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 μg/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® (AB Biodisk, Solna, Sweden) with automated testing by MicroScan Walk-Away® (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 μg/mL or 2.0 μg/mL in 90% and 86% of MRSA and methicillin-sensitive Staphylococcus aureus (MSSA) isolates, respectively. Automated testing revealed MICs of 2.0 μg/mL for 56% and 54% of MRSA and MSSA isolates, respectively. The manual MIC test by Etest® was >1 μg/mL in 87% of MRSA isolates and 86% of methicillin-susceptible S aureus isolates in which the automated MIC result was 1 μg/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® consistently revealed a one dilution higher vancomycin MIC compared to MicroScan®. Automated MIC results of invasive MRSA isolates should be confirmed by manual Etest® to ensure identification of those isolates with vancomycin MICs >1μg/mL that are at risk for vancomycin treatment failure or increased mortality.