Vancomycin MIC Susceptibility Testing of Methicillin-Susceptible and Methicillin-Resistant Staphylococcus aureus Isolates: A Comparison Between Etest® and an Automated Testing Method

Background: Vancomycin treatment failures and increased mortality have been reported in methicillin-resistant Staphylococcus aureus (MRSA) isolates with minimum inhibitory concentrations (MICs) >1 mu g/mL. Most of this data utilized manual testing to determine the MIC. Recent vancomycin treatment guidelines do not specify the optimal testing method to define the MIC. Methods: Over a twelve-month study period, we compared manual susceptibility testing by Etest (R) (AB Biodisk, Solna, Sweden) with automated testing by MicroScan Walk-Away (R) (Dade Behring, Inc., East Mississauga, Ontario) to determine the difference in the MICs among 383 sequential clinical S aureus isolates. Results: Manual testing demonstrated MICs of 1.5 mu g/mL or 2.0 mu g/mL in 90% and 86% of MRSA and methicillin-sensitive Staphylococcus aureus (MSSA) isolates, respectively. Automated testing revealed MICs of 2.0 mu g/mL for 56% and 54% of MRSA and MSSA isolates, respectively. The manual MIC test by Etest (R) was >1 mu g/mL in 87% of MRSA isolates and 86% of methicillin-susceptible S aureus isolates in which the automated MIC result was 1 mu g/mL. This same finding occurred in 94% (17/18) of S aureus isolates causing non-skin/skin structure infections. Among all subgroups of isolates, manual testing demonstrated statistically significant higher MICs compared to automated testing. Conclusions: MIC results generated by the Etest (R) consistently revealed a one dilution higher vancomycin MIC compared to MicroScan (R). Automated MIC results of invasive MRSA isolates should be confirmed by manual Etest (R) to ensure identification of those isolates with vancomycin MICs >1 mu g/mL that are at risk for vancomycin treatment failure or increased mortality.