Lithium is primarily prescribed as a mood stabilizer for Bipolar Disorder, a condition characterized by significant shifts in mood, energy, and activity levels. This salt compound is effective in treating acute manic episodes and preventing future mood swings. However, lithium has a narrow therapeutic index—a small window between an effective dose and a potentially harmful one. Therefore, careful medical supervision and regular blood testing are required to maintain the correct concentration in the bloodstream.
Defining Therapeutic Range Targets
Lithium concentration is measured in milliequivalents per liter (mEq/L). The target range shifts based on the patient’s clinical situation, requiring higher concentrations during severe mood episodes and lower concentrations for long-term stability. For acute mania treatment, the target serum concentration is generally 0.8 mEq/L to 1.2 mEq/L. Levels up to 1.5 mEq/L may be used in hospital settings for aggressive episodes, but this increases the risk of side effects.
Once acute symptoms stabilize, the goal shifts to a lower maintenance range to minimize long-term risks. The accepted maintenance therapeutic range is 0.6 mEq/L to 1.0 mEq/L. Some patients, particularly older individuals, may achieve stability at levels as low as 0.4 mEq/L. The final target level is always individualized by a healthcare provider, balancing symptom prevention against the patient’s tolerance for side effects.
The Process of Monitoring Lithium Levels
Monitoring lithium concentration requires a precise blood test known as a trough level measurement. The trough level is the lowest concentration of the drug in the bloodstream, measured just before the next dose is due. To standardize this measurement, the blood sample must be drawn exactly 10 to 12 hours after the patient’s last dose. This timing ensures a consistent reading that accurately reflects the lowest circulating level.
Before the initial test, the patient must take the medication consistently for about five days to reach a “steady state,” where absorption equals elimination. During the initial treatment phase or following a dosage change, blood levels are checked frequently, often once or twice a week, until the level is stable within the target range. Once stable on a maintenance dose, monitoring frequency decreases to every three to six months.
Recognizing and Addressing Lithium Toxicity
Because lithium has a narrow therapeutic index, levels exceeding the therapeutic range can quickly lead to toxicity requiring immediate medical attention.
Mild to Moderate Toxicity (1.5 mEq/L to 2.5 mEq/L)
Levels in this range are associated with early symptoms, including gastrointestinal issues and neurological signs.
- Nausea, vomiting, or diarrhea
- Fine hand tremor
- Fatigue
Sustained levels above 1.5 mEq/L increase the risk of severe side effects, even if initial symptoms are subtle.
Moderate Toxicity (2.5 mEq/L to 3.5 mEq/L)
As concentration climbs, symptoms escalate with more pronounced neurological effects. Patients may experience:
- Confusion and agitation
- Delirium
- Problems with coordination (ataxia) or slurred speech
Severe Toxicity (Above 3.5 mEq/L)
This concentration carries the risk of life-threatening complications. The severe stage can involve seizures, coma, hyperthermia, and permanent neurological damage.
Factors That Influence Lithium Concentration
Several physiological factors and medical conditions can cause lithium concentration to fluctuate unexpectedly. Lithium is processed by the kidneys, and its clearance is closely tied to the body’s salt and water balance.
Dehydration, caused by fever, vomiting, diarrhea, or excessive sweating, significantly reduces the body’s ability to excrete lithium. This leads to a rapid increase in blood levels. Maintaining consistent hydration and a normal salt intake is necessary to keep the concentration stable.
Certain common medications can also interfere with the kidney’s handling of lithium, causing levels to rise. These include:
- Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), such as ibuprofen, which reduce renal clearance.
- Diuretics, often prescribed for high blood pressure.
- Angiotensin-Converting Enzyme (ACE) inhibitors, also used for high blood pressure.
Patients starting any new medication that affects fluid or sodium balance must inform their prescribing physician so lithium levels can be monitored and adjusted as needed.