What baseline and follow‑up laboratory tests should be ordered when initiating lithium therapy, and how often should they be repeated?

Blood Investigations After Starting Lithium

Before initiating lithium therapy, obtain complete blood count, thyroid function tests (TSH, free T4), renal function tests (blood urea nitrogen and creatinine), serum calcium, urinalysis, and pregnancy test in females of reproductive age. 1, 2

Baseline Laboratory Testing

Prior to starting lithium, the following tests are essential:

• Complete blood count (CBC) to establish baseline hematologic parameters 1, 2
• Thyroid function tests including TSH and free T4, as lithium commonly causes subclinical hypothyroidism 1, 2
• Renal function tests including blood urea nitrogen and creatinine, given lithium's exclusive renal excretion and potential nephrotoxicity 1, 2
• Serum calcium to detect baseline abnormalities, as lithium can affect calcium metabolism 1, 2
• Urinalysis to assess baseline kidney function 1, 2
• Pregnancy test in all females of reproductive age, as lithium crosses the placenta 1, 2

Serum Lithium Level Monitoring

During the acute stabilization phase, check serum lithium levels twice weekly until both serum levels and clinical condition stabilize. 1, 3

• Blood samples must be drawn 8-12 hours after the previous dose when lithium concentrations are relatively stable 1, 3
• The FDA label emphasizes that lithium toxicity is closely related to serum levels and can occur at doses close to therapeutic levels 3
• Target therapeutic range is 0.6-1.2 mEq/L for maintenance therapy, though acute mania may require 1.0-1.5 mEq/L 3
• Once stabilized on maintenance therapy, monitor serum lithium levels at least every two months 3

Ongoing Laboratory Monitoring Schedule

After stabilization, monitor renal function tests, thyroid function tests, and urinalysis every 3-6 months, and serum calcium at least annually. 1, 2

• Renal function tests (BUN, creatinine) and urinalysis every 3-6 months to detect lithium-induced nephrotoxicity, which can progress to chronic kidney disease 1, 2
• Thyroid function tests every 3-6 months as lithium commonly causes hypothyroidism requiring ongoing surveillance 1, 2
• Serum calcium at least annually to monitor for hyperparathyroidism and calcium dysregulation 1, 2

Increased Monitoring Frequency

Increase monitoring frequency during intercurrent illness, as this can significantly alter lithium clearance and increase toxicity risk. 1, 4

• Medications affecting renal function (particularly NSAIDs, which are contraindicated) can alter lithium levels 1, 4
• Temporarily discontinue lithium during serious intercurrent illness that increases acute kidney injury risk, especially in patients with GFR <60 mL/min/1.73 m² 1, 4
• Suspend lithium before planned IV radiocontrast administration, bowel preparation, or major surgery 2

Signs Requiring Immediate Testing

Early signs of lithium toxicity include tremor, nausea, diarrhea, and polyuria-polydipsia—these warrant immediate serum lithium testing regardless of the regular monitoring schedule. 1, 2, 4

• If toxicity is suspected, obtain immediate serum lithium level and renal function tests 4
• Elderly patients often exhibit toxicity at serum levels ordinarily tolerated by younger patients 3

Critical Monitoring Caveat

Total reliance must not be placed on serum levels alone—accurate patient evaluation requires both clinical assessment and laboratory analysis. 3 The narrow therapeutic index means facilities for prompt and accurate serum lithium determinations must be available before initiating therapy 3. Regular clinical supervision coupled with standardized laboratory monitoring is essential for safe lithium use 5, 6.