Expansion
Narrow therapeutic index drugs where effect cannot be measured directly
Expansion
Monitoring is worthwhile when the drug has a narrow therapeutic index, the effect cannot be observed directly, there is wide interindividual variation, and the level correlates with effect or toxicity.
| Drug | Sampling |
|---|---|
| Digoxin | At least 6 hours post-dose, otherwise falsely high |
| Lithium | 12 hours post-dose |
| Phenytoin | Trough; interpret with albumin, since it is highly protein bound |
| Vancomycin | Trough before the dose |
| Gentamicin | Trough, and peak in some regimens |
| Ciclosporin, tacrolimus | Trough |
| Theophylline | 4 to 6 hours after a modified release dose |
Drugs monitored by effect rather than level: warfarin (INR), heparin (APTT), insulin (glucose), antihypertensives (blood pressure). This is generally preferable when possible.
Common errors
- Sampling before steady state, giving a falsely low result
- Sampling too soon after a dose, giving a falsely high result, the classic digoxin mistake
- Acting on a level without assessing the patient: digoxin toxicity can occur at a normal level if the patient is hypokalaemic
- Failing to note the timing on the request form, making the result uninterpretable