162. Identifying Determinants of Therapeutic Switch to Linezolid among Patients with Methicillin-Resistant Staphylococcus aureus Bloodstream Infections

BACKGROUND: In order to target future randomized controlled trials (RCT) of treatment of methicillin-resistant S aureus bloodstream infections (MRSA BSI), it will be important to understand the drivers of antibiotic selection. We aimed to determine factors associated with switching from vancomycin to inpatient linezolid administration during the management of MRSA BSI. METHODS: This retrospective cohort included all patients admitted to Veteran Affairs hospitals from 2007 to 2014 and had received vancomycin for MRSA BSI. Patients were considered to have switched to linezolid from vancomycin if they received at least 2 consecutive days of inpatient treatment and were not on concurrent vancomycin treatment. Cox proportional hazards models were used to identify factors that were associated with switch within 14 days and 30 days. Median with interquartile range (IQR), hazard ratio (HR) and 95% confidence intervals were reported. RESULTS: Among 7289 patients diagnosed with MRSA BSI during their index admission, 474 (6.5%) switched to linezolid during the admission. The median inpatient duration of vancomycin treatment among all patients was 13 days (IQR: 5–34) and among patients who switched was 16 days (IQR: 6–52). The median inpatient duration of linezolid treatment was 5 days (IQR: 1–13 days). Patients who switched to linezolid were more likely to have a MRSA isolate with MIC >=2 µg/mL (6.8% vs. 4.9%), diagnosis of respiratory tract infection (36.7% vs. 32.9%), or be obese (16.5% vs. 13.6%) than those who continued on vancomycin (P < 0.10). In risk-adjustment models, presence of a respiratory tract infection diagnosis was associated with greater likelihood of being switched to linezolid within 14- and 30-days (HR=1.29, 95% CI 1.01–1.64; HR=1.32, 95% CI 1.06–1.65). CONCLUSION: Less than 10% of patients initially treated with vancomycin for MRSA BSI were switched to linezolid in this real-world study. A diagnosis of respiratory tract infection was a major determinant of switching to linezolid. It is important to identify potential subsets of MRSA BSI patients so that future comparative effectiveness RCTs can be targeted to indications with clinical equipoise in real-world practice settings. DISCLOSURES: All authors: No reported disclosures.