1060. Impact of Laboratory Cessation of Extended-Spectrum β-Lactamase (ESBL) Reporting on Minimum Inhibitory Concentration (MIC) Distribution Trends and Associated Clinical Outcomes

BACKGROUND: The Clinical and Laboratory Standards Institute (CLSI) lowered the minimum inhibitory concentration (MIC) breakpoints of various β-lactam antimicrobials eliminating the need for confirmatory testing of extended-spectrum β-lactamase (ESBL) organisms. Our institution adopted the new CLSI breakpoints in June 2015. This multi-site study assessed the impact of laboratory cessation of ESBL reporting on the MIC distribution of commonly used antimicrobials and clinical outcomes. METHODS: This retrospective study included adult inpatients with positive blood cultures for Escherichia coli, Klebsiella pneumoniae, K. oxytoca, or Proteus mirabilis from June 2012 to June 2018. Patients were included in the pre-implementation group if they had an ESBL-positive blood culture from June 2012 to May 2015 and in the post-implementation group if they had a ceftriaxone-resistant organism from June 2015 to June 2018. Patients who died or transitioned to hospice within 48 hours of blood culture identification or before final susceptibilities were excluded. The primary outcome was MIC distribution of ceftriaxone, ceftazidime, cefepime, piperacillin/tazobactam, fluoroquinolones, and carbapenems. Secondary outcomes were antimicrobial prescribing patterns, 30-day all-cause mortality, 30-day re-infection rate, and time to microbiological clearance. RESULTS: A total of 249 patients were included (n = 40, pre-implementation; n = 209, post-implementation). Pitt Bacteremia Scores were significantly higher in the pre-implementation group (3.59 ± 2.85 vs. 2.21 ± 2.06; P = 0.0004). The median MIC distribution for each antimicrobial stayed within one dilution throughout the study timeframe. Carbapenem use decreased in the post-implementation group [n = 35 (87%) vs. n = 131 (63%)]. No significant differences were noted for other secondary outcomes: 30-day all-cause mortality (15% vs. 10%; P = 0.40), 30-day re-infection rate (2.5% vs. 4.3%; P = 1), and time to microbiological clearance (2.28 ± 1.2 vs. 2.41 ± 1.76 days; P = 0.72). CONCLUSION: Adoption of lowered CLSI breakpoints did not impact MIC distribution of select antimicrobials for Enterobacteriaceae; however, it has affected antimicrobial prescribing patterns. DISCLOSURES: All authors: No reported disclosures.