Pharmacokinetics, Pharmacodynamics and Therapeutic Drug Monitoring
Core concepts
Master the core pharmacokinetic parameters: absorption, bioavailability (F), volume of distribution (Vd), clearance (CL), half-life (t½) and their relationships (t½ ≈ 0.693 × Vd/CL). Understand zero- versus first-order kinetics, steady-state achievement (about 4–5 half-lives), loading doses versus maintenance doses, and the meaning of therapeutic index. For pharmacodynamics, know receptor agonism/antagonism, potency versus efficacy, and the dose-response curve.
Traps in questions
A common trap is confusing a loading dose (depends on Vd) with a maintenance dose (depends on CL). Another is sampling for TDM at the wrong time - trough levels must be drawn just before the next dose, at steady state. Watch for questions that apply first-order assumptions to drugs with saturable (zero-order) kinetics such as phenytoin and ethanol.
Worked rule of thumb
Use "load the volume, maintain the clearance": loading dose = (Vd × target concentration)/F; maintenance dose = (CL × target concentration)/F. For TDM, wait at least 4–5 half-lives before measuring a trough. For a patient started on a drug with a long half-life, a loading dose reaches target faster without changing steady-state concentration.