5. Therapeutic Drug Monitoring (TDM): Definition, types. Application in clinical practice.
Explains therapeutic drug monitoring as measuring drug levels in blood alongside the clinical picture to individualise dosing, and sets out three types: drug concentration, pharmacodynamic effect and pharmacogenetic testing, with examples such as INR for warfarin and TPMT before azathioprine. Walks through the target concentration approach within the therapeutic window, the criteria that make a drug suitable, and a table of common drugs with usual ranges. Closes with indications for sampling, steady-state timing and trough versus peak sampling.
Topic 5 of 30 in the MU Sofia syllabus for Clinical Pharmacology (Medicine, Year 5). The free AceMed note covers it in 5 sections, about 239 words (2 min of reading), with 15 flashcards.
In this note
Introduction
Types
The target concentration concept
Which drugs are suitable
When and how to sample
Introduction
Therapeutic drug monitoring (TDM) is the measurement of drug concentrations in blood, interpreted together with the clinical picture, to individualise the dose. It is teamwork between clinicians, laboratory specialists and clinical pharmacologists.
Types
PK-TDM: measures drug concentrations (the classic form).
PD-TDM: measures the effect (e.g. INR for warfarin, aPTT for heparin).
Pharmacogenetic TDM: tests the genes that govern drug handling (e.g. TPMT before azathioprine, HLA-B*57:01 before abacavir, CYP2C19 for clopidogrel).
The target concentration concept
The aim is a level inside the therapeutic window: above the minimum effective and below the minimum toxic concentration.
Choose a target concentration; calculate the loading and maintenance doses.
Start treatment.
Assess the clinical response and measure the concentration.
Recalculate and adjust the dose.
Which drugs are suitable
Concentration correlates well with the therapeutic or toxic effect.
Narrow therapeutic index; no active metabolites; concentration not predictable from the dose (variable PK, saturable kinetics); effect hard to measure clinically; often taken in overdose.
A sensitive, specific, practical assay exists.
Common drugs and target ranges
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