Predictors of high vancomycin MIC values among patients with methicillin-resistant Staphylococcus aureus bacteraemia

Background Recent evidence suggests that vancomycin demonstrates reduced activity against methicillin-resistant Staphylococcus aureus (MRSA) infections when vancomycin MIC values are at the high end of the susceptibility range (≥1.5 mg/L). However, scant research exists on factors predictive of high vancomycin MICs (≥1.5 mg/L) among MRSA bacteraemic patients. Empirical therapy decisions would greatly benefit from such information. Objectives To identify the parameters predictive of high vancomycin MICs (≥1.5 mg/L) among MRSA bacteraemic patients and to develop an evidence-based clinical prediction tool. Methods This observational cohort study included adult patients with MRSA bloodstream infections between January 2005 and May 2007. Demographics, co-morbid conditions, and microbiology and antibiotic exposure data were collected. Vancomycin MICs were determined by Etest. Stepwise logistic regression was used to identify independent predictors of high vancomycin MICs. Results Of the 105 patients who met the inclusion criteria, 77 patients (73.3%) exhibited a high vancomycin MIC (≥1.5 mg/L). In the bivariate analysis, prior vancomycin exposure within 30 days of index culture collection [15 patients (19.5%) versus 1 patient (3.6%), P = 0.05] and residence in an intensive care unit (ICU) at the onset of infection [27 patients (35.1%) versus 3 patients (10.7%), P = 0.02] were both significantly associated with a high vancomycin MIC value and both were independent predictors of high MICs in the logistic regression. Conclusions Patients with MRSA bloodstream infections in the ICU or with a history of vancomycin exposure should be considered at high risk of infection with strains for which vancomycin MICs are elevated. Appropriate and aggressive empirical therapy is required for these patients.