Effect of MIC interpretative breakpoint revision on cephalosporin and carbapenem susceptibility among ESBL-producing Enterobacteriaceae

Background: Minimum inhibitory concentration (MIC) breakpoints of selected cephalosporins and carbapenems against Enterobacteriaceae have been revised by major guidelines including CLSI and EUCAST mainly according to available pharmacokinetic/pharmacodynamic data. A decrease of breakpoint may obviate the need to detect specific resistance mechanisms such as extended-spectrum -lactamase (ESBL) and carbapenemase, which may be less correlated to treatment outcome than does the actual MIC of each agent.

Objective: To analyze cephalosporin and carbapenem MIC distributions among ESBL-producing Enterobacteriaceae at a university hospital against revised interpretative breakpoints.

Methods: MIC distributions of selected cephalosporins and carbapenems among 505 isolates of genotypically confirmed ESBL-producing Enterobacteriaceae were determined by E-test^TM method and analyzed according to interpretative breakpoints comparing between CLSI and EUCAST guidelines.

Results: ESBL-producing Enterobacteriaceae demonstrated a wide range of cephalosporin MIC (≤1 to ≥64). Up to 9.7% of isolates displayed MIC lower than a revised cephalosporin breakpoint. Most isolates remained susceptible to imipenem and meropenem while as high as 24.6% were not susceptible to ertapenem. Lowered breakpoints may result in a change in categorical interpretations.

Conclusion: ESBL-producing isolates could be reported as susceptible to a cephalosporin with revised breakpoints although clinical use is uncertain. A higher proportion of isolates would be reported as nonsusceptible to cephalosporins or carbapenems with lowered breakpoints and thus increasing use of broadspectrum antimicrobial agents should be monitored.