PH10.9-10 | PH10.{9-10} | Patient-Specific Dose Calculation and Therapeutic Drug Monitoring — SDL Guide — Summary & Reflection

KEY TAKEAWAYS

Patient-specific dose calculation requires systematic adjustment for four key variables: renal impairment (use Cockcroft-Gault CrCl and drug-specific dose tables), hepatic impairment (use Child-Pugh score for cirrhosis; adjust for high-extraction drugs), age extremes (mg/kg or BSA for children; start low, go slow in elderly), and pregnancy/lactation (use category ratings and benefit-risk assessment). Therapeutic drug monitoring is indicated for narrow-therapeutic-index drugs where plasma concentration correlates with efficacy and toxicity. Key TDM drugs: vancomycin, aminoglycosides, phenytoin, lithium, digoxin, cyclosporine. Levels must be drawn at the correct time (trough pre-dose; peak at the specified post-dose interval) after steady state is achieved (≥5 × half-life). Dose revision from TDM uses the proportional formula: New dose = Current dose × (Target level ÷ Measured level). The accumulation loop — toxicity worsening the organ function responsible for drug clearance — must be anticipated in nephrotoxic drugs.

REFLECT

Consider a patient you have recently seen prescribed an antibiotic, anticonvulsant, or anticoagulant. Was their renal function checked before prescribing? Was a TDM level ordered if appropriate? Were the results acted upon? What system-level changes — pharmacy review, electronic prescribing alerts, mandatory CrCl calculation fields — would reduce dose-related adverse drug events in the hospital environment where you train? These system questions are as important as the individual calculation skills — most prescribing errors arise not from ignorance of the formula but from failure to apply it in the workflow of a busy ward.