Therapeutic drug monitoring

Last Updated on by frcemuser


Therapeutic drug monitoring (TDM) is the individualization of drug dosage by maintaining plasma or blood drug concentrations within a target range (the ‘therapeutic window’)

This helps address the two major sources of variability in drug responses between individuals, the relationships between:

  • dose and plasma concentration (pharmacokinetic variability)
  • drug concentration at the receptor and the response (pharmacodynamic variability

Indications for therapeutic drug monitoring

  • to assess for drug toxicity or suspected overdose
  • if non-compliance is suspected
  • Adjust dosing for improving efficacy
  • after initiating treatment
  • after adjusting dose
  • if treatment is failing
  • when starting or stopping a potentially interacting drug
  • if there is a change in a patient’s physiology (eg. pregnancy, renal or hepatic impairment)
  • to confirm abstinence
  • to assist diagnosis – adverse drug effects may mimic disease state

Interpretation of drug levels

Interpretation of drug levels is dependent on:

  • time of sampling
  • Duration of treatment at the current dose and dosing schedule
  • individual characteristics that may affect the pharmacokinetics (eg. age, gender, concomitant disease, ethnicity)
  • concomitant medications
  • therapeutic range and pharmacokinetics of the drug

Time of sampling

  • For drugs with short half-lives compared to the dosing interval samples should be collected pre-dose
  • For drugs with long half-lives (e.g. phenytoin and amiodarone) it is OK to collect samples at any point in the dosage interval — this is also the case for digoxin at any point after the distribution phase (after 6 hours post-dose)
  • For most drugs wait until the steady state is achieved until checking levels.exceptions are drugs with long half-lives and which can cause severe toxicity, as the steady state may not be reached for months


Drugs Monitoring


TDM is always indicated for aminoglycosides and vancomycin due to the relatively narrow therapeutic windows

Patients with altered gentamicin pharmacokinetics, Example:

  • critically ill with severe sepsis or septic shock
  • renal replacement therapy
  • severe burns
  • cystic fibrosis
  • pregnancy
  • ascites
  • morbid obesity


Therapeutic drug monitoring

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