Wayne, Pennsylvania, USA-January 2012-The Clinical and Laboratory Standards Institute (CLSI) recently published updated editions of its antimicrobial susceptibility testing (AST) standards, Performance Standards for Antimicrobial Disk Susceptibility Tests; Approved Standard-Eleventh Edition (M02-A11) and Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard-Ninth Edition (M07-A9). In addition, the annual update of the well-known supplement to these documents, entitled Performance Standards for Antimicrobial Susceptibility Testing; Twenty-Second Informational Supplement (M100-S22), is available with the purchase of M02-A11 and M07-A9.

"Because antimicrobial resistance is acquired over time, updated standards and guidelines provide the tools to ensure that the latest knowledge can be used to provide consistent, quality data for patient care, and for the public health laboratory system," says Susanne Norris Zanto, MPH, MLS, SM, Deputy Laboratory Director and Lab Systems Improvement Manager, Montana DPHHS Laboratory Services Bureau, Helena, Montana, USA. "The Montana Public Health Laboratory has been able to distribute CLSI documents to encourage our clinical laboratory colleagues to make appropriate practice changes to address the public health challenge of emerging bacterial resistance."

Elizabeth Palavecino, MD, Associate Director of Pathology, Director Clinical Microbiology, Wake Forest Baptist Medical Center, Winston-Salem, North Carolina, USA, agrees. "It is important for CLSI to continuously review the M100 document because new data may support a breakpoint that is different from the one originally assigned to a particular antimicrobial agent, and the breakpoint may need to be modified," she explains. "In addition, the microbiological activity of an agent or its clinical indications may have changed and therefore the indication for testing and reporting may need to be reevaluated. It is critical for me, as director of a clinical microbiology lab, to know that the CLSI M100 document has been reviewed and updated to provide the most current information. This process assures me that I can check the latest version of these documents and give my advice on antimicrobial therapy to my clinical colleagues."

M100-S22 provides updates of the latest recommendations for detecting emerging resistance of aerobic and anaerobic 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.

"The microbial world is continually evolving and developing mechanisms of resistance. The timely updates of CLSI standards assist the clinical microbiology manager in providing the most accurate susceptibility data to health care providers, and the wealth of information included in the M100 document is priceless," says Beth Prouse, MS, MT(ASCP), Clinical Microbiologist, Peninsula Regional Medical Center, Salisbury, Maryland, USA. "The suggested groupings of antimicrobial agents, comments associated with the interpretive standards, and information included in the supplemental tables and appendixes are essential to any clinical laboratory performing antimicrobial susceptibility testing."

Dr. Palavecino adds, "I use the M100 document on a daily basis to review the breakpoints for different organism/drug combinations. If I receive a request for testing an agent not included in our panel, I know that I can count on the M100 document to find recommendations on whether to test and report the agent in question. It is reassuring for clinical microbiology labs to know that the latest versions of the document contain the most updated antimicrobial data, including new MIC breakpoints for new agents that may not be included in routine panels, but that may need to be tested and reported."

New or revised interpretive criteria are available for the following drugs:

  • Ertapenem disk diffusion and minimal inhibitory concentration (MIC) interpretive criteria for Enterobacteriaceae
  • Ciprofloxacin disk diffusion and MIC interpretive criteria for reporting against Salmonella typhi and extraintestinal Salmonella spp. only
  • Piperacillin, piperacillin-tazobactam, ticarcillin, and ticarcillin-clavulanic acid disk diffusion and MIC interpretive criteria for Pseudomonas aeruginosa
  • Doripenem for P. aeruginosa; methicillin-susceptible staphylococci; Haemophilus influenzae, Haemophilus parainfluenzae, Streptococcus pneumoniae (MIC only), Streptococcus spp. (©¬-hemolytic and viridans group MIC only), and anaerobes
  • Imipenem and meropenem for P. aeruginosa


  • New penicillin disk diffusion zone-edge test as an additional screening test for ¥Ã¢-lactamase production in the Staphylococcus aureus group
  • Cefamandole to the list of antimicrobial agents to which BLNAR (¥Ã¢-lactamase negative, ampicillin-resistant) strains of H. influenzae should be considered resistant

Quality control (QC) ranges added for:

  • Ceftaroline-avibactam
  • Ceftazidime-avibactam
  • Doxycycline
  • Finafloxacin
  • Fusidic acid
  • Omadacycline
  • Plazomicin
  • Solithromycin
  • Tedezolid

In addition to publishing CLSI document M100, new versions of its parent documents, M02 and M07, were also released. Every three years, these standards are updated by the CLSI Subcommittee on Antimicrobial Susceptibility Testing and several dedicated working groups to ensure that new methodology is included in the documents. This methodology is essential to correctly interpret and use the annual M100 supplement.

Performance Standards for Antimicrobial Disk Susceptibility Tests; Approved Standard-Eleventh Edition (M02-A11) describes the standard agar disk diffusion techniques used to determine the in vitro susceptibility of aerobic bacteria to antimicrobial agents.

Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard-Ninth Edition (M07-A9) outlines reference methods for the determination of MICs of aerobic bacteria by broth macrodilution, broth microdilution, and agar dilution.

"Disk diffusion is widely used in countries around the world because this is a standardized method that can be used without the need for expensive instrumentation. The updated information in M02 is an effective tool for clinical laboratories using this method for routine susceptibility and, therefore, it is important that this methodology be revised to include new mechanisms of resistance or breakpoint interpretation," explains Dr. Palavecino. "For laboratories using an automated antimicrobial susceptibility system, it is very reassuring that the disk diffusion and broth microdilution methodologies are standardized and available to confirm resistance patterns or unusual results obtained by the automated systems. Revision and standardization of these procedures is critically important because these methods can be used for evaluation and verification of new panels, upgraded instrumentation, or new reporting ranges, and clinical labs can have the assurance that the method has been standardized to include the most current antimicrobial data."

Updates to M02-A11 and M07-A9 include:

  • Information on a new subclass, cephems, including cephalosporins with anti-methicillin-resistant S. aureus (MRSA) activity
  • Nitroimidazoles as a new section, which includes the antimicrobial agents metronidazole and tinidazole
  • Oxazolidinones as a new section, which includes the antimicrobial agent linezolid
  • Streptogramins as a new section, which includes the antimicrobial agents quinupristin-dalfopristin and linopristin-flopristin
  • Additional recommendations for the use of nitrocefin-based tests or the penicillin disk diffusion zone-edge test for isolates of Staphylococcus
  • Additional information on ESBLs (extended-spectrum ¥Ã¢-lactamases) being inhibitor-susceptible enzymes
  • Additional information on how ¥Ã¢-lactam interpretive breakpoints are set at MIC values to recognize ESBL activity
  • Addition of penicillin zone-edge test method as an alternative method for detection of ¥Ã¢-lactamase in staphylococci

"To improve the public health laboratory system, the Montana Public Health Laboratory has distributed these CLSI guidelines to clinical laboratories in the state for the past five years," Zanto says. "A biennial survey of AST practices in these laboratories revealed knowledge increases of up to 64% in specific subject areas in a two-year period following distribution of CLSI standards and subsequent training.

In addition to the documents themselves, the Antimicrobial Susceptibility Testing Searchable CD-ROM, AST QC Flowchart Quick Guides, M100 Tables 1A-1C Quick Guide, and the Glossary of Antimicrobial Terms and Abbreviations Wall Chart for implementing CLSI's highly acclaimed AST standards and guidelines have also been updated.

CLSI and the Association of Public Health Laboratories (APHL) have announced an AST teleconference taking place on February 1, 2012, 1:00-2:30 PM Eastern (US) Time or February 2, 2012, 3:00-4:30 PM Eastern (US) Time. Janet Hindler, MCLS, MT(ASCP), Senior Specialist, Clinical Microbiology, UCLA Medical Center, Los Angeles, California, USA, will provide an overview of the M100-S22 tables, as well as discuss updates of the disk (M02-A11) and MIC (M07-A9) testing standards. There is also a beginner-level AST teleconference scheduled for January 19, 2012, 1:00-2:00 PM Eastern (US) Time, entitled, "The Basics: Using CLSI Antimicrobial Susceptibility Testing Standards." Register for the upcoming teleconferences at

CLSI is a volunteer-driven, membership-supported, nonprofit organization dedicated to developing standards and guidelines for the health care and medical testing community through a consensus process that balances the perspectives of industry, government, and the health care professions. For additional information, visit the CLSI website at or call 610.688.0100.

Posted by Sean Fenske, Editor-in-Chief, MDT