How to Measure Lithium Levels
Serum lithium concentrations should be measured as trough levels drawn 12 hours after the last lithium dose (typically in the morning before the first daily dose), using blood samples sent to a laboratory with facilities for prompt and accurate serum lithium determinations. 1
Timing of Blood Draw
The critical element is the 12-hour post-dose timing. Blood samples must be drawn immediately prior to the next dose when lithium concentrations are relatively stable (8-12 hours after the previous dose) 1. This standardized timing allows for:
- Consistent interpretation of therapeutic ranges
- Accurate comparison across serial measurements
- Proper assessment of steady-state concentrations
Frequency of Monitoring
During Acute Phase
Serum concentrations should be determined twice per week during the acute treatment phase and until both serum levels and clinical condition have stabilized 2, 1. This intensive monitoring is essential because lithium toxicity is closely related to serum concentrations and can occur at doses close to therapeutic levels 2.
During Maintenance Therapy
Once stabilized, serum lithium levels should be monitored at least every 2 months in uncomplicated cases receiving maintenance therapy during remission 1. For children and adolescents, monitoring should occur every 3-6 months along with renal and thyroid function tests 3.
Sample Collection and Handling
Standard venous blood samples are used, though emerging evidence supports alternative methods:
- Venous serum/plasma: The gold standard method 1
- Dried blood spots: Recent research demonstrates volumetric dried blood spots can reliably measure lithium with a blood spot/serum ratio of 0.78 and strong correlation (R=0.95) to traditional serum measurements 4
- Capillary dried plasma spots: Show 93-97% extraction yields with minimal hematocrit effects when properly corrected 5
Laboratory Methods
Lithium is measured using:
- Atomic absorption spectroscopy (most common clinical method) 6, 7
- Ion-selective electrodes on automated clinical analyzers 5
- Isotope dilution techniques for research applications 6
The detection limits are excellent (as low as 0.6 μg/L in research settings 8), well below therapeutic ranges.
Critical Pitfalls to Avoid
Do not rely solely on serum levels—accurate patient evaluation requires both clinical assessment and laboratory analysis 1. Key warnings include:
- Patients abnormally sensitive to lithium may exhibit toxic signs at levels of 1.0-1.5 mEq/L, which are typically therapeutic 1
- Elderly patients often respond to reduced dosages and may show toxicity at levels ordinarily tolerated by others 1
- Early signs of toxicity (diarrhea, vomiting, drowsiness, muscular weakness, lack of coordination) can occur at levels below 2.0 mEq/L 1
Age-Specific Therapeutic Ranges
Recent international consensus recommends lower therapeutic ranges for older adults 9:
- Ages 60-79 years: 0.4-0.8 mmol/L
- Ages 80+ years: 0.4-0.7 mmol/L
- Standard adults: 0.6-1.2 mEq/L for maintenance 1
When Timing Is Non-Standard
If blood cannot be drawn at exactly 12 hours post-dose, a validated equation (eLi12) can estimate the 12-hour level from measurements taken 3-24 hours after the last dose, with 97% accuracy showing closer approximation to true 12-hour levels than uncorrected values 10. This provides flexibility for patients while maintaining clinical accuracy.
Baseline and Ongoing Laboratory Assessment
Before initiating lithium, obtain 3:
- Complete blood count
- Thyroid function tests (TSH, T3, T4)
- Urinalysis
- Blood urea nitrogen and creatinine
- Serum calcium
- Pregnancy test in females of childbearing age
These should be repeated every 3-6 months during maintenance therapy 3.