Therapeutic Drug Monitoring (TDM)

Why do we need therapeutic drug monitoring (TDM)?

  • Some drugs have narrow therapeutic indices which can make it difficult to keep the drug concentration in the patients blood between suboptimal (low) and toxic (high) levels.
  • Situations where TDM may be useful:
    • uncertain drug adherence
    • drugs known to cause therapeutic and adverse effects
    • suboptimal tolerability
    • non-response at therapeutic doses
    • pharmacokinetic drug-drug interactions
    • identifying variation in inter-individual drug metabolism
  • Reduction of relapse rates by TDM is highly cost-effective, as relapses can lead to hospitalisation.

When should you measure antipsychotic drug levels?

  • During steady-state which is when the amount of drug entering is the same as that leaving within a given time frame.
  • It is recommended that drug plasma levels are sampled when a stable and effective treatment regime has been achieved. This optimal drug level can be used in the future to assess any deviations and adjust the the dosing accordingly.

Interpreting antipsychotic drug plasma levels

  • Act upon assay results in conjunction with reliable clinical observation (‘treat the patient, not the level’)

Magna Laboratories offers the following TDM assays:

  • Aripiprazole
    • Is recommended and primarily used in the treatment of schizophrenia and bipolar disorder
    • Main metabolite is dehydroaripiprazole which has similar activity to aripiprazole and accounts for 40% of total plasma level.
    • Linear dose-plasma level response.
  •  Haloperidol
    • Is used in the treatment of schizophrenia, tics in Tourette syndrome, mania in bipolar disorder, nausea and vomiting, delirium, agitation, acute psychosis, and hallucinations in alcohol withdrawal.
    • Monitoring is appropriate in psychoses management.
  • Olanzapine
    • Is recommended and primarily used in the treatment of schizophrenia and bipolar disorder.
    • Furthermore, given the large inter-patient variability in plasma olanzapine concentrations at the same dosages, olanzapine TDM can be considered very useful in assessing therapeutic efficacy and controlling adverse events.
    • Linear dose-plasma level response.
  • Quetiapine
    • Is an atypical antipsychotic used for the treatment of schizophrenia, bipolar disorder and major depressive disorder.
  • Risperidone
    • Is mainly used to treat schizophrenia, bipolar disorder and irritability in people with autism.
    • Efficacy is determined by active moieties, risperidone plus 9-hydroxyrisperidone.
    • It is common to find patients outside recommended therapeutic ranges.

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