DO NOT USE FOR CLINICAL PRACTICE
Please use current guidelines available on the UHNM intranet for patient treatment
Please use current guidelines available on the UHNM intranet for patient treatment
INDICATIONS
- Ask yourself if the result is likely to affect patient management
- Appropriate indications
- assist dose adjustment for optimal serum concentrations
- confirm suspected toxicity
- monitor effect of drug/drug or drug/disease interactions
- investigate treatment failure
- investigate suspected non-adherence
Advice
- Contact your ward’s clinical pharmacist or medicines information with
- details of the dose regimen, sample time and assay
- patient’s clinical details and drug treatment
- For advice on optimal use of antimicrobial agents, contact microbiologist
TIMING THE SAMPLE
- Unless toxicity suspected, assays usually little value until regular dosing with a steady state
- usually 4–5 half-lives after treatment began or dose was last altered
SENDING A SAMPLE
- Check appropriate forms and tubes on intranet
- Send all samples in tube appropriate for assay required
- Provide the following details
- dose, frequency and duration of treatment with drug
- time of last dose
- any impaired organ function (e.g. renal impairment, liver disease, cardiac failure)
- potentially interacting drugs (see individual drugs and BNF Appendix 1) including dose, frequency and duration of co-prescription
ASSAYS AVAILABLE
Clinical biochemistry
Carbamazepine
- Half-life: 35 hr (single dose); 10–20 hr (regular dosing)
When to sample
- Time from start: 2–3 weeks
- Change of dosage: 3–4 days
- Sample time: Pre-dose
- Assay Day
Notes
- Target range: 4–12 mg/L
Phenobarbital
- Half-life: 2-6 days
When to sample
- Time from start: 2–4 weeks (oral maintenance dose)
- Change of dosage: 2–4 weeks (oral maintenance dose)
- Sample time: any time
- Assay day
Notes
- Target range: 15–40 mg/L
- If urgent, contact laboratory
Phenytoin
- Half-life: 35 hrs
When to sample
- Time from start: 3–4 weeks (oral maintenance dose)
- Change of dosage: 3–4 weeks (oral maintenance dose)
- Sample time: any time
- Assay day
Notes
- Target range: 10–20 mg/L
- Dose-concentration relationship non-linear – see Phenytoin adjustment guideline
Digoxin
- Half-life: 40 hr
When to sample
- Time from start: 1–3 weeks (oral maintenance dose)
- Change of dosage: 1–3 weeks (oral maintenance dose)
- Sample time: At least 6 hr post-dose
- Assay day
Notes
- Target range: 0.8–2 microgram/L
- Hypokalaemia predisposes to toxicity. Monitor potassium
Theophylline
- Half-life: 6–8 hr
When to sample
- Time from start: 5 days
- Change of dosage: 5 days
- Sample time: IV anytime; Oral at least 4–6 hr post-dose
- Assay day
Notes
- Target range: 10–20 mg/L
- Theophylline is given by injection as aminophylline
Lithium
- Half-life: 8-45 hr
When to sample
- Time from start: 1 week (oral maintenance dose)
- Change of dosage: 1 week (oral maintenance dose)
- Sample time: At least 12 hr post-dose
- Assay day
Notes
- Target range: 0.6–1.2 mmol/L (BNF)
- NICE and other authorities make different recommendations
- If lithium toxicity suspected, stop lithium. Contact patient’s consultant psychiatrist
Tacrolimus
- Half-life: 10–20 hr
When to sample
- Time from start: ~1 week
- Change of dosage: ~1 week
- Sample time: Pre-dose
- Assay day
Notes
- Target range: dependent on use
Ciclosporin
- Half-life: 11 hr
When to sample
- Time from start: < 1 week
- Change of dosage: < 1 week
- Sample time: Pre-dose
- Assay day
Notes
- Target range: dependent on use
- Sodium valproate – discuss directly with laboratory concerned
Microbiology
Tobramycin (8-hrly dosing)
- Half-life: 2.5 hr
When to sample
- Time from start: 12 hrs
- Change of dosage: 12 hrs
- Sample time: Trough immediately pre-dose: Peak 1 hr post-dose
- Assay day
Notes
- Target range: Trough: <2 mg/L;
- Target range: Peak: 5–10 mg/L
- 8–10 mg/L for enterobacterial pneumonia
- 8–12 mg/L for exacerbation of bronchiectasis in cystic fibrosis patients
- Tobramycin range based on 8-hrly dosing
- All other drugs (e.g. teicoplanin, antiretroviral drugs) – discuss directly with laboratory concerned