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February 2016
Update on Antimicrobial Susceptibility Testing from the Clinical and Laboratory Standards Institute (CLSI)
by, Melvin P. Weinstein, MD
Professor of Medicine & Pathology
Chief, Division of Infectious Diseases, Allergy & Immunology
Vice Chair for Clinical Affairs, Department of Medicine
Co-Director, Microbiology Laboratory
Robert Wood Johnson University Hospital
Clinicians depend heavily on information from the clinical microbiology laboratory for treatment of their seriously ill patients.  The clinical importance of antimicrobial susceptibility test results demands that testing be performed under optimal conditions and that laboratories have the capability to provide results for the newest antimicrobial agents.  The CLSI Subcommittee on Antimicrobial Susceptibility Testing provides laboratories and clinicians with annual updates.  Several recent and important changes within CLSI are noteworthy for the Infectious Diseases community.  
First, CLSI has now made several pivotal documents available free of charge on the CLSI Website in a non-downloadable format.  This includes the crucial M100S Document (Performance Standards for Antimicrobial Susceptibility Testing) which contains the most current information for antibiotic selection and interpretation of susceptibility test results for the most common bacterial pathogens.  Several notable additions and changes in the new edition of M100S include:
• Ceftolozane-tazobactam breakpoints for Enterobacteriaceae and viridans streptococci
• Clarifications for testing cefazolin against Enterobacteriaceae
• Oritavancin MIC criteria for S. aureus, vancomycin-susceptible E. faecalis, beta-haemolytic streptococci, and viridans streptococci
• Televancin MIC and disk criteria for S. aureus, vancomycin-susceptible E. faecalis, and viridans streptococci
• Tedizolid MIC criteria for S. aureus, vancomycin-susceptible E. faecalis, S. pyogenes, S. agalactiae, and S. anginosus group 
The antifungal documents which contain interpretive criteria for yeasts (M27/M44S) and molds (M38/M51S) as well as the epidemiologic cutoff values for fungi (M57S) will also be available free of charge pending their revision/publication (anticipated November, 2016).  These documents can be accessed via the CLSI Home Page at  
Second, owing to concerns of potential conflict of interest and after careful consideration, employees of the pharmaceutical industry or individuals who derive significant portions of their income from the pharmaceutical industry may no longer serve as chairholder, vice-chairholder or voting members of the CLSI Subcommittees.  However, they are still permitted to serve on scientific working groups and to attend Subcommittee meetings.  The adoption of this policy reflects an attempt to create the optimal balance between ensuring that CLSI attracts, retains, and benefits from leading experts participating in the standards development consensus process while ensuring that potential conflicts have been appropriately disclosed and do not influence the standard setting process.  
For ID physicians who are not familiar with CLSI, it is a voluntary, non-profit, consensus guidelines and standards organization that has provided guidance for clinical laboratory testing for nearly 50 years.  For many years, IDSA has had a representative to CLSI who serves as a voting member on the AST Subcommittee (most recently, Melvin P. Weinstein, MD).  In addition, the CLSI Subcommittees on Antimicrobial Susceptibility Testing (AST) are led by IDSA members (Antibacterial Subcommittee Chair and Vice-Chair: Jean B.  Patel, PhD and Melvin P. Weinstein, MD, respectively, and Antifungal Subcommittee Chair and Vice-Chair: Barbara D. Alexander, MD and Gary W. Procop, MD, respectively).  The committees include ID physicians, clinical microbiologists, and PharmD’s from academia, government, and the diagnostics industry.  CLSI welcomes volunteers.  If you are interested in becoming involved, please email Ms. Tracy Dooley ( or one of the Chairs or Vice-Chairs mentioned above.

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