Effect of methodology on vancomycin MIC for MRSA isolates D-871.

Background: Determination of Vancomycin (VAN) MIC can influence the choice of agent to treat methicillin resistant Staphylococcus aureus (MRSA) infection. We studied the variability of VAN MIC from clinical isolates using E-test, Vitek-2 and broth microdilution (BMD) methodology. Methods: Clinical isolates were obtained from patients receiving vancomycin for confirmed MRSA infection from April 2012 to Dec 2013 in a large teaching hospital. VAN MIC values were determine prospectively on all isolates by gradient diffusion E-test and automated Vitek-2 (bioMérieux). Reading of E-test was performed independently by a senior medical scientist and medical microbiologist. CLSI reference BMD method was performed retrospectively on freeze stored (-20°C, 6-12 months) isolates. Results: From 111 patients 148 isolates were obtained. Sources were; skin and soft tissue 63.5%, respiratory 17.6%, blood/ CSF 10.1%, sterile body cavity 5.4% and other 3.4%. All MRSA isolates except one (E-test 3µg/mL) were susceptible to VAN and had an MIC of ≤2µg/mL irrespective of methodology. VAN MIC determination showed isolates were ≤1.0 µg/mL for 100% of BMD, 98.7% for Vitek-2, and E-test yielded higher MICs with 35.8% of isolates >1 µg /mL Variance was also noted between BMD 87.16%, Vitek-2 50.0%, and E-test 13.5% for MICs ≤0.5 µg/ mL (Figure 1). Conclusions: Considerable variation in MIC results using three methods was observed. E-test may overestimate MIC values compared to Vitek-2 and the gold standard of BMD. Susceptibility methodology needs to be considered in studies determining the effect of elevated MIC on clinical outcome. Clinical decisions using vancomycin in serious MRSA infections cannot be extrapolated between different methods.