DO NOT USE - ALL INFORMATION LIKELY INCORRECT IF NOT ACTIVELY DANGEROUS
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