Impact of CLSI and EUCAST Cefepime breakpoint changes on the susceptibility reporting for Enterobacteriaceae

Objective: We analyzed the effects of different cefepime MIC breakpoints on Enterobacteriaceae cefepime susceptibility and the presence of AmpC and extended-spectrum β-lactamase (ESBL) genes within the cefepime MIC interpretative categories. Methods: Using Enterobacteriaceae susceptibility data from 2013 comparisons of MIC breakpoints were performed using Pearson's chi-squared test. Molecular testing on a subset of isolates was done. Results: Among 3784 non-duplicate clinical isolates, cefepime susceptibility decreased from 97.6% to 96.1% to 93.7% for CLSI 2013, CLSI 2014, and EUCAST 2011, respectively. In ceftriaxone non-susceptible isolates, cefepime susceptibility decreased from 79% to 66% (P <. 0.0001) using CLSI 2013 and 2014, respectively, which was greater and statistically significant for Escherichia coli and Klebsiella spp. but not for Enterobacter spp. (P = 0.06). Isolates with MIC ≤1. μg/mL more often harbored AmpC (77%) than ESBL (18%) genes. Conclusions: Lower cefepime MIC breakpoints decrease cefepime susceptibility for isolates harboring ESBLs, while sparing the majority of those with AmpCs.