Vancomycin MIC as a predictor of outcome in MRSA bacteraemia in the UK context

Abstract

Objectives: Raised vancomycin MICs have been associated with poor outcomes for methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia in the USA and mainland Europe.We investigated if this also applies in the UK, where EMRSA-15 (clonal complex 22) dominates. Methods: Isolates from UK patients receiving vancomycin therapy for MRSA bacteraemia in 2008-10 were collected, along with clinical details. Outcomes (i.e. patient survival or bacteraemia resolution) were reported 28 days after vancomycin therapy ended. The relationship between clinical outcome and MIC-as determined by CLSI and BSAC agar dilution methods-was assessed. Results: Among 228 MRSA bacteraemias, 82%were caused by EMRSA-15; 65% of the patientswere male and the median age was 70.5 years. MICs correlated between methods, but CLSI agar dilution testing gave a mode at 1 mg/L with only 12% of results either side, whereas the BSAC method gave a mode straddling 0.7-1 mg/L with <4%outliers. Twenty-three percent of patients died, with MRSA contributory in half; another 17% had unresolved bacteraemia at 28 days. Neither death nor unresolved bacteraemia was significantly associated with higher vancomycin MICs by either method. Rifampicin co-therapy had no quantifiable effect on outcome. The patient's age was the only significant correlate of patient outcome (P<0.01); the underlying medical condition of the patient was important for the resolution of bacteraemia (P<0.01), though not for overall mortality. Conclusions: Subtle vancomycin MIC differences did not correlate with worse outcomes for vancomycin monotherapy or for vancomycin/rifampicin co-therapy in MRSA bacteraemia. Regardless of the exact MIC-outcome relationship, detecting such small MIC differences seems unlikely to be reliable in routine laboratories. © The Author 2013. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved.