Systematic Evaluation of Commercial Susceptibility Testing Methods for Determining the In Vitro Activity of Daptomycin versus Staphylococcus aureus and Enterococci

We systematically evaluated 5 methods for testing daptomycin versus 48Enterococcus faecalis , 51Enterococcus faecium , and 50Staphylococcus aureus isolates using (i and ii) broth microdilution (BMD) with 50-mg/liter calcium medium supplementation (reference method) and 30-mg/liter calcium medium supplementation (BMD30 method), (iii) Etest, and (iv and v) MicroScan panel 33 using 2 methods to prepare the bacterial inoculum (MicroScan turbidity and MicroScan Prompt). Isolates were categorized as susceptible (S) or nonsusceptible (NS) based on measured MICs. Essential (±1 dilution) agreement (EA) and categorical (S/NS) agreement (CA) for each method were compared to the reference method. ForE. faecium , categorical agreement was poor between the reference method and BMD30 as well as with the three commercial methods, with frequent false-NS results (30 for BMD30, 18 for Etest, 22 for MicroScan Prompt, and 25 for MicroScan turbidity). AllE. faecalis isolates were judged to be S by the reference method; two of these isolates were categorized as NS using the BMD30 method, and one was categorized as NS by all three commercial methods. AllS. aureus isolates were judged to be S using all five methods. MIC values determined by the comparator methods tended to be higher than those for the reference method, especially forE. faecium isolates. EAs between the reference BMD and BMD30, Etest, MicroScan Prompt, and MicroScan turbidity were 63%, 63%, 63%, and 56%, respectively, forE. faecium , 87%, 83%, 98%, and 80%, respectively, forE. faecalis , and all 100% forS. aureus .