Lithium monitoring rests on a trough level taken about 12 hours after the last dose, and a level drawn at any other interval is not comparable with the reference range printed beside it. A sample taken four hours after a dose can read alarmingly high in a patient who is perfectly stable; one taken 24 hours later can read reassuringly low in a patient who is not.
So the time of the last dose belongs on the request form, not in the patient's memory of the appointment. Where the timing is unknown, the level should be reported as uninterpretable and repeated rather than acted on.
The surrounding checks matter as much as the number: renal function and thyroid function at least six-monthly on stable treatment, calcium periodically, and a level within about a week of any dose change or of starting a drug that alters lithium handling — a thiazide, an ACE inhibitor or angiotensin receptor blocker, or a non-steroidal anti-inflammatory. Dehydration from any cause, including a febrile illness or a day of vomiting, does the same thing. Sick-day advice given once at initiation is not advice the patient still holds two years later.
- Write the time of the last dose on the request form, and treat a level without it as uninterpretable
- Take the sample about 12 hours post-dose — the reference range assumes that interval
- Recheck the level about a week after any dose change, and after starting a thiazide, ACE inhibitor, angiotensin receptor blocker or non-steroidal anti-inflammatory
- Check renal and thyroid function at least six-monthly on stable treatment, with calcium periodically
- Repeat sick-day advice at review: stop or hold if vomiting or unable to drink, and seek help rather than continuing regardless
Why it matters
Most lithium levels that trigger an alarming phone call are timing errors, not toxicity.
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