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ABSTRACTMethicillin-resistant Staphylococcus aureus(MRSA) is a common cause of bloodstream infection (BSI) and is often associated with invasive infections and high rates of mortality. Vancomycin has remained the mainstay of therapy for serious Gram-positive infections, particularly MRSA BSI; however, therapeutic failures with vancomycin have been increasingly reported. We conducted a comprehensive evaluation of the factors (patient, strain, infection, and treatment) involved in the etiology and management of MRSA BSI to create a risk stratification tool for clinicians. This study included consecutive patients with MRSA BSI treated with vancomycin over 2 years in an inner-city hospital in Detroit, MI. Classification and regression tree analysis (CART) was used to develop a risk prediction model that characterized vancomycin-treated patients at high risk of clinical failure. Of all factors, the Acute Physiology and Chronic Health Evaluation II (APACHE-II) score, with a cutoff point of 14, was found to be the strongest predictor of failure and was used to split the population into two groups. Forty-seven percent of the population had an APACHE-II score < 14, a value that was associated with low rates of clinical failure (11%) and mortality (4%). Fifty-four percent of the population had an APACHE-II score ≥ 14, which was associated with high rates of clinical failure (35%) and mortality (23%). The risk stratification model identified the interplay of three other predictors of failure, including the vancomycin MIC as determined by Vitek 2 analysis, the risk level of the source of BSI, and the USA300 strain type. This model can be a useful tool for clinicians to predict the likelihood of success or failure in vancomycin-treated patients with MRSA bloodstream infection.