Impact of vancomycin MIC creep on patients with methicillin-resistant Staphylococcus aureus bacteremia.

BACKGROUND/PURPOSE: To date, vancomycin is still the standard treatment for methicillin-resistant Staphylococcus aureus (MRSA) infections, but minimum inhibitory concentration (MIC) creep is becoming a major concern. The aims of this study were to investigate trends in vancomycin use and MIC values over the last decade at our institute and to evaluate the outcomes of bacteremic patients infected with MRSA isolates with reduced vancomycin susceptibility.

METHODS

Vancomycin use and density were evaluated using the defined daily doses (DDD) method. Patients with MRSA bacteremia were enrolled retrospectively. Patient demographic data and clinical outcomes were analyzed. The first isolate from each patient was collected for E-testing in order to determine vancomycin MIC. MIC trends were assessed as MIC(50), MIC(90), and the geometric mean.

RESULTS

Vancomycin use has increased over the last decade. One hundred and forty patients were enrolled and their respective isolates were retrieved, including isolates from 45 patients in 2001, 46 patients in 2005, and 49 patients in 2009. The geometric mean (± standard deviation) of the vancomycin MIC for MRSA isolates obtained in 2009 was 1.39 ± 0.30 μg/mL, which is significantly higher than the mean vancomycin MIC obtained in 2001 (1.19 ± 0.34 μg/mL, p < 0.01) and 2005 (1.99 ± 0.25 μg/mL, p < 0.001). There were no significant differences in terms of the in-hospital mortality rate between patients with MRSA isolates with MICs ≥ 1.5 μg/mL or < 1.5 μg/mL.

CONCLUSION

We identified a significant upward trend in the use of vancomycin and its MIC over the last decade. This study shows that patients infected with MRSA isolates with high MICs (≥1.5 μg/mL) do not have a significantly higher mortality rate compared with isolates with low MICs (<1.5 μg/mL).