title: Unacceptably high error rates in Vitek 2 testing of cefepime susceptibility in extended-spectrum-β-lactamase-producing Escherichia coli.
abstract: While a lack of concordance is known between gold standard MIC determinations and Vitek 2, the magnitude of the discrepancy and its impact on treatment decisions for extended-spectrum-β-lactamase (ESBL)-producing Escherichia coli are not. Clinical isolates of ESBL-producing E. coli were collected from blood, tissue, and body fluid samples from January 2003 to July 2009. Resistance genotypes were identified by PCR. Primary analyses evaluated the discordance between Vitek 2 and gold standard methods using cefepime susceptibility breakpoint cutoff values of 8, 4, and 2 μg/ml. The discrepancies in MICs between the methods were classified per convention as very major, major, and minor errors. Sensitivity, specificity, and positive and negative predictive values for susceptibility classifications were calculated. A total of 304 isolates were identified; 59% (179) of the isolates carried blaCTX-M, 47% (143) carried blaTEM, and 4% (12) carried blaSHV. At a breakpoint MIC of 8 μg/ml, Vitek 2 produced a categorical agreement of 66.8% and exhibited very major, major, and minor error rates of 23% (20/87 isolates), 5.1% (8/157 isolates), and 24% (73/304), respectively. The sensitivity, specificity, and positive and negative predictive values for a susceptibility breakpoint of 8 μg/ml were 94.9%, 61.2%, 72.3%, and 91.8%, respectively. The sensitivity, specificity, and positive and negative predictive values for a susceptibility breakpoint of 2 μg/ml were 83.8%, 65.3%, 41%, and 93.3%, respectively. Vitek 2 results in unacceptably high error rates for cefepime compared to those of agar dilution for ESBL-producing E. coli. Clinicians should be wary of making treatment decisions on the basis of Vitek 2 susceptibility results for ESBL-producing E. coli.
authors.affiliation: Department of Pharmacy Practice, Midwestern University, Chicago College of Pharmacy, Downers Grove, Illinois, USA Department of Pharmacy, Northwestern Memorial Hospital, Chicago, Illinois, USA.
authors.forename: Nathaniel J
authors.initials: NJ
authors.lastname: Rhodes
authors.affiliation: Department of Pharmacy, Northwestern Memorial Hospital, Chicago, Illinois, USA.
authors.forename: Chad L
authors.initials: CL
authors.lastname: Richardson
authors.affiliation: Midwestern University, Chicago College of Pharmacy, Downers Grove, Illinois, USA.
authors.forename: Ryan
authors.initials: R
authors.lastname: Heraty
authors.affiliation: Midwestern University, Chicago College of Pharmacy, Downers Grove, Illinois, USA.
authors.forename: Jiajun
authors.initials: J
authors.lastname: Liu
authors.affiliation: Department of Pathology, Clinical Microbiology Division, Northwestern Memorial Hospital, Chicago, Illinois, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Malczynski
authors.affiliation: Department of Pathology, Clinical Microbiology Division, Northwestern Memorial Hospital, Chicago, Illinois, USA.
authors.forename: Chao
authors.initials: C
authors.lastname: Qi
authors.affiliation: Department of Pharmacy Practice, Midwestern University, Chicago College of Pharmacy, Downers Grove, Illinois, USA Department of Pharmacy, Northwestern Memorial Hospital, Chicago, Illinois, USA mscheetz@nmh.org.
authors.forename: Marc H
authors.initials: MH
authors.lastname: Scheetz
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D000077723:Cefepime; D002511:Cephalosporins / Q000494:pharmacology*; D004926:Escherichia coli / Q000187:drug effects* / Q000201:enzymology* / Q000235:genetics; D004927:Escherichia coli Infections / Q000382:microbiology; D005798:Genes, Bacterial / Q000235:genetics; D008826:Microbial Sensitivity Tests / Q000379:methods* / Q000592:standards; D016133:Polymerase Chain Reaction; D015203:Reproducibility of Results; D001618:beta-Lactamases / Q000235:genetics / Q000378:metabolism*
chemical_list: D000900:Anti-Bacterial Agents; D002511:Cephalosporins; D000077723:Cefepime; D001618:beta-Lactamases
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