High vancomycin minimum inhibitory concentrations with heteroresistant vancomycin-intermediate Staphylococcus aureus in meticillin-resistant S. aureus bacteraemia patients.

Patients with high vancomycin minimum inhibitory concentrations (MICs) and heteroresistant vancomycin-intermediate Staphylococcus aureus (hVISA) infection are associated with treatment failure and poor outcomes. The main purpose of this study was to investigate the effect of hVISA on patient outcome, considering both the high vancomycin MIC and the existence of heteroresistant phenotypes. From January 2005 to December 2009, consecutive meticillin-resistant S. aureus (MRSA) isolates from 284 cases of MRSA bacteraemia receiving glycopeptide therapy were collected for further MIC and hVISA testing. The demographic distribution, clinical features and outcomes in bacteraemia patients with different vancomycin MICs and hVISA status in MRSA isolates were subsequently compared. Subjects were divided into three groups: low vancomycin MIC (<1.5mg/L) with vancomycin-sensitive S. aureus (VSSA) (n=50); high vancomycin MIC (≥1.5mg/L) with VSSA (n=218); and high vancomycin MIC with hVISA (n=16). Cox regression analysis demonstrated that the high MIC with VSSA group exhibited significantly higher 30-day mortality than the low MIC with VSSA group [odds ratio (OR)=2.349, 95% confidence interval (CI) 1.078-5.118]. The high MIC with hVISA phenotype was not associated with higher mortality but was independently associated with persistent MRSA bacteraemia (OR=5.996, 95% CI 1.438-25.005). To summarise, although hVISA is correlated with persistent bacteraemia, higher mortality in high vancomycin MIC infections could not be explained by the existing hVISA phenotype. Facing persistent bacteraemia under glycopeptide therapy for 7 days, clinicians should consider shifting to an alternative class of antibiotics to treat hVISA infection.