Susceptibility of enterococci to daptomycin is dependent upon testing methodology

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. (c) 2013 Elsevier Inc. All rights reserved.