Laboratory Studies
Therapeutic drug monitoring is readily available in most settings, and symptomatic patients should have their lithium levels measured. However, levels may not correlate with clinical symptoms due to the kinetic profile of lithium. A repeat level should be checked several hours later, after intravenous hydration, to disclose any trend. Serial levels may be warranted in cases of sustained-release tablets. The sample must be sent in a lithium-free tube; falsely elevated levels have been obtained in plasma samples from tubes with speckled green tops that contained lithium heparin as an anticoagulant. [11]
Urinalysis, electrolyte levels, and kidney function should also be checked. A low anion gap (see the Anion Gap calculator) or a low urine specific gravity may suggest lithium toxicity due to sodium loss. A thyroid function panel may also be considered in patients presenting with symptoms suggestive of hypothyroidism.
Co-ingestants should also be considered in cases of intentional overdose. An acetaminophen level should be obtained in every patient suspected of intentional overdose.
Lumbar puncture should be considered in patients with altered mental status and suspicion of central nervous system infection.
Imaging Studies
Consider a computed tomography (CT) scan of the head in individuals with severe movement disorders, seizures, stupor, or coma. CT scan may be needed to rule out other etiologies and to examine for trauma secondary to intoxication.
Electrocardiography
Chronic lithium toxicity is frequently associated with nonspecific and diffuse depressed ST segments and T-wave inversion unassociated with symptoms or significant sequelae. A retrospective study in 76 patients receiving lithium therapy found that corrected QT interval > 440 ms and diffuse T wave inversion were significantly more common in the 11 patients with serum lithium levels > 1.2 mEq/L. [12]
Lithium intoxication may result in dysrhythmias, including complete heart block. Serious cardiac toxicity is uncommon and generally occurs only in individuals with underlying heart disease.
