Therapeutic Drug Monitoring
The practice of measuring drug levels in the blood to optimise dosing — ensuring concentrations remain within the therapeutic window. TDM is essential for drugs with narrow therapeutic indices. Vancomycin is the peptide drug most commonly requiring TDM, with target trough levels guiding dose adjustment.
Technical Context
TDM is routinely performed for: vancomycin (target trough 15-20 μg/mL for serious infections, 10-15 μg/mL for less serious; AUC/MIC-based dosing increasingly preferred — target AUC24 400-600 mg·h/L for MRSA), cyclosporine (trough levels: target varies by indication and time post-transplant — typically 150-300 ng/mL early post-transplant, 100-200 ng/mL maintenance), and aminoglycoside antibiotics (if combined with peptide antibiotics). TDM is NOT routinely required for most peptide drugs: GLP-1 RAs (predictable PK, dose-titrated by clinical response), somatostatin analogues (clinical and biochemical response monitoring — IGF-1, not drug levels), and GnRH compounds (monitored by LH, FSH, and sex steroid levels rather than drug concentrations). TDM principles: sampling at steady state (after 4-5 half-lives), correct timing (trough = pre-dose; peak timing depends on route), validated assay methodology, and clinical correlation (drug levels interpreted alongside clinical response and toxicity signs).