What lithium level is therapeutic?

For maintenance prophylaxis of bipolar disorder, 0.6 to 0.8 mEq/L; for acute mania, up to 1.0. Levels of 1.2 to 1.5 are borderline toxic and above 1.5 frankly toxic. Troughs are drawn 12 hours after the evening dose, at steady state after about five days on a stable dose.

What is the Winter method for lithium dosing?

The Winter method estimates lithium clearance as 0.25 x creatinine clearance (converted to L/h), then sizes the daily dose as target level x clearance x 24 / bioavailability. Creatinine clearance is estimated on fat-free mass rather than total weight, because lithium distributes into lean tissue.

Why is lithium dosed on fat-free mass?

Lithium distributes into total body water and lean tissue, not adipose tissue. Dosing on total body weight in obesity overestimates clearance and overdoses. The Janmahasatian equation estimates fat-free mass from weight, height and BMI for the creatinine clearance calculation.

How is the dose adjusted from a measured level?

From a steady-state 12-hour trough on a known daily dose: patient clearance = (F x dose in mEq / 24) / measured level, then new daily dose = target x patient clearance x 24 / F. Confirm with a repeat steady-state trough after any change.

What are the signs of lithium toxicity?

Early: coarse tremor, ataxia, dysarthria, nausea and polyuria worsening. Severe: confusion, seizures, oliguria and coma. Toxicity is precipitated by dehydration, intercurrent illness, NSAIDs, thiazides and ACE inhibitors; any of these with symptoms needs an urgent level and clinical assessment.

How often should lithium levels be monitored?

Weekly after initiation or any dose change until stable, then every 3 months once stable, plus renal function, thyroid function and calcium at least annually. Check a level 5-7 days after any dose change, and urgently with dehydration, intercurrent illness or new interacting drugs.

Lithium Dosing Calculator

Size lithium carbonate doses with the Winter method: fat-free mass estimation, creatinine clearance, target-level dosing, per-dose amounts, time to steady state, and adjustment from a measured steady-state trough.

Lithium dosing tool

Age (years) Sex Actual body weight (kg) Height (cm) Serum creatinine (mg/dL) Target trough level (mEq/L) Dosing interval Formulation

Level-based adjustment (optional) Measured steady-state 12-h trough (mEq/L) Current total daily dose (mg lithium carbonate)

Key definitions

Winter method: A lithium dosing method that estimates lithium clearance as 0.25 x creatinine clearance (in L/h), then sizes the daily dose as target level x clearance x 24 / bioavailability. Creatinine clearance is estimated on fat-free mass.
Fat-free mass (Janmahasatian): Lean body mass estimated from weight, height and BMI. Lithium distributes into lean tissue and body water, not fat, so clearance estimates based on total weight overdose obese patients.
Steady-state trough: A lithium level drawn 12 hours after the evening dose, after about five half-lives (roughly 5 days) on an unchanged dose. The only level suitable for dose adjustment.
Therapeutic range: 0.6 to 0.8 mEq/L for maintenance prophylaxis, up to 1.0 for acute mania. Narrow by any standard: 1.2 to 1.5 is borderline toxic, above 1.5 frankly toxic.
mEq/L: Milliequivalents per litre, the unit for lithium concentrations. Lithium carbonate 300 mg contains about 8.1 mEq of lithium; the calculator converts between mg and mEq (36.95 mg per mEq).

Why lithium dosing needs a method

Lithium has the narrowest therapeutic index in psychiatry and one of the narrowest in all of medicine. The difference between 0.6 mEq/L, where it prevents bipolar relapse, and 1.5 mEq/L, where it poisons, is a factor of 2.5 in dose, and the clinical consequences of overshooting include irreversible neurological injury and death. Yet the dose that produces 0.8 in one patient produces 0.5 in another and 1.2 in a third, because lithium clearance is essentially renal clearance, and renal clearance varies enormously. Fixed dosing, the same milligrams for everyone, is therefore indefensible; the dose must be derived from the patient's kidneys.

The Winter method does this with unusual elegance. Lithium is handled by the kidney much like sodium and creatinine: filtered freely, substantially reabsorbed. Empirically, lithium clearance runs at about a quarter of creatinine clearance, hence liCl = 0.25 x CrCl (converted to L/h). From clearance, the maintenance dose is determined: daily mEq = target x clearance x 24 / bioavailability. The method's distinctive step is using fat-free mass rather than total body weight in the Cockcroft-Gault estimate, via the Janmahasatian equation, because lithium's distribution follows lean tissue. In a patient of normal build the correction changes little; in obesity it prevents the systematic overdosing that total-weight estimates produce.

Worked example: a 40-year-old man, 75 kg, 180 cm, creatinine 1.0 mg/dL, target 0.8 mEq/L, twice-daily immediate-release. BMI is 23.1, fat-free mass 59.5 kg, creatinine clearance 82.7 mL/min, lithium clearance 1.24 L/h. The daily dose is 23.8 mEq, which is 880 mg of lithium carbonate per day, or 440 mg every 12 hours. Half-life is 23.3 hours, so steady state takes about 4.9 days: the first meaningful trough is drawn on day 5 or 6, not before.

Targets by phase of illness

The target level is a clinical decision the calculator implements but does not make. For maintenance prophylaxis of bipolar disorder, 0.6 to 0.8 mEq/L is the standard, with some patients, particularly the elderly or those sensitive to side effects, maintained at 0.4 to 0.6 for depression prophylaxis. For acute mania, targets run higher, 0.8 to 1.0, accepting more side effects for faster control. Above 1.0 the risk-benefit turns: 1.0 to 1.2 is the high therapeutic zone needing close monitoring, 1.2 to 1.5 borderline toxic, above 1.5 toxic. The calculator labels the chosen target with its band so the prescriber sees the territory explicitly.

Formulation affects the arithmetic through bioavailability: immediate-release lithium carbonate is taken as F = 1.0, extended-release as 0.9. The per-dose amount follows from the interval: the same daily milligrams split every 12 hours or given once daily. Once-daily dosing, usually in the evening, is common in maintenance for convenience and possibly lower polyuria; the 12-hour trough is then drawn before the next evening dose, which is why the standard sampling time is defined that way.

Adjusting from a measured level

Once a steady-state trough exists, estimation gives way to measurement. Patient clearance = (F x daily dose in mEq / 24) / measured level, in L/h; the new daily dose = target x patient clearance x 24 / F. Example: a patient on 900 mg/day (24.4 mEq/day) immediate-release with a steady-state trough of 0.55 mEq/L targeting 0.8. Patient clearance = (1.0 x 24.4/24)/0.55 = 1.85 L/h; new dose = 0.8 x 1.85 x 24 = 35.5 mEq/day = 1,311 mg/day, practically 1,200 to 1,350 mg/day in available tablet strengths, confirmed with a repeat trough in 5 to 7 days.

The validity conditions are strict and frequently violated. The level must be a true 12-hour trough at steady state: drawn early, during absorption or before five half-lives, it misleads. Renal function must be stable; a level drawn during gastroenteritis with dehydration describes a clearance the patient will not have next week. Missed or doubled doses in the days before sampling corrupt the result. When any of these apply, treat the level as suspect and repeat rather than adjusting on it.

Toxicity, precipitants and monitoring

Lithium toxicity is a clinical diagnosis supported by a level, not a level alone, because chronic accumulation produces symptoms at lower levels than acute overdose. Early signs, coarse tremor, ataxia, dysarthria, worsening nausea, increasing polyuria, should prompt an urgent level and dose hold. Severe toxicity brings confusion, seizures, oliguria and coma, and needs emergency care; haemodialysis removes lithium effectively and is indicated in severe or deteriorating cases.

The classic precipitants all reduce lithium clearance or volume: dehydration and intercurrent illness, NSAIDs (which can raise levels 20 to 40%), thiazide diuretics, ACE inhibitors and ARBs, and low salt intake. Starting any of these in a lithium-treated patient requires a level within a week and counselling to maintain fluid and salt intake during illness. Baseline and ongoing monitoring includes renal function, thyroid function and calcium at least annually, weight and polyuria review, and pregnancy planning discussion, since lithium carries teratogenic risk, notably Ebstein's anomaly, that needs preconception specialist input.

Limitations and pitfalls

The Winter method assumes stable adult renal function and average lithium handling. It does not apply in acute kidney injury, where creatinine lags behind true filtration; in pregnancy, where clearance rises substantially; in the elderly with sarcopenia, where creatinine-based estimates flatter true function; or in dialysis. The fat-free mass correction improves obesity dosing but remains an estimate, and the 0.25 clearance factor is a population average around which individuals vary.

Tablet strengths constrain the prescription: lithium carbonate commonly comes in 300 and 400 mg tablets (and 200/400 mg extended-release), so calculated doses must be rounded to achievable regimens and confirmed with levels. The calculator does not manage drug interactions quantitatively beyond the standard monitoring advice; NSAIDs and thiazides in particular need proactive level checks, not arithmetic. And it cannot assess adherence, the commonest cause of an unexpected level: a low trough in a patient missing doses is not an indication to increase the dose.

Key takeaways

Dose lithium from the kidneys: Winter method, liCl = 0.25 x CrCl, daily mEq = target x clearance x 24 / F.
Use fat-free mass (Janmahasatian) for the creatinine clearance estimate, not total body weight.
Target 0.6 to 0.8 mEq/L for maintenance, up to 1.0 for acute mania; above 1.2 is toxic territory.
Draw troughs 12 hours post-dose at steady state (about 5 days); adjust from the patient's own clearance.
Watch precipitants: dehydration, NSAIDs, thiazides, ACE inhibitors; check a level within a week of any change.
Monitor renal, thyroid and calcium function long-term; toxicity is clinical as well as numerical.

Frequently asked questions

What lithium level is therapeutic?: 0.6 to 0.8 mEq/L for maintenance prophylaxis; up to 1.0 for acute mania. 1.2 to 1.5 is borderline toxic, above 1.5 toxic. Draw 12-hour troughs at steady state.
What is the Winter method?: Lithium clearance = 0.25 x creatinine clearance (L/h); daily dose = target x clearance x 24 / bioavailability. Creatinine clearance is estimated on fat-free mass.
Why is lithium dosed on fat-free mass?: Lithium distributes into lean tissue and body water, not fat. Total-weight dosing overdoses obese patients; the Janmahasatian equation estimates fat-free mass from weight, height and BMI.
How is the dose adjusted from a measured level?: Patient clearance = (F x daily mEq / 24) / measured trough; new dose = target x clearance x 24 / F. Requires a true steady-state 12-hour trough and stable renal function.
What are the signs of lithium toxicity?: Coarse tremor, ataxia, dysarthria, nausea, worsening polyuria early; confusion, seizures, oliguria, coma late. Dehydration, NSAIDs, thiazides and ACE inhibitors precipitate it.
How often should levels be monitored?: Weekly after initiation or dose changes until stable, then every 3 months; renal, thyroid and calcium function at least annually; urgently with dehydration, illness or new interacting drugs.
Can lithium be used in pregnancy?: Only with specialist preconception planning: it carries teratogenic risk including Ebstein's anomaly, and clearance rises markedly in pregnancy, requiring dose increases and close monitoring. Never stop abruptly without psychiatric guidance.
What if a dose is missed?: Take it when remembered unless the next dose is near; never double up. A single missed dose rarely matters clinically, but repeated misses destabilise levels and should prompt an adherence conversation, not a dose increase.

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References

Ng F, Mammen OK, Wilting I, et al. The International Society for Bipolar Disorders (ISBD) consensus guidelines for the safety monitoring of bipolar disorder treatments. Bipolar Disord. 2009;11(6):559-595.
Janmahasatian S, Duffull SB, Ash S, et al. Quantification of lean bodyweight. Clin Pharmacokinet. 2005;44(10):1051-1065.

Medical disclaimer

This calculator is an educational tool for clinicians, students and informed readers. It does not provide medical advice. Lithium has a narrow therapeutic index: dosing must be confirmed with therapeutic drug monitoring and regular renal, thyroid and electrolyte checks under qualified supervision. Never start, stop or change a prescribed medicine on the basis of a calculator result alone; if you are unwell, seek professional care promptly.