Antimicrobial resistance to cefotaxime and ertapenem in Enterobacteriaceae: the effects of altering clinical breakpoints

Introduction: The Clinical and Laboratory Standards Institute (CLSI) updated its antimicrobial susceptibility testing interpretation criteria for Enterobacteriaceae . This study assessed the effects of clinical breakpoint changes in the CLSI 2009 to 2012 guidelines on antibiotic susceptibility testing reports. Methodology: In total, 2,076 non-duplicate clinical isolates of Enterobacteriaceae were analyzed. The disk diffusion method was used for susceptibility testing. The CLSI 2009-12 clinical breakpoints were applied to determine susceptibility of cefotaxime and ertapenem. Combined-disk testing was used for phenotypic confirmation of extended-spectrum beta–lactamase (ESBL) production. Results: In total, Enterobacteriaceae resistance rates to cefotaxime increased from 13.1% using the CLSI 2009 guidelines to 23.6% with the CLSI 2010-12 guidelines, and the resistance rates to ertapenem were 0.4%, 1.0% and 0.8% with CLSI 2009, 2011 and 2012, respectively. Based on the 2010-12 CLSI criteria, all ESBL-producing Escherichia coli and Klebsiella pneumoniae were resistant to cefotaxime. Marked differences in susceptibility to ertapenem between the 2009 CLSI criteria and 2012-12 CLSI criteria were noted in ESBL-producing K. pneumoniae . Conclusions: Breakpoints changes in the updated CLSI guidelines resulted in higher resistance rates to cefotaxime and ertapenem. In addition, the effects were different in individual Enterobacteriaceae species. As a result, clinicians may opt to use alternative antimicrobial agents. Upon implementation of the newer CLSI guidelines, laboratories should be aware of the possible consequences and closely monitor the effects.