Therapeutic Drug Monitoring (TDM)

Facilitates the prevention and detection of therapeutic failure and toxicity.
TDM involves the measurement and interpretation of 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).

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