In June 2010, the Clinical and Laboratory Standards Institute (CLSI) published new MIC and disk diffusion interpretive criteria (i.e., breakpoints) for the Enterobacteriaceae for five cephalosporins, including:
CLSI also published initial breakpoints for doripenem. The revised breakpoints are shown in Table I (PDF). 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.
The new CLSI breakpoints simplify susceptibility testing by obviating the need for extended-spectrum beta-lactamase (ESBL) testing, (i.e., testing isolates using cefotaxime and ceftazidime, with and without the beta-lactamase inhibitor, clavulanic acid) to enhance the detection of strains with low-level cephalosporin resistance.
The modified Hodge test (MHT), which was used to detect carbapenemase activity, also is no longer necessary to detect low-level carbapenem resistance, although both ESBL testing and the MHT may be performed to detect these specific resistance mechanisms for infection control purposes.
The decision to revise the CLSI breakpoints came after several years of intense discussion and debate and was ultimately based on:
“Prior to reporting patient test results, the laboratory is responsible for establishing the performance specifications for each modified FDA-cleared or approved test system, each test system not subject to FDA clearance or approval, and each test system for which the manufacturer does not provide performance specifications. The establishment of method performance specifications should provide evidence that the accuracy, precision, analytical sensitivity, and analytical specificity of the procedure is adequate to meet the clients' needs as determined by the laboratory director and clinical consultant. “Modified by the laboratory,” means any change to the assay that could affect its performance specifications for sensitivity, specificity, accuracy, or precision, etc. Laboratory modification of the manufacturer's instructions that could affect performance specifications include but are not limited to:
IDSA members are encouraged to review the new/revised CLSI breakpoints (PDF), assess their impact on patient management, and work with the laboratory to implement the breakpoints as appropriate to optimize treatment of infectious diseases in your institution.
This alert focuses on when and how the new (revised) CLSI breakpoints will be implemented in clinical microbiology laboratories and what role IDSA members can play. For additional information, contact CLSI at firstname.lastname@example.org.
Clinical and Laboratory Standards Institute. 2011. Performance standards for antimicrobial susceptibility testing; Twenty-first informational supplement; M100-S21. Clinical and Laboratory Standards Institute, Wayne, PA.Clinical and Laboratory Standards Institute. 2010. Performance standards for antimicrobial susceptibility testing; Twentieth informational supplement; M100-S20. June 2010 Update. Clinical and Laboratory Standards Institute, Wayne, PA.Clinical and Laboratory Standards Institute. 2010. Performance standards for antimicrobial susceptibility testing; Twentieth informational supplement; M100-S20. Clinical and Laboratory Standards Institute, Wayne, PA.
This alert was developed under the auspices of IDSA’s Antimicrobial Resistance Work Group (ARWG).
IDSA’s Board of Directors wishes to thank additional contributors including: Ruth Lynfield, MD, FIDSA (ARWG chair); Janet Hindler, MCLS MT (ASCP); James Johnson, MD, FIDSA; James Jorgensen, PhD; Jean Patel, PhD, D(ABMM) (CDC Representative); David Schwartz, MD; Edward Septimus, MD, FIDSA; Fred Tenover, PhD, D(ABMM), FIDSA; and Melvin Weinstein, MD (IDSA Liaison to CLSI).
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