Does a patient need to fast before a lithium level lab test?

Lithium Lab Testing: No Fasting Required

Patients do not need to fast before having their lithium levels drawn. The critical factor for accurate lithium monitoring is the timing of the blood draw relative to the last dose, not fasting status 1.

Proper Timing for Lithium Level Testing

The FDA drug label for lithium clearly states that blood samples for serum lithium determination should be drawn immediately prior to the next dose when lithium concentrations are relatively stable (8-12 hours after the previous dose) 1. This timing is crucial for accurate monitoring:

• For multiple daily doses: Draw blood 12 hours after the last dose
• For once-daily dosing: Draw blood 24 hours after the last dose 2

This timing difference is important because serum lithium levels at 12 hours can be approximately 1.3 times higher than at 24 hours in patients taking once-daily dosing 2.

Frequency of Monitoring

Lithium monitoring frequency should follow this schedule:

• Acute phase: Twice per week until serum level and clinical condition stabilize 1
• Maintenance phase: At least every two months for uncomplicated cases 1
• Elderly patients: May require more frequent monitoring as they often respond to reduced dosage and may exhibit toxicity at levels normally tolerated by others 1

Therapeutic Range and Clinical Considerations

The therapeutic range for lithium is narrow:

• Acute mania: 1.0-1.5 mEq/L 1
• Long-term maintenance: 0.6-1.2 mEq/L 1

Some patients may exhibit toxicity at levels as low as 1.0 mEq/L, particularly elderly patients 1. This underscores the importance of monitoring both serum levels and clinical response.

Common Pitfalls to Avoid

1. Incorrect timing of blood draws: Sampling at random times rather than at trough levels leads to inaccurate results 3
2. Inadequate monitoring frequency: Studies show that approximately 25% of providers don't check lithium levels as frequently as recommended 4
3. Failure to monitor kidney and thyroid function: Lithium can affect these organs, so creatinine and TSH should be monitored alongside lithium levels 4
4. Ignoring elevated levels: Research shows that about one-third of doctors make no response to elevated lithium concentrations within six weeks 5
5. Assuming fasting is required: Unlike some lab tests, lithium levels do not require fasting

Best Practice Recommendations

1. Schedule lithium level draws at the appropriate trough time (12 or 24 hours after last dose)
2. Maintain consistent timing for all follow-up tests
3. Ensure regular monitoring of kidney and thyroid function
4. Adjust dosage based on both serum levels and clinical response
5. Document both the time of last dose and time of blood draw

Remember that while serum lithium levels are essential for monitoring, clinical evaluation remains equally important, as noted in the FDA label: "Total reliance must not be placed on serum levels alone. Accurate patient evaluation requires both clinical and laboratory analysis" 1.