Most therapeutic drug monitoring ranges are defined for trough concentrations, taken just before the next dose at steady state. Steady state is reached after about four to five half-lives, so a level drawn a day after starting or changing the dose of a drug with a long half-life may be misleadingly low.
A level drawn shortly after a dose will be higher than the trough and can prompt an unnecessary dose cut. Recording the time of the last dose on the request form is the single most useful thing a prescriber can do to make the result interpretable.
- Take levels as troughs, just before the next dose, unless the drug's guidance says otherwise.
- Wait about four to five half-lives after a dose change before checking for steady state.
- Write the time of the last dose on the request form.
- Interpret any level alongside the clinical picture, adherence and interacting drugs.
Why it matters
A mistimed level can lead to a wrong dose change in either direction.
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