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January 2011
Patient Care and Science
CLSI Publishes New Breakpoints for Antimicrobial Susceptibility Testing

The Clinical and Laboratory Standards Institute (CLSI) has published new breakpoints for antimicrobial susceptibility testing. IDSA members are encouraged to review the new/revised CLSI breakpoints, assess their impact on patient management, and work with laboratories to implement the breakpoints as appropriate to optimize treatment of infectious diseases in your institution.

In June 2010, CLSI published new minimal inhibitory concentration (MIC) and disk diffusion interpretive criteria (i.e., breakpoints) for the Enterobacteriaceae for five cephalosporins, including first-generation and third-generation (extended-spectrum) cephalosporins, aztreonam, and for three carbapenems (imipenem, meropenem, and ertapenem). CLSI also published initial breakpoints for doripenem. The new MIC breakpoints are one to three doubling dilutions lower than the original breakpoints, and the new disk diffusion criteria include larger zone diameters than those in previous guidelines. Thus, many organisms that would have been categorized previously as susceptible using the former breakpoints may now be considered intermediate or resistant.

Please visit IDSA’s website for an alert with more information, including the critical issue of when and how the revised CLSI breakpoints will be implemented in clinical microbiology laboratories, particularly in the United States, and what role IDSA members can play in this process. This alert was developed under the auspices of IDSA’s Antimicrobial Resistance Work Group.

Please review the entire alert on antimicrobial susceptibility testing, including the additional references, before proceeding with any changes. For additional information, please contact CLSI at
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