Diagnostic Use

In acute poisoning , initial plasma lithium level does not correlate well with toxicity – should rely more on history, physical examination and other investigations.

In the setting of chronic use or acute poisoning -on-chronic use, plasma lithium more closely correlate with clinical toxicity .

It is important to determine the optimum range for each individual patient. Lower levels in elderly may be suitable in some patients (0.4-0.6), e.g. elderly. A few patients remain well with levels 0.8 mmol/L to avoid recurrence. Serum level guidelines are the same regardless of the lithium formulation used.

Levels taken anywhere within post-dose timeframe of 10 – 14 hours are similar enough to allow assessment of dose/response. Routine monitoring should be done after 5 days, then weekly, after initiation and after each dose change until concentrations are stable, then 3 monthly unless clinical settings change (see NZ Formulary website).

Routine 3 monthly checks are recommended for stable patients, but results are only applicable if the patient is adherent and remaining in a stable state. Levels should be checked if there is a change in clinical state, including developing side effects.

For acute episodes of mania, a target lithium concentration of 0.8 – 1.2 mmol/L is recommended, reducing to 0.6 – 0.8 mmol/L once euthymia is achieved. Slow and careful reduction in dose to maintenance phase is recommended, rather than a sudden shift. Abrupt reduction increases risk of relapse.

For premenopausal women, due to the influence of menstrual cycle on both lithium level and symptoms, dosing over at least one full cycle is encouraged before reducing dose. Effective contraception is needed if at risk. Specialist oversight and more frequent monitoring is needed if lithium is continued in pregnancy. Visit NZ Formulary https://nzf.org.nz/nzf_2216 (accessed Feb 2023).

Lithium concentrations >2.0 mmol/L require urgent treatment and specialist advice (NZ National Poison Centre). Appropriate action may be influenced by symptoms, other laboratory findings, and factors such as age and comorbidities.

Toxic effects include: tremor, ataxia, dysarthria, nystagmus, renal impairment and convulsions.

Other tests should be considered at baseline and then with periodic monitoring at least 6 monthly – Ca, TSH, U/E, creatinine/eGFR, or earlier if there is evidence of deterioration, or starting an interacting drug (e.g. ACEi, NSAIDs, diuretics).

Factors commonly precipitate / predispose patients to increased risk of toxicity include:

1. Dehydration from e.g. reduced fluid intake, diuretic use, fluid loss from gastrointestinal tract, nephrogenic diabetes insipidus, hypercalcaemia

2. Medications e.g. NSAIDs, Angiotensin converting enzyme inhibitors

3. Age >50yrs

4. Thyroid dysfunction

5. Pre-existing renal impairment

References:

1. Australian and NZ clinical practice guidelines for treatment of bipolar disorder. Aus NZ J Psychiatry 2004;38:280-305

2. Mokhlesi B et al. Adult toxicology in critical care: Part II: Specific poisonings. Chest 2003; 123:897-922

3. Dawson AH et al Therapeutic drug monitoring in drug overdose. J Clin Pharmacol. 1999; 48:278-83

4. New Zealand Formulary (NZF). NZF v126. 2022. Available from: https://nzf.org.nz/nzf_2219 (Accessed December, 2022)

Reference Intervals

Units: mmol/L

* Sample taken approximately 12 hour after last dose. Optimum range should be individualised.

^ Especially if >1.5 mmol/L. Levels >2.0 mmol/L require urgent treatment and specialist advice.

Please refer to Diagnostic Use and Interpretation in Test Guide.