310. The Impact of Microscan versus Vitek-2 for Automated Susceptibility Testing on the Utilization of Vancomycin Alternatives for the Treatment of Methicillin-Resistant Staphylococcus aureus Bacteremia

Background Automated susceptibility testing (AST) provides minimum inhibitory concentrations (MIC) to guide effective antibiotic therapy. AST is critical for methicillin-resistant Staphylococcus aureus (MRSA) bacteremia, as susceptible MIC values ≥ 1.5 µg/mL are associated with vancomycin (VAN) failure. The Microscan (MS) instrument may report elevated MIC values compared to Vitek-2 (VTK), thus impacting treatment. This study aimed to evaluate the impact of MS versus VTK on VAN alternative use in the treatment of MRSA bacteremia in a Texas health system. Methods This was a retrospective cohort study of patients admitted to the Ascension Seton health system in Austin, TX. Patient eligibility included: age ≥18 years, ≥1 positive MRSA blood culture, ≥72 hours of MRSA therapy, and VAN use within 48 hours of positive culture. Patients were stratified into the MS group (May 2013-Dec 2016) and VTK group (Jun 2017-Mar 2020). The primary outcome was therapy switch from VAN to VAN alternatives. Secondary endpoints include S. aureus MIC, 30-day all-cause mortality, 30 and 90-day readmission, and length of hospital stay (LOS). Outcomes were compared between groups using appropriate bivariable comparisons, as well as multivariable logistic regression and propensity score-adjusted logistic regression. Results A total of 199 patients were included: 91 in the MS group and 108 in the VTK group. Switch to VAN alternative was 56% vs. 19% (p< 0.0001) for MS and VTK, respectively. The median (interquartile range) MIC value reported was 2 μg/mL (2 – 2) and 1 μg/mL (0.5 – 1) for MS and VTK, respectively (p< 0.0001). Thirty-day readmission (19% vs. 20%, p=0.7647) and 30-day mortality (10% vs. 9%, p=0.5262) were comparable between MS and VTK groups, respectively. Hospital LOS significantly decreased in the VTK period (16 days vs. 12 days, p=0.0153). The MS group was the only independent positive predictor of VAN alternative therapy: logistic regression, OR 5.64 (95% CI 1.67–18.99) and propensity score adjusted, OR 4.21 (95% CI 1.32–13.48). Conclusion Since implementation of VTK from MS, Ascension Seton hospitals experienced a decreased median VAN MIC for MRSA bacteremia as well as therapy switches from VAN to VAN alternatives without affecting other patient health outcomes. Disclosures All Authors: No reported disclosures