Factors associated with post-operative conversion to methicillin-resistant Staphylococcus aureus positivity or infection in initially MRSA-negative patients.

Background: Hospital-acquired methicillin-resistant Staphylococcus aureus (MRSA) is associated with morbid, invasive infections and has been implicated in nearly every type of nosocomial infection. Our aim was to identify the risk factors for patient conversion from MRSA negativity pre-operatively to MRSA positivity post-operatively.

Methods: We retrospectively reviewed all patients at the Veterans Affairs-Boston Health Care System who underwent clean or clean-contaminated surgical procedures during the years 2008 and 2009 and had documented pre-operative nasal polymerase chain reaction (PCR) testing for MRSA. We abstracted post-operative MRSA microbiologic testing results, MRSA infections, surgical site infections (SSIs), surgical prophylaxis data, and SSI risk index, as calculated using the Veterans Affairs Surgical Quality Improvement Project (VASQIP) database variables. All patients who had a negative nasal MRSA PCR result in the 31-day pre-operative period and did not have any positive MRSA clinical swab or culture in the 1-year pre-operative period were defined as MRSA-negative. These patients were classified as converters to MRSA positivity if they had at least one documented positive nasal MRSA PCR swab, culture, nosocomial infection, or SSI within 31 days post-operatively.

Results: Among 4,238 eligible patients, 3,890 (92%) qualified as MRSA-negative pre-operatively. A total of 1,432 (37%) of these patients were assessed in the VASQIP database, of whom 34 (2%) converted to MRSA positivity post-operatively. On multivariable logistic regression analysis of the VASQIP sample, age (odds ratio [OR] 1.049; 95% confidence interval [CI] 1.016, 1.083), SSI risk index (OR 2.863; 95% CI 1.251-6.554), and vancomycin prophylaxis alone or in combination (OR 3.223; 95% CI 1.174-8.845) were significantly associated with conversion to MRSA positivity.

Conclusion: In pre-operatively MRSA-negative patients, age, SSI risk index, and vancomycin prophylaxis were significant factors for conversion to MRSA positivity post-operatively. Alternatives to vancomycin prophylaxis in non-colonized patients and optimization of patients' SSI risk factors should be considered before elective surgery.