Therapeutic Drug Monitoring

The answer first

Therapeutic drug monitoring (TDM) is the measurement of a drug concentration in plasma or serum, with the dose adjusted to maintain the concentration within a defined therapeutic range that maximises efficacy and minimises toxicity. TDM is mandatory for drugs with a narrow therapeutic index, unpredictable pharmacokinetics, concentration-dependent toxicity, or a clear concentration-response relationship — including anti-infectives (vancomycin, aminoglycosides, antifungals, antiretrovirals), anticonvulsants (phenytoin, valproate, carbamazepine), cardiac agents (digoxin, amiodarone, flecainide), immunosuppressants (tacrolimus, cyclosporin, sirolimus, mycophenolate), lithium, methotrexate (high-dose), biologics (infliximab, adalimumab drug and anti-drug antibody levels), and a growing number of oncology agents. The physician must understand the indications, the sampling (trough versus peak, time after dose, sample handling), the interpretation (in clinical context), and the dose adjustment. [5] [10]

The FRACP candidate must be able to defend three positions without hedging: (1) TDM is a clinical tool, not a number — interpret the concentration in the context of the patient (renal function, age, drug interactions, organ function, clinical response); (2) the trough concentration (just before the next dose) is the standard sample for most drugs, taken after steady state is reached (four to five half-lives), with a few notable exceptions (random methotrexate, peak and trough aminoglycosides); and (3) dose adjustment uses the linear pharmacokinetic equation for most drugs at therapeutic doses (new dose = old dose × target concentration / measured concentration), with non-linear kinetics (phenytoin, the Michaelis-Menten model) requiring a different calculation. Lead with the decision, then the evidence, then the trap. [5] [11]

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