title: Susceptibility of enterococci to daptomycin is dependent upon testing methodology.
abstract: An increase in daptomycin nonsusceptible enterococci (DNSE) was noted in our institution (8.3% 2008 to 34.5% 2011) using MicroScan methods which may overestimate DNSE prevalence. DNSE (N = 150) from the clinical laboratory (2008-2011) underwent susceptibility testing using broth microdilution (BMD), Etest, Sensititire, MicroScan prompt (MSP), and MicroScan turbidity (MST) with only 20% of isolates confirmed as nonsusceptible. Categorical and essential agreement were highest with MSP and MST, but both missed the majority of resistant isolates (70% and 87% missed). Etest MIC values were statistically higher, more likely to be nonsusceptible, had the lowest very major error rate (37%), and the highest falsely nonsusceptible rate (22%). Sensititre MIC values were not statistically different from BMD, but missed 57% of DNSE. PFGE analysis did not define a clonal outbreak. These findings suggest that MicroScan methods overestimate nonsusceptibility, and the lack of correlation between methods raises questions regarding which method is most effective at confirming nonsusceptibility.
authors.affiliation: Department of Pathology and Microbiology, College of Medicine, University of Nebraska Medical Center, 984031 Nebraska Medical Center, Omaha, NE 68198-4031, USA.
authors.forename: Kendall A
authors.initials: KA
authors.lastname: Bryant
authors.forename: Amity L
authors.initials: AL
authors.lastname: Roberts
authors.forename: Mark E
authors.initials: ME
authors.lastname: Rupp
authors.forename: James R
authors.initials: JR
authors.lastname: Anderson
authors.forename: Elizabeth R
authors.initials: ER
authors.lastname: Lyden
authors.forename: Paul D
authors.initials: PD
authors.lastname: Fey
authors.forename: Trevor C
authors.initials: TC
authors.lastname: Van Schooneveld
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D017576:Daptomycin / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D013293:Enterococcus faecalis / Q000187:drug effects* / Q000302:isolation & purification / Q000502:physiology; D016984:Enterococcus faecium / Q000187:drug effects* / Q000302:isolation & purification / Q000502:physiology; D005189:False Positive Reactions; D016908:Gram-Positive Bacterial Infections / Q000188:drug therapy / Q000382:microbiology; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods / Q000592:standards*; D012680:Sensitivity and Specificity
chemical_list: D000900:Anti-Bacterial Agents; D017576:Daptomycin
keywords: Daptomycin; Enterococcus; Susceptibility testing