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Background Polymyxins are important antimicrobial agents for the treatment of infections due to carbapenem-resistant and other multidrug-resistant organisms. Recently, the CLSI and EUCAST have set breakpoints for colistin (EUCAST and CLSI) and polymyxin B (CLSI) with slight differences in recommendations. However, there are issues unique to the polymyxin class that warrant additional guidances. Herein, we assess data related to breakpoint setting and make additional recommendations for polymyxin AST interpretive criteria. Methods Data sources included longitudinal (2011–2017) US surveillance reference broth microdilution (BMD) MIC distributions (128,573 isolates) for colistin and polymyxin B (PB), published data on accuracy of various AST methodologies, in vivo pharmacokinetic/pharmacodynamic (PK-PD) models, prior polymyxin guidelines and agency package insert dosing recommendations, and population PK-PD and toxicodynamic (TD) data. Epidemiological cut-off, PK-PD (and TD), and clinical data were all considered for susceptible (S) breakpoint determinations. Results Data demonstrate that the most commonly utilized AST methodologies (disk diffusion, Etest, and automated MIC susceptibility panels), as well as agar dilution testing cannot reliably detect resistance; and BMD is the preferred AST. Importantly, colistin S is a reliable surrogate for PB S with cross-S accuracy at > 99% of isolates in each pathogen group. Breakpoint recommendations can be found in the Table with emphasis on applying combination therapy. Key recommendations include an S breakpoint of ≤2 mg/L for each pathogen (both colistin and PB). However, based on a lack of preclinical efficacy in murine pneumonia models, PK/PD concerns, and poor clinical outcome data, we strongly suggest that no breakpoints are applied for pneumonia and that alternative therapies should be used where available. Additionally, due to a lack of significant renal excretion, PB will also have no S breakpoint recommendation for lower urinary tract infections. Conclusion The polymyxins have compromising characteristics that make them suboptimal antimicrobials when used alone, and additional caveats are required for AST breakpoint interpretive criteria and stewardship programs. Disclosures All authors: No reported disclosures.

open forum infectious diseases

1534. Polymyxin Antimicrobial Susceptibility (AST) Testing and Breakpoints for P. aeruginosa, A. baumannii, and Enterobacteriaceae: Recommendations from the United States Committee on Antimicrobial Susceptibility Testing (USCAST)