Impact of CLSI Break Point Changes Over the Past Decade on Antimicrobial Susceptibility in Gram-Negative Bacteria

Background: Over the past decade, the CLSI has updated susceptibility break points for several antimicrobial agents. The purpose of this study was to evaluate the impact of these changes against gram-negative bacteria at our academic medical center. Methods: In this retrospective, IRB-approved study, we collected consecutive, nonduplicate clinical isolates of Enterobacter cloacae , Escherichia coli , Klebsiella aerogenes , K. oxytoca , K. pneumoniae , and Pseudomonas aeruginosa for the past decade (2010–2019) at our academic medical center and 3 adult ICUs. Susceptibility testing was performed using the BD Phoenix automated system. For these isolates, susceptibilities for 7 β-lactams (aztreonam, ceftriaxone, ceftazidime, cefepime, piperacillin/tazobactam, ertapenem, and meropenem) and 2 fluoroquinolones (levofloxacin, ciprofloxacin) were calculated based upon CLSI break points in 2010 and current CLSI break points in 2020. Any change >5% in susceptibility was deemed significant for this analysis. Results: In 17.5% of Enterobacteriales isolates tested, at least 1 antimicrobial demonstrated significant decline. Ertapenem was the most commonly affected antimicrobial (45% of the isolates) followed by ceftriaxone (35%) and cefepime (25%). Susceptibilities of aztreonam, ceftazidime, and meropenem were not affected for any of the Enterobacteriales. The most common organism demonstrating a significant impact on change in susceptibility among the Enterobacteriales was E. cloacae (41.7% of the time) followed by E. aerogenes (20.8%), K. oxytoca (12.5%), K. pneumoniae (8.3%) and E. coli (4.2%). Most of the impact was observed hospital-wide (33.3%), followed closely by the MICU (28.6%), the NSICU (23.8%) and the CVICU (14.3%). For P. aeruginosa , the impact of the antimicrobial break-point changes on susceptibility was more pronounced than the Enterobacteriales. Overall, 93.8% of the time there was a significant decline in antimicrobial susceptibility. Each antimicrobial (ciprofloxacin, levofloxacin, meropenem, and piperacillin/tazobactam) demonstrated a significant decline in susceptibility hospital-wide and in each ICU except for the susceptibility of meropenem in the NSICU. Conclusions: Changes in break points had a significant impact on the susceptibility of all antimicrobials for P. aeruginosa at our institution, both hospital-wide and in the adult ICUs. Although the impact was less for the Enterobacteriales, ertapenem, ceftriaxone, and cefepime demonstrated signi