Decreasing prevalence of isolates with vancomycin heteroresistance and vancomycin minimum inhibitory concentrations ≥2 mg/L in methicillin-resistant Staphylococcus aureus over 11 years: potential impact of vancomycin treatment guidelines

Abstract

We evaluated vancomycin MIC (V-MIC) and the prevalence of intermediately susceptible (VISA) and heteroresistant (hVISA) isolates trends in methicillin-resistant Staphylococcus aureus bacteremia among 720 adults (≥18years) inpatients over 4 study periods (2002–2003, 2005–2006, 2008–2009, and 2010–2012). V-MIC (Etest) and the prevalence of hVISA and VISA (determined by population analysis profile–area under the curve) were stratified according to the study period. Mean vancomycin MIC was 1.78±0.39, 1.81±0.47, 1.68±0.26, and 1.54±0.28mg/L in 2002–2003, 2005–2006, 2008–2009, and 2010–2012, respectively (P<0.0001). We noted a steadily decreasing prevalence of isolates with V-MIC ≥2mg/L (50.0%, 45.2%, 35.4%, and 18.7%; P<0.0001) and hVISA (9.7%, 6.6%, 3.0%, and 2.1%; P=0.0003). VISA prevalence remained low (0–2%). These changes coincided with steadily increasing vancomycin trough levels (9.9±7.8, 11.1±8.4, 16.6±7.8, and 19.7±5.9mg/L in 2002–2003, 2005–2006, 2008–2009, and 2010–2012, respectively; P<0.0001). These changes imply that adherence to vancomycin treatment guidelines may suppress the development of less susceptible isolates.