Background: Contact isolation of methicillin-resistant Staphylococcus aureus(MRSA) carriers is designed to protect non-carriers from MRSA infection. The Cleveland Clinic does not place MRSA carriers in contact isolation. The purpose of this study was to examine the value of contact isolation by comparing the risk of MRSA infection in MRSA carriers and non-carriers, in the absence of contact isolation.
Methods: Adult patients hospitalized at Cleveland Clinic from Jan 1, 2008 to December 31, 2015, who were tested for S. aureuscolonization by PCR or culture of a nasal swab at least once during the hospitalization were screened for inclusion. Only the first hospitalization per patient was considered. Included patients were divided into MRSA carriers and non-carriers, based on the result of their first nasal MRSA test result. Among these patients, the risk of subsequent MRSA bloodstream infection (BSI) and non-bacteremic MRSA infection during the same hospitalization were determined, and compared, for non-carriers versus carriers.
Results: Of 74595 patients identified, 1223 were excluded because they had a S. aureus infection within 3 days of admission to the hospital. Of the remaining 73372 patients, 5% were MRSA carriers. One hundred twenty (0.2%) of 69452 non-carriers developed an MRSA infection during the same hospitalization compared to 82 (2.1%) of the 3920 MRSA carriers (RR 0.08, 99% CI 0.06 0.12, p-value 3.62 × 10-50). Relative risks were very similar when analyzed separately for bacteremic and non-bacteremic infection. A Monte Carlo simulation with 1000 trials simulating corrections for false positive and false negative nasal MRSA tests found very similar results. The magnitude of the effect was similar across subgroups of age, sex, year, and hospital length of stay (figure).
Conclusion: Non-carriers are at much lower risk of developing MRSA infection during a hospitalization than are MRSA carriers. The absolute risk of MRSA infection in non-carriers is one-fifth of 1%, even in the absence of protection by contact isolation of MRSA carriers. These findings suggest that the focus of preventing MRSA infections should be on protecting MRSA carriers, not the non-carriers.
Figure: Relative Risk, across subgroups, of MSSA carriers developing MRSA infection compared to MRSA carriers.
S. Gordon, None