Therapeutic Drug Monitoring (TDM)
Facilitates the prevention and detection of therapeutic failure and toxicity.
TDM involves the measurement and interpretation of drug concentrations.
Measuring drug concentrations
- Accurate recording of drug doses and times of administration is necessary to interpret results. Put details in request form.
- Timing of samples:
- Take trough plasma samples (just before next dose) at steady state where possible. Half-life and time to steady state may be very different in some patients. Half-life is determined by clearance, and therefore longer in patients with impaired clearance.
- Renal impairment increases the half-life of renally cleared drugs.
- Half-life often longer in frail elderly patients.
- Patients presenting acutely are often not at steady state.
- If toxicity is suspected, take samples as clinically required.
- Some drugs, such as aminoglycosides, require more than one sample and steady state is not relevant.
- Ensure you use the correct tube (see Test Manager below).
Interpreting drug concentrations
- Routine TDM support is provided through the ward/clinical pharmacists and the Medicines Information Service (
80900).- For aminoglycoside and vancomycin TDM, see specific guidance.
- For immunosuppressants in transplant, contact the relevant transplant team.
- For other advice on TDM or interpretation of results:
- During work hours: Clinical Pharmacology registrar (pager 8524)
- After hours: If urgent before working hours, on-call pharmacist.
- Key resources:
- CHL Test Manager includes a comprehensive list of every medicine it tests; with information about blood sample tube, sample timing, half-life, and steady-state.
- Therapeutic Drug Monitoring (TDM) profiles are available for selected medicines on the Christchurch Medicines Information Service website.
- Pharmacokinetic Table (notes when TDM is available at Health New Zealand Canterbury hospitals)
- For information on poisoning and overdose, see Hospital HealthPathways, TOXINZ, or contact the National Poisons Centre ( 0800 764 766).
Topic Code: 93303
