2430. Impact of USCAST Proposed Breakpoint Changes to Aminoglycosides, Cyclines, and Levofloxacin on Carbapenem-Resistant Enterobacteriaceae at a US Tertiary Referral Academic Medical Center

BACKGROUND: USCAST is one of many national committees that establish standards for testing and interpreting antimicrobial susceptibility. While working closely with EUCAST, USCAST has proposed updated breakpoints for the aminoglycosides, fluoroquinolones, and tigecycline and is discussing updated breakpoints for the tetracycline antimicrobials. A majority of US hospitals currently utilize FDA or CLSI breakpoints. This study sought to determine the impact of the proposed updated breakpoints on a population of carbapenem-resistant Enterobacteriaceae at a US tertiary referral academic medical center. METHODS: Carbapenem-resistant Enterobacteriaceae (n = 122) from January 2012 to January 2017 were identified as part of routine patient care for study inclusion. Amikacin, gentamicin, tobramycin, levofloxacin, minocycline and tigecycline were evaluated in duplicate on at least two separate occasions by broth microdilution according to CLSI guidelines. The most conservative minocycline breakpoint (≤1 mg/L) being discussed by USCAST was utilized for analysis. McNemar’s test determined significant susceptibility changes between USCAST and FDA/CLSI breakpoints for all CRE and for K. pneumoniae and Enterobacter spp. RESULTS: K. pneumoniae (n = 58; 48%) and Enterobacter spp. (n = 40; 33%) comprised the majority of CRE. P < 0.05 are significant and indicate differences between CLSI/FDA and USCAST susceptibility. CONCLUSION: Implementation of the proposed USCAST susceptibility breakpoints will impact clinician antimicrobial choice regarding the treatment of infections caused by CRE. Amikacin and tigecycline susceptibility markedly decreased when utilizing the proposed USCAST breakpoints. DISCLOSURES: All authors: No reported disclosures.