Predicting methicillin resistance and the effect of inadequate empiric therapy on survival in patients with Staphylococcus aureus bacteremia

Background: The restriction of vancomycin hydrochloride use is recommended as a measure to decrease the emergence of vancomycin resistance in gram-pos itive organisms; however, vancomycin also is the treatment of choice for me thicillin-resistant Staphylococcus aureus (MRSA) infections. If vancomycin use is restricted to patients with documented infections due to methicillin -resistant organisms, then patients with MRSA infections may not initially receive vancomycin. This study was performed to determine factors that pred ict MRSA bacteremia and if ineffective empiric antibiotic therapy increased the risk of death in patients with S aureus bacteremia. Methods: We conducted a retrospective cohort study of all patients with cli nically significant S aureus bacteremia (132 episodes in 128 patients) diag nosed between October 1, 1995, and January 1, 1998, at an urban acute care Veterans Affairs medical center (approximately 200 acute care beds) in Balt imore, Md. During the study period, vancomycin was a restricted antibiotic. Empiric use had to be approved by an attending physician specializing in i nfectious diseases. Results: Compared with patients who had methicillin-sensitive S aureus bact eremia, patients with MRSA bacteremia were significantly older (70 vs 58 ye ars; P<.01), more likely to have a history of MRSA (47% vs 6%; P<.01) and a nosocomial infection (76% vs 50%; P<.01), and less likely to use injection drugs (8% vs 32%; P<.01). In addition, compared with patients who had meth icillin-sensitive S aureus bacteremia, patients with MRSA bacteremia were s ignificantly less likely (45% vs 98%; P<.01) to receive effective antibioti c therapy during the first 48 hours of hospitalization. However, the risk o f death due to ineffective empiric therapy was less than 1 (relative risk, 0.82; 95% confidence interval, 0.36-1.88) and did not change significantly when adjusted for age, occurrence of sepsis, or nosocomial infection. Conclusions: The results of this study support the safety of the restrictio n of vancomycin use in patients with clinically significant S aureus bacter emia. However, patients with a history of MRSA are more likely to have futu re MRSA infections and should receive empiric therapy using vancomycin for possible S aureus infections, particularly for nosocomial infections.