A lithium level is only interpretable if it is taken about 12 hours after the last dose, usually in the morning before the next dose. A level drawn a few hours after a dose looks falsely high and may prompt an unnecessary reduction.
The common causes of toxicity in a stable patient are new prescriptions and dehydration. NSAIDs, ACE inhibitors, angiotensin receptor blockers and thiazide diuretics all raise lithium levels. Diarrhoea, vomiting or a febrile illness can do the same. Kidney and thyroid function, and calcium, need regular checks on long-term treatment.
- Take lithium levels about 12 hours after the last dose.
- Recheck the level within about a week of starting an NSAID, ACE inhibitor, angiotensin receptor blocker or thiazide.
- Tell patients to seek advice early if they have vomiting, diarrhoea or cannot keep fluids down.
- Monitor kidney and thyroid function and calcium regularly on long-term lithium.
Why it matters
Most lithium toxicity in stable patients follows a new prescription or an intercurrent illness, both of which can be anticipated.
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