A guide to therapeutic drug monitoring of β-lactam antibiotics.

Therapeutic drug monitoring (TDM) opens the door to personalized medicine, yet it is infrequently applied to β-lactam antibiotics, one of the most commonly prescribed drug classes in the hospital setting. As we continue to understand more about β-lactam pharmacodynamics (PD) and wide inter- and intra-patient variability in pharmacokinetics (PK), the utility of TDM has become increasingly apparent. For β-lactams, the time that free concentrations remain above the minimum inhibitory concentration (MIC) as a function of the dosing interval (%fT>MIC) has been shown to best predict antibacterial effect. Many studies have shown that β-lactam %fT>MIC exposures are often suboptimal across a wide variety of disease states and clinical settings. A limitation to implementing this practice is the general lack of understanding on how to best operationalize this intervention and interpret the results. The instrumentation and expertise needed to quantify β-lactams for TDM is rarely available locally, but certain laboratories advertise these services and perform them regularly. Familiarity with the modalities and nuances of antimicrobial susceptibility testing is crucial to establishing β-lactam concentration targets that meet the relevant exposure thresholds. Evaluation of these concentrations is best accomplished using population PK software and Bayesian modeling, for which a multitude of programs are available. While TDM of β-lactams has shown an ability to increase the rate of target attainment, there is currently limited evidence to suggest that it leads to improved clinical outcomes. Although consensus guidelines for β-lactam TDM do not exist in the United States, guidance would help to promote this important practice and better standardize the approach across institutions. Herein, we discuss the basis for β-lactam TDM, review supporting evidence, and provide guidance for implementation in specific patient populations.