CBC with Differential at Lithium Initiation
Baseline complete blood count with differential is recommended before initiating lithium therapy, regardless of patient history. 1
Baseline Laboratory Requirements
The American Academy of Child and Adolescent Psychiatry establishes that baseline assessment before lithium initiation must include: 1
- Complete blood cell counts with differential
- Thyroid function tests (TSH, free T4)
- Blood urea nitrogen (BUN)
- Serum creatinine
- Electrolytes
This baseline CBC with differential requirement applies universally to all patients starting lithium, not selectively based on history. 1
Rationale for Universal CBC with Differential
Lithium causes predictable hematologic effects that require baseline documentation:
- Lithium induces leukocytosis in the majority of patients, with mean increases of approximately 2,237 cells/mm³ above baseline. 2
- This leukocytosis is drug-related but not dose-dependent or concentration-dependent. 2
- Without baseline values, distinguishing lithium-induced leukocytosis from pathologic processes (infection, leukemia) becomes impossible clinically. 2
The differential count specifically helps identify the neutrophil predominance pattern typical of lithium (rather than left shift suggesting infection or blast cells suggesting malignancy). 3
Special Considerations for High-Risk Patients
Patients with impaired renal function (eGFR <60 mL/min/1.73 m²) require the same baseline CBC with differential, plus intensified ongoing monitoring: 1
- Standard monitoring frequency increases from every 3-6 months to monthly when eGFR <30 mL/min/1.73 m². 1
- Renal impairment affects lithium clearance but does not change the need for baseline hematologic assessment. 4, 5
Patients with history of blood disorders warrant baseline CBC with differential to:
- Document pre-existing cytopenias that might be incorrectly attributed to lithium later
- Identify contraindications to therapy (severe baseline neutropenia, thrombocytopenia)
- Establish individualized monitoring thresholds 6
Ongoing Monitoring Schedule
After baseline assessment, the American Academy of Child and Adolescent Psychiatry recommends: 1
- During acute phase: Check lithium levels twice weekly until stable (CBC monitoring frequency not specified for acute phase but clinical judgment applies)
- Maintenance therapy: Monitor lithium levels, renal function, and thyroid function every 3-6 months
- After dose adjustments or clinical changes: Increase monitoring to every 1-2 weeks 1
Common Pitfall to Avoid
Do not skip baseline CBC with differential even in apparently healthy patients. The leukocytosis induced by lithium is so consistent that without baseline values, you cannot distinguish normal lithium effect from emerging pathology during treatment. 2 This is particularly critical since lithium patients require decades of treatment, during which time comparison to baseline becomes essential for detecting true hematologic complications. 4, 7