Etest Versus Vitek 2 Vancomycin Minimum Inhibitory Concentration Testing Methods for Methicillin-Resistant Staphylococcus aureus

Purpose: To evaluate the discordance between Vitek 2 and Etest vancomycin minimum inhibitory concentration (MIC) testing methods in methicillin-resistant Staphylococcus aureus (MRSA) isolates. Methods: Inclusion criteria consisted of MRSA isolates with blood, respiratory, or wound origin of culture, inpatient or emergency department location at time of culture and isolates with a Vitek 2 MIC reported. Isolates were subjected to Etest and the resulting MICs were compared to the corresponding Vitek 2 MICs, and both the rate and degree of discordance were evaluated. Results: Seventy-five MRSA isolates met the inclusion criteria. Etest resulted in an MIC value greater than that of Vitek 2 in 64 isolates (85.3%). Furthermore, of the 35 isolates with an MIC of ≤0.5 mcg/mL via Vitek 2, Etest reported 27 isolates with an MIC >1 mcg/mL. Of the 39 isolates with a Vitek 2 MIC of 1 mcg/mL, Etest reported 30 isolates with an MIC >1 mcg/mL. Conclusion: We have demonstrated that Etest commonly reports MICs greater than 1 mcg/mL, even among isolates with MICs as low as ≤0.5 mcg/mL, according to Vitek 2. The clinical significance of these findings is unknown and would be an area of further study.