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July/August 2016
CLSI Updates on Antimicrobial Susceptibility Testing Are Now Available
IDSA recognizes the importance of access to up-to-date antimicrobial susceptibility breakpoints for ID physicians and is pleased to announce that the Clinical Laboratory Standards Institute (CLSI) Performance Standards for Antimicrobial Susceptibility Testing (M100 document) with all the most current breakpoints for common bacteria is now available for free in an online searchable version.  IDSA is also continuing policy and advocacy efforts to improve and speed up the process through which the Food and Drug Administration (FDA) sets breakpoints.
At its June 2016 meeting, the CLSI AST Subcommittee, which includes an IDSA liaison, addressed the following issues:
+ Due to recent reports of Enterococcus faecium treatment failures with daptomycin, recently defined resistance mechanisms, and modifications in pharmacokinetics/pharmacodynamics (PK/PD) with altered dosing strategies, the committee moved to re-examine the current susceptibility only breakpoint of ≤ 4µg/mL for daptomycin versus Enterococcus spp.    
+ Colistin clinical breakpoints were revised for Acinetobacter sp. and Pseudomonas aeruginosa at ≤ 2 µg/mL susceptible, ≥ 4 resistant (the intermediate category was eliminated). A recommendation was also made to add a comment to the report to remind clinicians to use colistin with an initial loading dose, at maximum doses and in combination with other agents.
+ Two Epidemiologic Cutoff Values (ECV or ECOFF) for clinically relevant bacteria were established.  An ECV is not a clinical breakpoint and is only based on wild type distribution of characterized isolates within a species against a specific antimicrobial. A clinical breakpoint differs from an ECV because breakpoints are also based on pharmacokinetic/pharmacodynamics, animal model efficacy and clinical outcomes. ECVs can be important for detection of the emergence of resistance.  
  • Epidemiologic Cutoff Value (ECV) of ≤ 1 µg/mL was established for N. gonorrhoeae and azithromycin to detect early signals of resistance.  
  • The committee felt that there was still not enough data to set a clinical breakpoint for colistin and Enterobacteriaceae due to issues of testing accuracy, outstanding data on PK/PD and resistance prediction. However, due to the potential for increased colistin/ polymyxin resistance in light of plasmid mediated spread, an ECV was established for Enterobacteriaceae (≤ 2 μg/mL) to detect non-wild type isolates which may harbor resistance mechanisms.  
The CLSI Subcommittee on Antimicrobial Susceptibility Testing appreciates input from IDSA members.  More information about how to contact CLSI and participate in its work is available here
About CLSI:
The Clinical Laboratory Standards Institute (CLSI) is a non-profit membership organization that develops clinical laboratory testing standards based on input from and consensus among industry, government, and health care professionals. CLSI meetings are open to all interested parties.  CLSI has a subcommittee on antimicrobial susceptibility testing (AST), and its decisions are made by a group of volunteers representing the government, industry, and the professions that evaluate contemporary issues and construct the recommendations included in CLSI documents. The subcommittee is comprised of microbiologists, infectious diseases physicians (including an IDSA liaison), pharmacists, public health specialists, and others. Meetings are twice a year at different locations around the country. 

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