Vancomycin MICs for Methicillin-Resistant Staphylococcus aureus Isolates Differ Based upon the Susceptibility Test Method Used

Abstract

ᰔRecent studies have questioned treatment of serious methicillinresistant Staphylococcus aureus (MRSA) infections with vancomycin because of treatment failures, despite vancomycin MICs in the susceptible category (MIC Յ 2 g/ml) (2, 3, 5, 6, 7). Some of these studies have used broth microdilution for determining vancomycin MICs (5), while others have used the Etest commercial product (AB Biodisk, Solna, Sweden) (2, 7). We sought to determine if substantive differences might result from using different susceptibility test methods for determining vancomycin MICs.A group of 101 previously characterized strains of MRSA recovered between 2002 and 2006 from bacteremic patients in our institution were selected. Fifteen to 25 isolates per year were chosen, with MICs in the range of 0.5 to 2 g/ml, based upon initial tests. Isolates were stored frozen at Ϫ70°C in skim milk and subcultured twice prior to being tested. Each isolate was tested by the CLSI broth microdilution and the CLSI agar dilution methods (1) and by the Etest method using two different brands of Mueller-Hinton agar plates (BBL agar [BD Microbiology Systems, Cockeysville, MD] and Remel agar [Remel, Lenexa, KS]). All tests were performed at the same time from the same inoculum suspension.Results are depicted in Table 1. Vancomycin MICs generated by Etest were consistently one twofold dilution higher than MICs determined by CLSI broth or agar dilution; i.e., the modal vancomycin MIC determined by both the CLSI broth and agar dilution methods was 1 g/ml, while the modal vancomycin MIC by Etest was 2 g/ml with both Mueller-Hinton agar brands. In fact, 89 to 98% of MICs were 1.5 or 2 g/ml by Etest, but only 3 to 12% of vancomycin MICs were 2 g/ml when determined by the CLSI broth or agar dilution method.Our findings raise several issues. Pharmacodynamic studies have shown that the area under the vancomycin concentration curve-to-MIC ratio (AUC/MIC) is the optimal method for predicting vancomycin efficacy against S. aureus infections (7). Furthermore, the probability of achieving an optimal AUC/MIC ratio has been shown to be much lower when the vancomycin MIC is 2 g/ml than when it is 1 g/ml (4). Therefore, even a one dilution difference in the MIC can substantially affect the MIC/AUC ratio and the ability to optimize treatment (7). If differences of only one dilution are relevant to predicting clinical outcomes of MRSA infections, the MIC method used is a critical part of the equation.The differences between MICs generated by the Etest and

Reduced Vancomycin Susceptibility inStaphylococcus aureus, Including Vancomycin-Intermediate and Heterogeneous Vancomycin-Intermediate Strains: Resistance Mechanisms, Laboratory Detection, and Clinical Implications

“…In a study by Prakash et al (263), 101 MRSA isolates were tested by agar dilution and microbroth MICs according to CLSI criteria as well as by standard vancomycin Etest performed with two different brands of Mueller-Hinton agar (BBL and Remel). While there was reasonable correlation between the CLSI agar dilution and broth microdilution methods, the Etest results were consistently one twofold dilution higher than results for the other methods, changing the modal vancomycin MIC from 1 g per ml (broth and agar dilution) to 2 g per ml (Etest) (263). Similar results were recently reported for 1,800 randomly selected MRSA isolates, where Etest MICs were 0.5 to 1.5 log 2 dilution steps higher than the reference broth MIC results (291).…”

“…However, the change in breakpoints will not help detect heteroresistant strains with a vancomycin broth MIC of Յ2 g per ml. Additionally, given the potential for differences in the vancomycin MIC results based on the methodology used (263,291), a vancomycin broth MIC using reference methodology such as CLSI broth microdilution should be used as the definitive test for the definition of VISA.…”

“…Possible methods for defining the vancomycin MIC include CLSI-approved methods (agar dilution and broth MIC), Etest MIC using a 0. (130,174,263,291,338). In a study by Leonard et al, 100 hVISA and 50 VSSA isolates were tested by vancomycin broth and Etest MICs.…”

Vancomycin Susceptibility Trends and Prevalence of Heterogeneous Vancomycin-Intermediate Staphylococcus aureus in Clinical Methicillin-Resistant S. aureus Isolates

“…Etest reported vancomycin MICs higher than those reported by the two broth microdilution assays. This was expected and has been described elsewhere (33,36). Etest strips contain a gradient of vancomycin concentrations, including an intermediate reading of 1.5 g/ml, as opposed to the broth microdilution assays, where the concentrations are in 2-fold increments.…”

Vancomycin MICs of the resistant mutants of S. aureus ATCC43300 vary based on the susceptibility test methods used

“…13 It has been reported that vancomycin MICs generated by Etest were consistently one twofold dilution higher than those determined by Clinical and Laboratory Standards Institute (CLSI) broth dilution method. 14,15 In addition, the BHIA plates being recommended to screen the S. aureus reduced susceptibility to vancomycin in the CLSI. 16 Thus, we tested the susceptibility of the resistant mutants of S. aureus ATCC43300 by three different MIC-testing methods in this paper.…”