Statistical analyses were performed using the R statistical package (3.0.2) (R Core Team, Vienna, Austria).

Our objective was to evaluate the sensitivities and specificities of various ceftriaxone MICs in determining the optimal threshold for identifying an organism as a potential ESBL producer. If the ceftriaxone MIC threshold for ESBL identification is too high, appropriate infection control and treatment strategies may not be deployed. If the ceftriaxone MIC threshold is lower than necessary, this practice has the potential to lead to the overuse of carbapenems or to needlessly increase the workload of microbiology laboratories (for institutions where confirmatory testing is still performed).

For these reasons, many clinical microbiology laboratories use ceftriaxone (or other third-generation cephalosporin) MIC thresholds to indicate to clinicians that an organism is a possible ESBL producer or to trigger additional confirmatory phenotypic testing. In addition, the absence of institutional epidemiological data on ESBLs can hamper efforts to control their spread within health care facilities ( 6). Without a method of alerting clinicians to the possibility that an agent may be an ESBL producer, potentially suboptimal therapy (e.g., cefepime or piperacillin-tazobactam) may be inadvertently prescribed. A portion of these organisms retain in vitro susceptibility to piperacillin-tazobactam and cefepime ( 2), even though these agents are generally considered inferior to carbapenem therapy for the treatment of invasive infections by ESBL-producing organisms ( 3, – 5). However, identification of organisms producing ESBLs still has important clinical implications. This was in part due to the large workload imposed on clinical microbiology laboratories with phenotypic confirmatory ESBL testing ( 1). In 2010, the Clinical and Laboratory Standards Institute (CLSI) lowered the ceftriaxone (and cefotaxime) breakpoint from 8 μg/ml to 1 μg/ml, and with that change, the recommendation for screening for extended-spectrum β-lactamase (ESBL) production became optional ( 1).

Mic library for proteus 8