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J Antimicrob Chemother. 2012 Mar;67(3):622-32. doi: 10.1093/jac/dkr524. Epub 2011 Dec 13.

Effects of clinical breakpoint changes in CLSI guidelines 2010/2011 and EUCAST guidelines 2011 on antibiotic susceptibility test reporting of Gram-negative bacilli.

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Institut für Medizinische Mikrobiologie, Universität Zürich, Zürich, Switzerland.



The aim of this study was to analyse the effects of clinical breakpoint changes in CLSI 2010 and 2011 guidelines and EUCAST 2011 guidelines on antibiotic susceptibility testing (AST) reports.


In total, 3713 non-duplicate clinical isolates of Enterobacteriaceae, Pseudomonas aeruginosa, Stenotrophomonas maltophilia and Acinetobacter baumannii were analysed. Inhibition zone diameters were determined for β-lactams, carbapenems, fluoroquinolones, aminoglycosides and trimethoprim/sulfamethoxazole. CLSI 2009-11 and EUCAST 2011 clinical breakpoints were applied.


Changes in resistance as defined per the guidelines affected individual species and drug classes differently. The cefepime resistance rate in Escherichia coli and Enterobacter cloacae increased from 2.1% and 1.3% to 8.2% and 6.9%, respectively, applying CLSI 2009-11 versus EUCAST 2011 guidelines. Ertapenem resistance rates in E. cloacae increased from 2.6% with CLSI 2009 to 7.2% for CLSI 2010 and 2011, and to 10.1% when applying EUCAST 2011. Cefepime and meropenem resistance rates in P. aeruginosa increased from 12.2% and 20.6% to 19.8% and 27.7%, respectively, comparing CLSI 2009-11 with EUCAST 2011. Tobramycin and gentamicin resistance rates in A. baumannii increased from 15.9% and 25.4% to 34.9% and 44.4% applying CLSI 2009-11 versus EUCAST 2011.


Higher resistance rates reported due to breakpoint changes in CLSI and EUCAST guidelines will result in increasing numbers of Gram-negative bacilli reported as multidrug resistant. AST reports classifying amoxicillin/clavulanic acid, cefepime or carbapenem resistance will lead clinicians to use alternative agents. Upon implementation of the EUCAST guidelines, laboratories should be aware of the implications of modified drug susceptibility testing reports on antibiotic prescription policies.

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