Phys · pharmacological
Therapeutic Drug Monitoring
Also known as Therapeutic Drug Monitoring · therapeutic drug monitoring
Consultant-physician depth guide to Therapeutic Drug Monitoring for FRACP DWE/DCE preparation — presentation, differentials, investigations, management, complications and exam angles.
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- Missed urgency or delayed escalation in Therapeutic Drug Monitoring turns a salvageable presentation into preventable harm
- Treating the label without confirming the mechanism leads to wrong therapy in Therapeutic Drug Monitoring
- Ignoring multimorbidity and drug interactions while managing Therapeutic Drug Monitoring is a classic exam and clinical trap
- Failing to document the shared plan and safety-net advice after Therapeutic Drug Monitoring loses follow-through
- Using recalled thresholds without a cited source is forbidden — verify before acting
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. [1]
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. [1]
References5ShowHide
- [1]Kang JS, Lee MH Overview of therapeutic drug monitoring. Korean J Intern Med, 2009.PMID 19270474
- [2]Rybak MJ, Le J, Lodise TP, Levine DP, et al. Therapeutic Monitoring of Vancomycin for Serious Methicillin-resistant Staphylococcus aureus Infections: A Revised Consensus Guideline and Review by the American Society of Health-system Pharmacists, the Infectious Diseases Society of America, the Pediatric Infectious Diseases Society, and the Society of Infectious Diseases Pharmacists. Clin Infect Dis, 2020.PMID 32658968
- [3]Krishnan N, Perazella MA Diagnosis and Management of Acute and Chronic Lithium-Associated Nephrotoxicity. J Am Soc Nephrol, 2026.PMID 42247265
- [4]Morales Junior R, Telles JP, Chua NGS, Kunicki PK, et al. Mapping Therapeutic Drug Monitoring Research in Low- and Middle-Income Countries: A Scoping Review (2015-2024) Ther Drug Monit, 2026.PMID 42457203
- [5]Asir B, Al-Soleiti M, Singh B Bipolar Disorder in Older Adults: A Comprehensive Overview of Pharmacotherapy Patterns and Clinical Considerations Drugs Aging, 2026.PMID 42467341