Vancomycin MICs for Staphylococcus aureus Vary by Detection Method and Have Subtly Increased in a Pediatric Population Since 2005

Vancomycin MICs for Staphylococcus aureus isolates in a pediatric hospital with a high rate of staphylococcal infections were examined for any increase over a 7-year period. A broth microdilution scheme allowed direct comparison of the MICs generated by this method to MICs generated by Etest. MICs generated by both methods were determined with the same inoculum suspension. One hundred sixty-five S. aureus isolates were selected on the basis of the patients having been bacteremic or having received vancomycin as the definitive therapy for their infections. Of the 165 isolates, 117 were methicillin-resistant S. aureus and 48 were methicillin-susceptible S. aureus. Forty-seven were acquired in the hospital (nosocomial), 56 were community acquired, and 62 were community onset-health care associated. All but one isolate tested by broth microdilution had MICs of <1.0 μg/ml, while 96% of these same isolates tested by Etest had MICs of ≥1 μg/ml. A significant increase in MICs that occurred after study year 4 (2004 to 2005) was demonstrated by the Etest (P < 0.00007) but not by broth microdilution. MICs were not different for isolates of community or health care origin, regardless of methodology. The proportion of isolates with Etest MICs of <1 and ≥1 μg/ml between children with bacteremia for ≤5 days and >5 days (P = 0.3) was not different. We conclude that MICs for pediatric isolates have increased slightly since 2005 and therapeutic decisions based on vancomycin MICs need to be made by considering the methodology used.Recent studies have reported a steady increase in vancomycin MICs for Staphylococcus aureus that may be, in part, due to the increase in the use of vancomycin in response to community-acquired (CA) methicillin-resistant S. aureus (MRSA) (18). Also, some studies report that vancomycin MICs between 1.5 and 2.0 μg/ml are predictors of a poor therapeutic response in adults (15). The decrease in vancomycin susceptibility is difficult to assess by percentage reporting because the MIC increases are subtle, would all be classified as susceptible by using 2009 Clinical and Laboratory Standards Institute (CLSI) interpretive breakpoints, and are only detected by using a more closely spaced (arithmetic) dilution scheme versus the standard geometric dilution scheme (16). We report the first study of vancomycin MIC trends for S. aureus isolates from children comparing Etest and modified broth microdilution (BMD) schemes.