Lithium citrate syrup ↔ carbonate ↔ mEq
Convert between lithium citrate oral solution, lithium carbonate, and milliequivalents of lithium ion.
High-consequence calculationLithium has a very narrow therapeutic index. Confirm the formulation and the intended dose, and ensure serum concentration monitoring is in place.
What is this for?
Lithium is available as carbonate tablets and capsules and as a citrate oral solution. The two are dosed differently by weight but interchange on the basis of lithium ion content: 5 mL of the citrate syrup contains 8 mEq of lithium, equivalent to 300 mg of lithium carbonate.
How to use it
- Select what you are starting from: a carbonate dose in milligrams, a syrup volume in millilitres, or milliequivalents.
- Enter the amount.
- All three equivalent expressions are shown in the result panel.
Worked example
A patient takes lithium carbonate 900 mg daily but cannot swallow tablets. What volume of lithium citrate solution provides the equivalent dose?
Answer: 900 mg ÷ 300 mg = 3 units; 3 × 5 mL = 15 mL of citrate solution, supplying 24 mEq of lithium.
Clinical pearls & pitfalls
- The reference equivalence to hold in memory is 5 mL syrup = 300 mg carbonate = 8 mEq lithium. Everything else scales from it.
- Lithium citrate solution is absorbed faster than carbonate tablets and produces a higher, earlier peak. Concentrations should be rechecked after switching formulations even though the daily dose is equivalent.
- Modified-release carbonate products are not interchangeable with immediate-release at the same dose without reassessment, because the peak-to-trough profile differs.
- Sodium and lithium compete for renal reabsorption. Sodium depletion from any cause increases lithium reabsorption and raises the concentration — one reason thiazides, NSAIDs, and ACE inhibitors are all significant interactions.
- Toxicity can occur within the "therapeutic" range, particularly in older adults. Treat the patient and the symptoms, not only the number.
Assumptions & limitations
- A dose-equivalence conversion only. It does not account for the differing absorption profiles of the formulations.
- Does not adjust for renal function. Lithium is renally cleared and requires dose reduction and closer monitoring in impairment.
- Does not cover the modified-release carbonate products, which have their own dosing considerations.
- The therapeutic ranges quoted are conventional; individual targets vary with indication, age, and tolerability.
References
- Product labelling for lithium carbonate tablets, capsules, and extended-release tablets, and lithium citrate oral solution.
- Malhi GS, et al. Lithium therapy and its interactions. Aust Prescr. 2020;43(3):91-93.
- Yatham LN, et al. CANMAT and ISBD guidelines for the management of patients with bipolar disorder. Bipolar Disord. 2018;20(2):97-170.