Vancomycin MIC creep in methicillin-resistant Staphylococcus aureus (MRSA) isolates from 2006 to 2010 in a hospital in China

Purpose: To assess whether vancomycin minimum inhibitory concentration (MIC) creeps among clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) in a regional hospital in China. Furthermore, to analyze the causes of vancomycin MIC creeps and the relationship between vancomycin MICs and the outcome among patients with MRSA infection. Materials and Methods: All clinical isolates of MRSA from 2006-2010 were retrieved and tested by the broth microdilution procedure to determine their vancomycin MIC. Meanwhile, related patient records were analyzed. Results: While all isolates were susceptive to vancomycin, the percentage of isolates with a vancomycin MIC = 1 mg/L increased significantly from 2006 (37.0%) to 2010 (75.7%). Meanwhile, vancomycin usage density (DDDs/1000 bed-days) had increased significantly from 2006-2010. Mean linear correlation analysis showed a statistically significant positive correlation (r = 0.905, P < 0.05) between the consumption of vancomycin and the percentage of MRSA isolates with a vancomycin MIC = 1 mg/L. Clinical records revealed high vancomycin MIC was associated with a higher microbiologic failure rate in MRSA bloodstream infections. Conclusions: The data demonstrated vancomycin MIC creep among clinical isolates in our hospital, and the MIC creep may be caused by the increasing usage of vancomycin. Furthermore, the analysis strongly suggested this shift of vancomycin MIC within the susceptible range may be associated with an increasing probability of treatment failure.