Clinical meaning
Therapeutic drug monitoring and baseline laboratory assessment are grounded in pharmacokinetic and pharmacodynamic principles that determine drug safety and efficacy. Medications with narrow therapeutic indices (lithium, warfarin, digoxin, aminoglycosides, phenytoin, vancomycin) have small margins between therapeutic and toxic serum concentrations, making routine monitoring essential. Drug metabolism occurs primarily through hepatic cytochrome P450 (CYP) enzymes, with genetic polymorphisms producing ultra-rapid, extensive, intermediate, or poor metabolizer phenotypes that dramatically alter drug clearance. CYP2D6 metabolizes many psychotropics and opioids; CYP2C9 and VKORC1 variants affect warfarin sensitivity; CYP2C19 polymorphisms alter clopidogrel activation. Renal excretion governs clearance of lithium, digoxin, and aminoglycosides, so declining glomerular filtration rate (estimated by Cockcroft-Gault or CKD-EPI equations) mandates dose reduction. Steady-state serum concentration is reached after approximately 4-5 half-lives of consistent dosing, which determines when trough levels should first be drawn. Trough levels drawn immediately before the next dose represent the lowest concentration in the dosing interval. Baseline organ function assessment (hepatic transaminases, renal function, CBC) establishes the pre-treatment reference against which drug-induced toxicity is measured: drug-induced liver injury (DILI) manifests as AST/ALT elevation greater than 3 times the upper limit of normal; nephrotoxicity presents as rising creatinine and declining GFR; bone marrow suppression causes cytopenias. QTc interval monitoring identifies medications that prolong cardiac repolarization (antipsychotics, fluoroquinolones, methadone), with QTc exceeding 500 milliseconds conferring significant risk for torsades de pointes, a polymorphic ventricular tachycardia. Drug-drug interactions occur through enzyme induction (increasing metabolism and reducing drug levels), enzyme inhibition (decreasing metabolism and increasing toxicity risk), and pharmacodynamic potentiation (additive QT prolongation, serotonergic effects, CNS depression).