Lithium Unit Converter
Lithium Unit Converter
Convert lithium between mmol/L, mEq/L and mg/L — mmol/L and mEq/L are the same number because lithium is monovalent — and read the result against the rule that makes it interpretable: the sample must be taken 12 hours after the dose.
Lithium converter
Mass ⇄ molarSerum lithium 0.7 mmol/L, 12 hours after the dose
Why mmol/L and mEq/L are the same number
mg/L = mmol/L × 6.941
mmol/L = mg/L ÷ 6.941
- lithium is monovalent
- the lithium ion carries a single positive charge, so one millimole supplies one milliequivalent. The two units are numerically identical for lithium, exactly as they are for sodium and potassium and unlike calcium or magnesium, where a millimole supplies two milliequivalents
- atomic weight 6.941
- the standard atomic weight of lithium. It is the lightest metal, which is why a therapeutic concentration in mass units is a fraction of a milligram per litre of anything else you might measure
- mg/L — offered here, used nowhere
- no clinical laboratory reports serum lithium in mg/L. It is on this page because the conversion is occasionally asked for in teaching, in toxicology reference tables and in older literature, and because it is easier to show the number than to argue about it. If a report says mg/L, check it: it is far more likely to be mmol/L
- the dose is a salt, the level is an ion
- lithium carbonate 400 mg contains about 10.8 mmol of lithium, and different salts and modified-release preparations are not interchangeable milligram for milligram. Brands are prescribed by name for that reason. The level measures the ion, whatever the salt
Worked example
Serum lithium 0.7 mmol/L, 12 hours after the dose
0.7 mmol/L = 0.70 mEq/L — identical, because lithium is monovalent
0.7 × 6.941 = 4.86 mg/L, which is arithmetically correct and clinically useless: no laboratory reports lithium this way
0.70 mmol/L sits inside the BNF's 0.4–1.0 mmol/L and just below NICE's 0.6–0.8 mmol/L target for a first course
Now change one thing that is not the dose. Start a thiazide, or let the patient become dehydrated with a diarrhoeal illness, and the same daily dose can push this level towards 1.2–1.5 mmol/L within days
And change the sampling time instead. Draw the blood four hours after the dose rather than twelve and the number is substantially higher — not because anything has changed in the patient, but because absorption and distribution are incomplete. That level cannot be compared with any target range
The three units, and which of them anyone actually uses
| Unit | 0.4 mmol/L | 0.7 mmol/L | 1.0 mmol/L | 1.5 mmol/L | Used clinically? |
|---|---|---|---|---|---|
| mmol/L | 0.40 | 0.70 | 1.00 | 1.50 | Yes — universally |
| mEq/L | 0.40 | 0.70 | 1.00 | 1.50 | Identical figures; the older North American convention |
| mg/L | 2.78 | 4.86 | 6.94 | 10.41 | No laboratory reports lithium in mg/L |
Published targets, all on a 12-hour sample
| Source | Target (mmol/L = mEq/L) | Applies to |
|---|---|---|
| BNF | 0.4 – 1.0 | General target, with the lower end for maintenance and for older people |
| NICE CG185 | 0.6 – 0.8 | A person starting lithium for the first time |
| NICE CG185 | 0.8 – 1.0 | A person who has relapsed previously on lithium, or who has continuing subthreshold symptoms with functional impairment |
| Commonly quoted caution threshold | above 1.0 | Toxicity becomes more likely; above 1.5 it is likely, and above 2.0 it is a medical emergency |
What raises a lithium level without the dose changing
| Cause | Mechanism |
|---|---|
| Thiazide diuretics | Sodium depletion increases proximal tubular reabsorption of lithium, which the kidney handles like sodium. One of the most reliable ways to make a stable patient toxic |
| NSAIDs, including over-the-counter ibuprofen | Reduced renal prostaglandin synthesis lowers renal blood flow and lithium clearance. Often bought rather than prescribed, so it is missed unless asked about directly |
| ACE inhibitors and angiotensin receptor blockers | Reduced glomerular filtration and sodium loss both reduce lithium clearance, sometimes substantially and sometimes weeks after starting |
| Dehydration — vomiting, diarrhoea, fever, heat, poor intake, diuretic abuse | Volume depletion drives avid sodium and lithium reabsorption. The commonest precipitant of toxicity in practice |
| Any acute deterioration in renal function | Lithium is cleared almost entirely by the kidney and has essentially no metabolism |
| Low-sodium diet, or a sudden change in salt intake | Lithium handling follows sodium handling |
One number, two identical units, and a sampling time that decides whether any of it means anything
Serum lithium is reported in millimoles per litre by every clinical laboratory, and a level in milliequivalents per litre is the same number. That is because lithium exists in serum as a monovalent cation: one millimole of a singly charged ion supplies one milliequivalent, so no arithmetic separates the two units. The conversion to a mass concentration is straightforward as well — lithium’s atomic weight is 6.941, so 0.7 mmol/L is 4.86 mg/L — but it should be said plainly that no laboratory reports lithium that way. The mg/L figure appears on this page because it is occasionally asked for in teaching and in toxicology tables, not because it is a unit anyone should be working in. A report that appears to give a lithium concentration in mg/L is far more likely to be mmol/L, and is worth checking.
The important content of a lithium page is not the conversion but the sampling time. A lithium level is taken 12 hours after the last dose, at steady state, on an unchanged dose for at least five days. Every target range in the BNF and in NICE CG185 was defined on that basis. Lithium is absorbed over hours and distributes slowly into tissues, so a sample taken four hours after a dose is substantially higher than the 12-hour figure and cannot be compared with any published range; one taken at eighteen hours is lower. A level drawn at the wrong time is worse than no level at all, because it invites a dose change that the patient does not need. Record the time of the last dose and the time of the sample on the request, and if either is unknown, repeat it properly rather than interpret it.
Lithium is monitored because its therapeutic index is narrow. The gap between a concentration that prevents relapse and one that causes harm is small: NICE’s targets sit between 0.6 and 1.0 mmol/L, toxicity becomes more likely above 1.0, is likely above 1.5, and above 2.0 mmol/L it is a medical emergency. No other commonly prescribed drug is quite so unforgiving of a modest change in clearance, and lithium’s clearance can change without the dose changing at all. Thiazide diuretics are the classic offender, because the kidney handles lithium much as it handles sodium and sodium depletion drives reabsorption of both. NSAIDs, very often bought over the counter rather than prescribed, reduce renal prostaglandin synthesis and lithium clearance with it. ACE inhibitors and angiotensin receptor blockers do the same by a different route. And dehydration from any cause — a diarrhoeal illness, vomiting, fever, hot weather, simply drinking too little — is the commonest precipitant of all. Each of these is a question to ask before a level is interpreted as an adherence problem or a dosing problem.
Two things this page deliberately does not do. It does not interpret a raised level or grade toxicity — the lithium level interpreter exists for that, and duplicating it here would be worse than linking to it. And it does not turn a number into a dose. Lithium dosing is a clinical decision that takes in the level, the sampling time, renal and thyroid function, calcium, the current mental state, the patient’s own history of relapse and every other medicine they are taking. Lithium salts are prescribed by brand because different preparations are not interchangeable milligram for milligram, and changes are made in small steps with a repeat level about a week later. Long-term monitoring — the level itself, plus renal, thyroid and calcium checks — continues for as long as the drug does.
Frequently asked questions
Are mmol/L and mEq/L the same for lithium?
Yes, exactly the same number. Lithium circulates as a monovalent cation, so one millimole carries one milliequivalent of charge and a level of 0.7 mmol/L is 0.7 mEq/L. It is the same relationship as for sodium and potassium, and unlike calcium or magnesium, where one millimole supplies two milliequivalents. If you have a lithium level in mEq/L, you can read it against a mmol/L target range without converting anything.
Can lithium be reported in mg/L?
Arithmetically yes — multiply mmol/L by 6.941, so 0.7 mmol/L is 4.86 mg/L — but no clinical laboratory reports serum lithium that way. The conversion is here for teaching and for older or toxicological sources. If a report appears to give lithium in mg/L, treat that as a reason to check the units rather than to convert, because it is much more likely to be mmol/L.
Why must a lithium level be taken 12 hours after the dose?
Because every published target range was defined on a 12-hour post-dose sample at steady state. Lithium is absorbed over hours and distributes slowly into tissues, so a level taken four hours after a dose is much higher than the 12-hour figure and one taken at eighteen hours is lower. Comparing a mistimed sample with a target range invites a dose change the patient does not need, which is why a level drawn at the wrong time is worse than no level. Record the dose time and the sampling time, and repeat rather than reinterpret.
What is the target lithium level?
The BNF gives 0.4–1.0 mmol/L on a 12-hour sample, with the lower end for maintenance and for older people. NICE CG185 is more specific: 0.6–0.8 mmol/L for someone taking lithium for the first time, and 0.8–1.0 mmol/L for someone who has relapsed previously on lithium or who has continuing subthreshold symptoms with functional impairment. All of these assume a 12-hour sample on a stable dose, and none of them replaces a clinical judgement about whether this patient’s dose is right.
What raises a lithium level without the dose changing?
Anything that reduces lithium clearance, because the kidney handles lithium much like sodium. Thiazide diuretics are the classic cause, along with NSAIDs (often bought over the counter), ACE inhibitors and angiotensin receptor blockers, and any cause of dehydration or sodium depletion — vomiting, diarrhoea, fever, hot weather, a low-salt diet or simply drinking too little. Deteriorating renal function does the same. A stable patient can become toxic within days through any of these while taking exactly the dose they have always taken.
Does an in-range lithium level mean the patient is safe?
It means the concentration was acceptable at the moment the sample was taken. Because the therapeutic index is narrow, a level inside the range gives little margin: a new thiazide or NSAID, a diarrhoeal illness or a spell of hot weather can move it into the toxic range within days on an unchanged dose. Symptoms of toxicity — coarse tremor, vomiting and diarrhoea, ataxia, slurred speech, drowsiness, confusion — can also appear at concentrations only slightly above target and occasionally within it, so they are assessed clinically rather than read off a number.
Related calculators
References
- National Institute for Health and Care Excellence. Bipolar disorder: assessment and management. Clinical guideline CG185. London: NICE; 2014 (updated 2023). Aim for a serum lithium level of 0.6–0.8 mmol/L in people starting lithium for the first time; consider 0.8–1.0 mmol/L for people who have relapsed previously while taking lithium or who have subthreshold symptoms with functional impairment.
- Joint Formulary Committee. Lithium carbonate — Monitoring requirements. British National Formulary. London: BMJ Group and Pharmaceutical Press. Serum-lithium concentration 0.4–1.0 mmol/L, measured 12 hours after the dose.
- Finley PR. Drug interactions with lithium: an update. Clin Pharmacokinet. 2016;55(8):925–941.
- Hedya SA, Avula A, Swoboda HD. Lithium toxicity. In: StatPearls. Treasure Island FL: StatPearls Publishing; 2024.
- Severus E, Taylor MJ, Sauer C, et al. Lithium for prevention of mood episodes in bipolar disorders: systematic review and meta-analysis. Int J Bipolar Disord. 2014;2:15.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.