Vancomycin MIC creep progresses in methicillin-resistant Staphylococcus aureus despite the national antimicrobial stewardship campaign: Single facility data in Japan.

Background: Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of nosocomial and community infections, and vancomycin (VCM) is widely recommended as a first-line therapeutic drug. Minimum inhibitory concentrations (MICs) of VCM ≤2 μg/mL are defined as susceptible, but increases in these levels, known as "VCM MIC creep" have been reported. The aim of this study was to investigate VCM MIC creep during the promotion of a national antimicrobial stewardship campaign.

Methods: We collected data from 2013 to 2020 on S. aureus isolated at the clinical microbiology laboratory at Okayama University Hospital, Japan. We calculated the annual proportions of MRSA isolation rates by MIC levels for nosocomial and community samples and estimated annual percentage changes in the antimicrobial use density of the VCM.

Results: Of the 1,716 MRSA isolates, no strains showed intermediate or resistant ranges of VCM MIC levels. By 2020, the proportion of MRSA with an MIC of ≤0.5 μg/mL decreased to 35.4%, while that with an MIC of 1 μg/mL increased to 64.1% over time. The annual percentage changes of the VCM antimicrobial use density significantly increased without any trend change point (average 8.1%, p = 0.035). There was no clear correlation between the VCM AUD and annual proportion of nosocomial MRSA with MIC 1 μg/mL (correlation coefficient 0.48; p value = 0.24).

Conclusion: We demonstrated a deteriorating situation of VCM MIC creep among MRSA strains isolated at our university hospital during the national antimicrobial stewardship campaign.