Therapeutic drug monitoring

Key Information

Appropriate Tests


Monitoring is of value with drugs whose effect is related to the plasma level, and which have a low therapeutic index.

Plasma total drug levels are usually measured, but specialised laboratories may assay plasma free drug levels, or salivary levels, in some circumstances.

Active metabolite assays are sometimes needed.

Anticoagulant drugs are generally monitored by their effect on coagulation tests; see Anticoagulant monitoring.

Some drugs must be assayed at a specific time in relation to therapy eg, trough level just prior to next dose.

Some drugs do not show a correlation between plasma levels and therapeutic activity (eg, sulthiame); monitoring may still be useful to identify non-compliance.



Inadequate therapeutic response

  • Inadequate dose
  • Poor compliance
  • Fast removal (short half life)
  • Low bioavailability
  • Drug interactions



  • Excessive dose
  • Slow removal (long half life)
  • Increased bioavailability
  • Drug interactions


Available tests include



Phenytoin level, Carbamazepine level, Phenobarbitone, Valproate level, Primidone, Clonazepam, Sulthiame, Ethosuximide, Lamotrigine.

Cardiac drugs

Digoxin level, Quinidine, Procainamide, Lignocaine, Amiodarone level, Mexiletine.

Psychotropic drugs

Tricyclic antidepressants, Lithium.

Anti-inflammatory drugs

Gold, Salicylate.


Antibiotic assay; Gentamycin level, Tobramycin, Amikacin, Vancomycin level, Flucytosine, Chloramphenicol.


Theophylline, Cyclosporine, Methotrexate, Tacrolimus.

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