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Essential New CLSI Antimicrobial Susceptibility Testing Updates Scheduled for Release in January 2007

Performance Standards for Antimicrobial Susceptibility Testing; Seventeenth Informational Supplement (M100-S17); and Methods for Antimicrobial Susceptibility Testing of Anaerobic Bacteria (M11-A7)

Interpretive Criteria and Quality Control Ranges Assist Clinical Microbiologists with Day-to-Day Challenges

Each January, the well-known CLSI antimicrobial susceptibility testing standard, Performance Standards for Antimicrobial Susceptibility Testing (M100), provides updates of the latest recommendations for detecting emerging resistance of aerobic bacteria—arranged in tabular format. The “breakpoints” included in the supplement are defined as specific values on the basis of which bacteria can be assigned to the clinical categories of susceptible, intermediate, or resistant. This essential information assists clinicians with drug selection, interpretation, and quality control using the procedures standardized in CLSI documents Performance Standards for Antimicrobial Disk Susceptibility Tests; Approved Standard—Ninth Edition (M2-A9); and Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard—Seventh Edition (M7-A7). Janet F. Hindler, MCLS, MT(ASCP), Senior Specialist, Clinical Microbiology, UCLA Medical Center, explains, “For over three decades, NCCLS and now CLSI consensus standards have been the primary resources for antimicrobial susceptibility testing in the clinical laboratory. The annual updates provide the most current recommendations to ensure accurate detection and reporting of emerging resistance.”

Recommendations for Detecting Emerging Resistance

Emerging resistance is among the most significant challenges to managing patient care for infectious diseases. 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 requires that these tests be done under optimal conditions and that laboratories have the capability to provide results for the newest antimicrobial agents. James H. Jorgensen, PhD, Professor of Pathology and Medicine, The University of Texas Health Science Center, says, “As the era of antimicrobial resistance continues to evolve, it is critical that clinical microbiology laboratories perform the most reliable tests for prompt detection of emerging resistance, and understand how to communicate those results clearly to their clinicians. The frequently updated CLSI microbiology documents represent the best sources for this important information.”

Key Updates in Performance Standards for Antimicrobial Susceptibility Testing; Seventeenth Informational Supplement (M100-S17), to Publish January 2007

Performance Standards for Antimicrobial Susceptibility Testing; Seventeenth Informational Supplement is scheduled for publication in January 2007. This edition includes updated tables from CLSI’s newest disk (M2-A9) susceptibility and MIC (M7-A7) testing standards. In addition, a new feature in the document provides three pages of adhesive index tabs, which can be inserted for quick access to each table.

Updates include:

  • new disk diffusion and MIC interpretive criteria for colistin and polymyxin B;
  • new antimicrobial agents and QC ranges;
  • preparation of stock solutions for antimicrobial agents provided with activity expressed as units;
  • added information on the development of resistance and testing of repeat isolates;
  • modified recommendations for testing and reporting designated clusters of agents with similar interpretive results and clinical efficacy;
  • recommendations for reporting critical results; and
  • suggestions for verification and confirmation of susceptibility testing results for Neisseria meningitides.

Standard Reference Methods for Anaerobic Bacteria

CLSI also produces standards for testing the in vitro susceptibility of anaerobic bacteria in its document, Methods for Antimicrobial Susceptibility Testing of Anaerobic Bacteria (M11). Susceptibility testing of anaerobes is recommended for surveillance purposes and for specific clinical situations. Two endpoint-determining susceptibility testing methods for anaerobic bacteria are described in the M11 standard. The agar dilution method (Wadsworth) remains the reference standard, and is well suited for surveillance testing and research. It is also the standard to which other methods are compared. Broth microdilution is well suited for the clinical laboratory, but is currently limited to testing of Bacteroides fragilis group organisms and selected antibiotics. Diane Citron, M(ASCP), R.M. Alden Research Laboratory, describes M11 as, “…a helpful, practical resource for reference laboratories that are going to be testing many organisms using the agar dilution method to evaluate the potency of new drugs. It is one of the best step-by-step descriptions of the agar dilution method published anywhere. The preparation and procedures for the day of testing are comprehensively outlined. It can be used for testing aerobic bacteria as well.”

The recent and varied trends in antibiotic resistance, the spread of resistance genes, and the potential for poor clinical outcomes when using an ineffective antibiotic argue strongly for more susceptibility testing of anaerobic organisms. David W. Hecht, MD, Loyola University Medical Center, concurs: “With the increasing resistance among anaerobic bacteria, the importance of doing susceptibility testing has never been greater.”

Key Updates in Methods for Antimicrobial Susceptibility Testing of Anaerobic Bacteria; Approved Standard—Seventh Edition (M11-A7), to Publish January 2007

Antimicrobial Susceptibility Testing of Anaerobic Bacteria; Approved Standard—Seventh Edition (M11-A7) is scheduled for publication in January 2007. M11-A7 provides reference methods for the determination of minimal inhibitory concentrations (MICs) of anaerobic bacteria by agar dilution and broth microdilution. The document is complete with updated tables for AST of anaerobic bacteria.

Updates include:

  • new QC and interpretive criteria for moxifloxacin;
  • new QC strain Clostridium difficile, with QC ranges for various antimicrobial agents;
  • alternative methods provided for generating an anaerobic environment; and
  • new antimicrobial agents and QC ranges to test for and report.

Regarding the importance of the addition of the Clostridium difficile QC strain, Dr. Hecht explains, “There is a lot of attention being brought to Clostridium difficile as one of the anaerobes. The addition of that quality control strain in this document is important for testing that organism, as well as testing agents active against many other gram-positive anaerobes.”  Citron agrees: “The new QC strain is very helpful for the new gram-positive drugs that are being developed. So, the new Clostridium difficile QC strain addresses the agar dilution QC values for testing these new antibiotics.” Dr. Hecht adds, “The additional updates and revisions in this edition make it as easy as possible for clinical microbiologists to understand and use the document.”

Practical Application

When asked about the practical application of M11, Dr. Hecht says, “For my hospital, we use agar dilution testing methods directly from the document to test anaerobic isolates—not only here, but for 12 other hospitals.” He adds, “M11 was used specifically to develop, test, and validate the new QC strain for Clostridium difficile included in this latest edition.”

Citron states, “I have been using the techniques in M11 for years. I trained at Wadsworth, which is one of the primary AST methods described in the document. I absolutely reference this document, use all of the QC ranges, and record all of our QC ranges to make sure that they follow the values in the CLSI document.” She adds, “Much of the research work we do is for companies that will be taking their drugs to FDA for approval for treating anaerobic infections. All of our data must be acceptable to the FDA, and that only occurs if we are in accordance with a reference method and are using the standard values in the CLSI documents.”

For additional information about M100-S17 and M11-A7, and to pre-order copies of the documents, visit www.clsi.org, call +610.688.0100, or e-mail customerservice@clsi.org. [ return to top ]

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