Impact of adopting minimum inhibitory concentration as the determinant of susceptibility to cephalosporins and carbapenems in multi-drug resistant Enterobacteriaceae.

The Clinical and Laboratory Standards Institute has recommended that Enterobacteriaceae susceptibility to most cephalosporins and carbapenems be reported according to minimum inhibitory concentration (MIC) alone. We analyzed our record of multi-drug resistant Enterobacteriaceae to assess the impact of these changes. We compared susceptibilities of ceftriaxone-resistant Enterobacteriaceae when using the 2009 and new 2010 MIC standards. Vitek2® (BioMerieux), was used to assess the changes in susceptibility. Klebsiella pneumoniae, Proteus sp., and Escherichia coli were the major species from urine, sputum, blood, and other sterile sites. The new breakpoint for cephalosporins increased resistance in E. coli and P. mirabilis. Many Proteus categorized as resistant by extended-spectrum beta-lactamase (ESBL) detection or inferred resistance have MICs to ceftriaxone ≤ 1 mcg/ml. New carbapenem breakpoints increased resistant Klebsiella pneumoniae and Proteus mirabilis. The increased ceftriaxone resistance from lowering breakpoints was almost balanced by the loss of resistance in ESBL isolates with MICs ≤ 1 mcg/ml. MIC-based susceptibility for multi-drug resistant Enterobacteriaceae increases the number of resistant isolates. Inferring mechanisms of resistance has a disproportionate effect on the susceptibility of Proteus mirabilis to cephalosporins, and the MIC-based standard has an almost equivalent but opposite effect on Proteus mirabilis susceptibility to carbapenems.