What is the primary purpose of therapeutic drug monitoring in pharmacokinetics?

• A. To measure PK parameters directly without regard to levels.
• B. To adjust therapy based solely on reported symptoms without drug levels.
• C. To tailor dosing by measuring drug levels and maintaining within the therapeutic window.
• D. To replace clinical judgment in dosing decisions.

Answer: (C)

Therapeutic drug monitoring uses measured drug concentrations to guide dosing so a patient’s exposure stays within a defined therapeutic window. This is crucial for drugs with narrow therapeutic indices and when there is substantial variability in how individuals process medications. By timing a concentration sample—often a steady-state trough—you can estimate how much drug exposure a patient is receiving and adjust the dose or dosing interval to achieve an effective, safe level.

Clinical response and symptoms are important, but they can be imperfect indicators on their own. Drug levels provide objective data to tailor therapy to the individual, balancing efficacy and safety. This approach is not about measuring PK parameters in isolation, nor about relying solely on symptoms, nor about replacing clinical judgment; it’s about using concentration data in concert with clinical context to personalize dosing.