION: MRSA colonization of nares, either present at admission to the 
hospital or acquired during hospitalization, increases the risk for MRSA 
infection. Identifying MRSA colonization at admission could target a high-risk 
population that may benefit from interventions to decrease the risk for 
subsequent MRSA infection.