Background:
Failure of antibiotic treatment increases mortality of critically ill patients. This study investigated the association between the treatment resolution of methicillin-resistant Staphylococcus aureus (MRSA) bacteremia and vancomycin pharmacokinetic variables.

Methods:
A total of 28 critically ill patients were included in this study. All data were collected from medical, microbiology and pharmacokinetic records. The clinical response was evaluated on the basis of clinical and microbiological parameters. The 24-h area under the curve (AUC_0-24) was estimated from a single trough level using established equations.

Results:
Out of the 28 patients, 46% were classified as responders to vancomycin treatment. The trough vancomycin concentration did not differ between the responders and non-responders (15.02 ± 6.16 and 14.83 ± 4.80 μg mL^-1; P = 0.929). High vancomycin minimum inhibitory concentration (MIC) was observed among the non-responders (P = 0.007). The ratio between vancomycin trough concentration and vancomycin MIC was significantly lower in the non-responder group (8.76 ± 3.43 vs. 12.29 ± 4.85 μg mL^-1; P = 0.034). The mean ratio of estimated AUC_0-24 and vancomycin MIC was 313.78 ± 117.17 μg h mL^-1 in the non-responder group and 464.44 ± 139.06 μg h mL^-1 in the responder group (P = 0.004). AUC_0-24/MIC of ≥ 400 μg h mL^-1 was documented for 77% of the responders and 27% of the non-responders (c² = 7.03; P = 0.008).

Conclusions:
Ratio of trough concentration/MIC and AUC_0-24/MIC of vancomycin are better predictors for MRSA treatment outcomes than trough vancomycin concentration or AUC_0-24 alone. The single trough-based estimated AUC may be sufficient for the monitoring of treatment response with vancomycin.