Site of infection rather than vancomycin MIC predicts vancomycin treatment failure in methicillin-resistant Staphylococcus aureus bacteraemia

Background: Therapeutic use of vancomycin is characterized by decreased susceptibilities and increasing reports of clinical failures. Few studies have examined the clinical outcomes of patients with methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia treated with vancomycin. The primary objective was to compare clinical outcomes of patients with MRSA bacteraemia treated according to standard of care practices. Methods: Patients were included if: (i) admitted to University of New Mexico Hospital between 2002 and 2009; (ii) >= 18 years of age; (iii) had one blood culture positive for MRSA; and (iv) received vancomycin. Clinical outcomes were defined as cure, failure (relapse of infection 30 days after completion of therapy, death or change in therapy) or unevaluable. Patient demographics, source of bacteraemia, treatment regimen, and microbiological characteristics were determined. Results: Two hundred patients with MRSA bacteraemia were included. Sixty-one patients were unevaluable, leaving 139 patients for the final analysis. Seventy-two (51.8%) patients were cured and 67 (48.2%) experienced vancomycin failure. Vancomycin MIC90 was 2 mg/L for both groups by Etest. Patients with endocarditis (P=0.02) or pneumonia (P=0.02) were more likely to fail therapy. Panton-Valentine leucocidin, loss of agr functionality and strain type were not predictors of outcomes in this study. Conclusions: High failure rates were observed in patients with MRSA bacteraemia treated with vancomycin, despite high vancomycin troughs and low rates of nephrotoxicity. Predictors of vancomycin failure included endocarditis and pneumonia. In these situations, vancomycin provides suboptimal therapy.