Tacrolimus has a narrow therapeutic window and variable absorption, so dosing is guided by whole-blood trough levels. Take the sample immediately before the next dose, so the level reflects the trough and not a peak. A level drawn after the morning dose will mislead.
Wait until steady state, usually after about three days at a stable dose, before adjusting, and change one thing at a time. Check for interacting drugs when a level changes unexpectedly: azole antifungals, macrolides and some calcium channel blockers raise levels, while rifampicin and some anticonvulsants lower them. Diarrhoea can also raise levels.
- Draw the trough immediately before the next dose.
- Wait for about three days at a stable dose before adjusting.
- Review interacting drugs when levels change unexpectedly.
- Remember that diarrhoea can raise tacrolimus levels.
Why it matters
A badly timed sample can lead to a dose change in the wrong direction.
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