Lithium is a highly effective medication used primarily as a mood stabilizer for conditions like bipolar disorder, helping to manage acute episodes and prevent future recurrence of symptoms. It also helps reduce the risk of suicidal thoughts and behaviors. However, lithium has a narrow therapeutic window; the effective concentration is very close to the toxic concentration. This small margin requires routine and careful monitoring of the lithium level in the bloodstream for patient safety and treatment success.
Establishing the Initial Monitoring Schedule
The frequency of testing is highest when a person first begins lithium treatment or whenever the dosage is changed. After starting the medication, the first serum lithium level test is typically scheduled five to seven days later. This waiting period allows the drug to reach a steady-state concentration, where the amount taken in equals the amount eliminated. Testing continues at a high frequency, often weekly, until the desired therapeutic level is achieved and remains stable.
All monitoring during this phase must be a “trough level,” representing the lowest concentration of the drug in the blood during a dosing interval. To ensure accuracy, the blood sample must be drawn exactly 12 hours after the last dose was taken. This specific timing provides the most reliable measure for determining the correct dosage and prevents misleading results.
Routine Long-Term Maintenance Testing
Once a patient is stable on a consistent dose and has achieved a steady-state therapeutic level, the monitoring frequency can be reduced. For most compliant patients showing no signs of instability, the standard recommendation is to check the lithium level every three months for the first year.
After the first year of stability, monitoring frequency is often extended, with levels typically checked every six months. However, certain factors require a patient to remain on a more frequent three-month schedule:
- Advanced age.
- The presence of other medical conditions.
- Taking other medications known to interact with lithium.
- Indication of poor adherence.
- A history of prior instability.
Urgent Testing Due to Acute Changes
The planned schedule must be immediately abandoned if specific health changes or symptoms develop, prompting the need for urgent, unscheduled testing. The most serious trigger is the appearance of signs of potential lithium toxicity, which include:
- Severe nausea.
- Persistent vomiting or diarrhea.
- A coarse hand tremor.
- Confusion.
- Lack of coordination.
Any significant, acute change in a patient’s health status also requires urgent monitoring because lithium is excreted through the kidneys. Conditions causing dehydration, such as a severe flu, stomach illness, or high fever, reduce the body’s ability to clear lithium. When less fluid is available, the lithium concentration in the blood rises quickly, raising the risk of toxicity.
Furthermore, starting or stopping certain medications necessitates an immediate level check, as they can alter how the body processes lithium. Common interacting drug classes include non-steroidal anti-inflammatory drugs (NSAIDs), diuretics, and some blood pressure medications like ACE inhibitors. In any of these acute situations, the patient must contact their healthcare provider right away, as testing may be required within 24 hours to prevent a medical emergency.
Interpreting the Test Results and Concurrent Checks
The results from a lithium blood test provide a snapshot of the drug concentration, measured in milliequivalents per liter (mEq/L) or millimoles per liter (mmol/L). The generally accepted therapeutic range for maintenance treatment is between 0.6 and 1.2 mEq/L. Maintaining levels within this window balances efficacy with minimizing adverse effects.
Levels above 1.5 mEq/L indicate a significant risk of intoxication, with severe toxicity likely at concentrations exceeding 2.0 mEq/L. Because lithium can affect other organ systems over time, concurrent checks of kidney and thyroid function are mandatory.
Long-term lithium use is associated with potential changes to these organs. Kidney function is checked using tests for creatinine and blood urea nitrogen (BUN), while thyroid function is checked via a thyroid-stimulating hormone (TSH) test. These concurrent function tests are typically performed every six months to a year, and are a mandatory part of the overall safety protocol.