[Methicillin-resistant Staphylococcus aureus bacteremia. Predictor factors for an isolate with a vancomycin minimal inhibitory concentration > or =2 mg/l]

Abstract

A greater rate of treatment failures with vancomycin in methicillin-resistant Staphylococcus aureus (MRSA) bacteremia has been reported recently when the minimum inhibitory concentration (MIC) is > or =2 mg/l. This study has aimed to evaluate if there are clinical and/or epidemiological factors that predict isolation of a MRSA strain with MIC of vancomycin of > or =2 mg/L in the bacteremia episodes collected during a 15 year period (January 1991 to December 2005) in a tertiary urban hospital. During the study period, a total of 478 episodes of MRSA bacteremia were studied prospectively. The following clinical variables were recorded for each one: age, gender, comorbidity, previous administration of vancomycin or another antibiotic, prognosis of baseline diseases, bacteremia focus, shock, empiric antibiotic received and mortality. The MIC of vancomycin of 419 strains (88%) was determined with the E-test. In 216 (52%) of the isolations the MIC of vancomycin was 1.50 mg/L, in 110 (26%) of the cases it was < or =1 mg/l and in 93 (22%) 2 mg/l. Uni-and multivariate analyses were made, comparing the clinical variables of the patients infected by strains with MIC of vancomycin > or =2 mg/l regarding the MIC strains < or =1 mg/l. In the last 3 years of the study (2003-2005) the proportion of the strains with MIC of vancomycin > or =2 mg/l was significantly greater than those isolated with MIC < or = 1 mg/L (44 % vs 3 %; p<0.001). In the multivariate analysis, the only clinical characteristic associated independently to the isolation of a strain with MIC > or =2 mg/l was the nosocomial-acquired infection OR (95 % CI): 1.94 (1.04-3.63); p=0.04. Although the isolation of a MRSA strain with MIC of vancomycin > or =2 mg/l is more frequent in the nosocomial-acquired bacteremia episodes, in the clinical practice, it is not a useful predictive parameter because the frequency of isolation of these strains in the community is also high.