Therapeutic drug monitoring targets for vancomycin, aminoglycosides, antiepileptics, immunosuppressants and biologics assume a sample taken at a defined time, usually immediately before the next dose. A sample drawn at the wrong time can look high or low and lead to a wrong dose change.
The commonest errors are a 'trough' drawn after the dose has started, and a level taken before steady state has been reached.
- Record both the time of the last dose and the time of sampling on the request.
- Draw troughs within the 30 minutes before the next dose for intermittently dosed drugs.
- Wait for steady state — usually four to five half-lives — before interpreting a level after a dose change.
- Question an unexpected result before changing the dose; check timing first.
Why it matters
A mistimed sample is a common cause of unnecessary dose changes and toxicity.
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