Can J Hosp Pharm. 2021 Fall;74(4):334-343. doi: 10.4212/cjhp.v74i4.3195.
BACKGROUND: Vancomycin remains widely used for methicillin-resistant Staphylococcus aureus (MRSA) infections; however, treatment failure rates up to 50% have been reported. At the authors' institution, monitoring of trough concentration is the standard of care for therapeutic drug monitoring of vancomycin. New guidelines support use of the ratio of 24-hour area under the concentration-time curve to minimum inhibitory concentration (AUC24/MIC) as the pharmacodynamic index most likely to predict outcomes in patients with MRSA-associated infections.
OBJECTIVES: To determine the discordance rate between trough levels and AUC24/MIC values and how treatment failure and nephrotoxicity outcomes compare between those achieving and not achieving their pharmacodynamic targets.
METHODS: This retrospective cohort study involved patients with MRSA bacteremia or pneumonia admitted to the study hospital between March 1, 2014, and December 31, 2018, and treated with vancomycin. Data for trough concentrations were collected, and minimum concentrations (C min) were extrapolated. The AUC24/MIC values were determined using validated population pharmacokinetic models. The C min and AUC24/MIC values were characterized as below, within, or above pharmacodynamic targets (15-20 mg/L and 400-600, respectively). Discordance was defined as any instance where a patient's paired C min and AUC24/MIC values fell in different ranges (i.e., below, within, or above) relative to the target ranges. Predictors of treatment failure and nephrotoxicity were determined using logistic regression.
RESULTS: A total of 128 patients were included in the analyses. Of these, 73 (57%) received an initial vancomycin dose less than 15 mg/kg. The discordance rate between C min and AUC24/MIC values was 21% (27/128). Rates of treatment failure and nephrotoxicity were 34% (43/128) and 18% (23/128), respectively. No clinical variables were found to predict discordance. Logistic regression identified initiation of vancomycin after a positive culture result (odds ratio [OR] 4.41, 95% confidence interval [CI] 1.36-14.3) and achievement of target AUC24/MIC after 4 days (OR 3.48, 95% CI 1.39-8.70) as modifiable predictors of treatment failure.
CONCLUSIONS: The relationship between vancomycin monitoring and outcome is likely confounded by inadequate empiric or initial dosing. Before any modification of practice with respect to vancomycin monitoring, empiric vancomycin dosing should be optimized.