Evaluation of MicroScan WalkAway and Vitek 2 for determination of the susceptibility of extended-spectrum β-lactamase-producing Escherichia coli and Klebsiella pneumoniae isolates to cefepime, cefotaxime and ceftazidime.

The aim of this study was to evaluate the accuracies of these automated susceptibility test systems with cefepime, cefotaxime and ceftazidime using the new CLSI and EUCAST guidelines in extended-spectrum β-lactamase (ESBL)-producing Escherichia coli and Klebsiella pneumoniae. A total of 220 ESBL-producing clinical isolates were collected from 12 hospitals in Korea. Susceptibility testing for cefepime, cefotaxime and ceftazidime was performed by MicroScan WalkAway, Vitek 2 and the CLSI broth microdilution test. ESBL genotypes were determined by PCR amplification. The proportion of isolates classified as susceptible to cefepime and ceftazidime with the CLSI and EUCAST guidelines was 35.0% versus 2.3% for cefepime (P < 0.001) and 21.8% versus 8.2% for ceftazidime (P < 0.001), respectively, and the susceptible isolates were mainly the CTX-M-9 group or SHV-type ESBL producers. All of the isolates were resistant to cefotaxime. Against the total of 220 ESBL-producing isolates, using the CLSI (EUCAST) criteria, very major/major error rates of MicroScan and Vitek 2 were as follows: 1.9%/20.8% (1.8%/20.0%) and 27.4%/0% (12.2%/0%) for cefepime and 2.6%/8.3% (1.2%/0%) and 4.5%/0% (2.3%/0%) for ceftazidime, respectively. The very major error rates of MicroScan and Vitek 2 with cefotaxime were 0.9% and 1.4%, respectively. The errors were mainly major errors for MicroScan and very major errors for Vitek 2. A substantial portion of ESBL-producing isolates were susceptible to cefepime and ceftazidime by using the CLSI and EUCAST breakpoints. Unfortunately, the error rates of the two automated susceptibility systems were not acceptable for cefepime and ceftazidime.