Standards In Action: M2, M7, M100
CLSI Antimicrobial Susceptibility Testing Standards

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.

A variety of laboratory methods can be used to measure the in vitro susceptibility of bacteria to antimicrobial agents.  Many clinical microbiology laboratories routinely use an agar disk diffusion method for testing common, rapidly growing, and certain fastidious bacterial pathogens.  Other laboratories employ standard broth dilution (macrodilution and microdilution) and agar dilution techniques, which enable the discovery of the minimal inhibitory concentration of drug needed to treat an infection.

CLSI offers standards for antimicrobial susceptibility testing, which serve as sources for new antimicrobial information, methods for detecting new or changing resistance mechanisms, and updated breakpoints.  These documents have been simultaneously updated for publication this month:

• M2-A9—Performance Standards for Antimicrobial Disk Susceptibility Tests; Approved Standard—Ninth Edition, which contains the current CLSI-recommended methods for disk susceptibility testing.
• M7-A7—Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard—Seventh Edition, which describes standard broth dilution (macrodilution and microdilution) and agar dilution techniques for measuring the in vitro susceptibility of bacteria to antimicrobial agents.
• M100-S16—Performance Standards for Antimicrobial Susceptibility Testing; Sixteenth Informational Supplement, which provides updated tables for the CLSI antimicrobial susceptibility testing standards M2-A9 and M7-A7. 

These documents play a vital role for both microbiology laboratories, and the manufacturers of the commercial tests and instruments used by those laboratories.

“We may look at the M100 supplement in the laboratory every day or multiple times a week in order to answer certain difficult questions that come up in the lab,” says Richard B. Thompson, PhD, Professor of Pathology at Northwestern University Feinberg School of Medicine.  “We may use the supplement when we have discussions with the clinicians in the hospital who are taking care of the patient, and are wondering about a breakpoint, or the activity of a drug.”

Clinical Microbiology Laboratory Technical Specialist Violeta Rekasius, MT, ASCP, of the Loyola University Medical Center, says “the M100 tables guide all decisions in the clinical microbiology lab.”

To facilitate access to the most commonly used tables, Ms. Rekasius says, the Loyola University Medical Center Clinical Microbiology Laboratory color codes and posts on a bulletin board tables including proper zone sizes, MICs, and interpretations, quality control strains and acceptable limits, appropriate antimicrobial choices, and proper testing conditions.  This practice of customizing tables for quick reference—replicated in a number of clinical microbiology laboratories—has prompted CLSI to include index tabs with new editions of the M100 supplemental tables.  Users will now be able to apply the self-adhesive tabs to the first page of each table for easy reference.

Paul Schreckenberger, PhD, Professor of Pathology at Loyola University, points to the utility of the M100 in addressing new antibiotics.

“Most laboratories use a variety of susceptibility testing methods,” Dr. Schreckenberger says.  “This is where the M100 tables become extremely helpful because they provide breakpoint interpretations for newer antibiotics and also provide guidance for testing on the automated susceptibility testing instruments.”

"M2 and M7 are kept in the lab as references and are used primarily for teaching purposes,” Ms. Rekasius says, “while the M100 tables are used quite frequently to check various parameters that are required in daily lab work in order to provide proper results to clinicians.” 

Barbara Zimmer, PhD, Director, Clinical and Scientific Affairs at Dade Behring MicroScan, attests to the vital role played by the standards in the industry sector.

"For manufacturers of diagnostic equipment which give antibiotic susceptibility testing results,” Dr. Zimmer says, “the M2 and M7 reference methods are considered the gold standard."

Along with products used by microbiology laboratories to identify pathogenic bacteria, Dade Behring's MicroScan division produces methods for laboratories to determine the minimum dose of antimicrobial agents needed to treat infection.  Because CLSI's M7 standard specifically addresses reference methods for determining minimal inhibitory concentrations (MICs) of aerobic bacteria, Dr. Zimmer says, "all of (MicroScan's) studies are done in comparison to results generated by methods prepared according to CLSI M7 as a predicate device."  

The newly approved M2-A9 and M7-A7 documents, which include the M100-S16 supplement, are now available for purchase though Clinical and Laboratory Standards Institute.  To purchase, please contact customer service at +610.688.0100 or visit our online store.

  [ return to top ]