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Romney M. Humphries, Janet A. Hindler, Should Laboratories Test Methicillin-Resistant Staphylococcus aureus for Elevated Vancomycin Minimum Inhibitory Concentrations by Etest as a Driver of Treatment Changes?, Clinical Infectious Diseases, Volume 55, Issue 4, 15 August 2012, Pages 612–613, https://doi.org/10.1093/cid/cis469
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Tothe Editor—In a recent review and meta-analysis published in Clinical Infectious Diseases, van Hal and colleagues evaluated vancomycin treatment outcomes for methicillin-resistant Staphylococcus aureus (MRSA) relative to vancomycin minimum inhibitory concentrations (MICs) [1]. The authors suggest that laboratories specifically consider performing vancomycin MICs by Etest when MRSA are isolated from blood, and that a MIC on the high end of the susceptible range (eg, 2.0 µg/mL) be used as indication for treatment with an alternative anti-MRSA agent, especially in patients with persistent disease. However, the interstudy variability in susceptibility test methods among the studies evaluated to reach this conclusion is significant, and thus we feel these data must be interpreted with caution before a single vancomycin MIC value be used alone to make treatment decisions.
It is well accepted that vancomycin MIC results are influenced by both test method [2] and media type and manufacturer [3, 4]. The majority of studies evaluated by van Hal (15 of 22) performed Etest, and authors all stated that Etest was performed per the manufacturer's instructions. However, only 6 studies indicated what media was used for Etest and only 2 used BBL brand Mueller Hinton agar (MHA) [5, 6], as recommended by the manufacturer. Three studies used brain heart infusion (BHI) agar [7–9], an enriched medium that is associated with vancomycin MICs 1–1.5 dilutions higher than those obtained on MHA (R. M. H. and J. A. H., unpublished observations). A second concern is the widespread use of stocked clinical isolates for vancomycin testing among the studies evaluated. Only 1 study [10] explicitly stated that Etest MICs were evaluated at the time of MRSA isolation. Vancomycin MICs are known to decline by 0.25–0.5 µg/mL following 6 months storage at −80°C, even for isolates that test in the susceptible MIC range [11].
