Impact of reduced vancomycin susceptibility on the therapeutic outcome of MRSA bloodstream infections

BACKGROUND: The aim of this study was to determine whether clinical outcome of patients with methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia was correlated with vancomycin susceptibility of the corresponding strains. METHODS: A retrospective study on MRSA bacteraemia was performed at a teaching hospital between January 1998 and October 2005 by linking vancomycin susceptibility profiles of patients' isolates with hospitalization data. RESULTS: A total of 20 out of 209 MRSA bacteraemia patients were treated with vancomycin for at least 5 days with adequate trough levels, and fulfilled the study's inclusion and exclusion criteria. Twenty-two S. aureus isolates from these patients' blood cultures were identified as MRSA, including two hetero-VISA from separate patients and two VISA with vancomycin MIC of 4 mg/L from one patient. Between patients who showed 'good' vancomycin response and patients who did not, there was a significant difference (p < 0.01) in their corresponding MRSAs' vancomycin susceptibility expressed by 'area under curve' (AUC) of population analysis. Significant correlations were found between AUC and initial vancomycin therapeutic response parameters of 'days till afebrile' (r = 0.828, p < 0.01) and 'days till CRP ≦ 30% of maximum' (r = 0.627, p < 0.01) CONCLUSION: Our study results caution healthcare personnel that early consideration should be given to cases with a poor vancomycin treatment response that could signify the involvement of MRSA with reduced susceptibility to vancomycin.