Therapeutic Drug Monitoring and Drug Level Interpretation

Therapeutic drug monitoring (TDM) is the clinical practice of measuring drug concentrations in biological fluids — most commonly plasma or serum — to optimise dosing for individual patients. TDM is indicated when a drug has a narrow therapeutic index (small difference between effective and toxic concentrations), when pharmacokinetic variability between patients is high, when clinical endpoints are difficult to assess directly, or when toxicity is serious and reversible only if caught early.

The therapeutic range is the concentration interval associated with maximum probability of therapeutic response and minimum probability of toxicity in a population. It is derived from population studies and is a statistical construct, not an absolute boundary. Some patients achieve excellent outcomes below the range; others require concentrations above it. The TDM result must always be interpreted in the clinical context — the correct question is not "is the level in range?" but "does this level explain the patient's clinical picture?"

Timing of sample collection is critical to meaningful TDM. Trough samples (taken immediately before the next dose at steady state) are the most reproducible and clinically interpretable, as they represent the lowest concentration during a dosing interval. Peak samples are taken at a defined time after dosing (the absorption phase must be complete) and are useful for drugs where efficacy relates to peak concentration (e.g., aminoglycosides). Pre-dose (trough) levels are standard for digoxin, phenytoin, carbamazepine, lithium, cyclosporin, tacrolimus, and methotrexate. Steady state must be reached before sampling (allow 4–5 half-lives), unless a loading dose was given.

The AUC (area under the concentration-time curve) is increasingly recognised as the most clinically meaningful TDM parameter because it integrates total drug exposure over time. AUC/MIC targets for antibiotics (e.g., vancomycin AUC24/MIC ≥ 400) and AUC-based targets for immunosuppressants (cyclosporin AUC0-4h or tacrolimus AUC0-12h) better predict outcomes than trough-only monitoring. In practice, limited sampling strategies (measuring 2–3 concentrations at specific times post-dose) and Bayesian software generate AUC estimates without full 12-hour profiles.

Common TDM drugs and their NZ clinical targets: Digoxin (0.6–1.0 µg/L for heart failure; sample >6 h post-dose to allow distribution); Phenytoin (40–80 µmol/L total, 4–8 µmol/L free; adjust for albumin); Lithium (0.6–0.8 mmol/L maintenance, 0.8–1.0 mmol/L acute; trough at 12 h post-dose); Vancomycin (AUC24 400–600 mg·h/L; trough >10 mg/L to prevent resistance); Gentamicin once-daily (peak >10 × MIC, trough <1 mg/L); Cyclosporin (C0 100–300 µg/L or C2 800–1400 µg/L depending on protocol); Tacrolimus (C0 5–15 µg/L organ-specific); Methotrexate (level at 24, 48, 72 h post high-dose; target <0.1 µmol/L at 48 h for leucovorin rescue guidance).

Interpreting an unexpected level requires systematic analysis: Was it taken at the right time? Is the patient at steady state? What is the renal and hepatic function? What interacting drugs are present? Has adherence been confirmed? Has there been a recent dose change? Only after confirming sample timing and clinical context should the dose be adjusted.