What is the best course of action for a patient with hypercobalaminemia (elevated vitamin B12 level), a history of leukocytosis, and a psychiatric condition, who is potentially taking medications such as lithium?

Management of Hypercobalaminemia in a Psychiatric Patient with Leukocytosis

Immediate Action Plan

Your plan to repeat the B12 level next week and consult a hospitalist if it remains elevated is appropriate, but you should simultaneously evaluate for underlying serious conditions that commonly cause hypercobalaminemia, particularly hematologic malignancies and solid tumors. 1

Persistently elevated B12 levels (>1,000 pg/mL on two measurements) have been associated with solid tumors, hematologic malignancy, and increased risk of cardiovascular death 1. Given your patient's history of leukocytosis, this warrants urgent attention.

Critical Differential Diagnosis to Consider

Most Likely Causes in Your Patient

First, verify medication and supplement history:

• Review ALL supplements including multivitamins, as patients often don't recognize B12 content 2
• Lithium itself causes leukocytosis 3, which may explain the elevated white blood cell count independent of any hematologic malignancy
• Lithium is specifically listed as a medication commonly associated with leukocytosis 3

Second, rule out serious hematologic conditions:

• The combination of leukocytosis and elevated B12 is characteristic of chronic myelogenous leukemia (CML) 4
• Polycythemia vera presents with leukocytosis, thrombocytosis, and increased serum vitamin B12 level 5
• Primary bone marrow disorders should be suspected when patients present with extremely elevated white blood cell counts or concurrent abnormalities in red blood cell or platelet counts 3

Key Clinical Features to Assess

When you repeat labs next week, obtain:

• Complete blood count with differential to evaluate all three cell lines 3
• Peripheral blood smear to look for immature cells or abnormal morphology 4
• Active B12 (holotranscobalamin) provides better assessment of biologically available B12 than total serum B12 in complex cases 2

Physical examination should specifically assess for:

• Splenomegaly (characteristic of CML and polycythemia vera) 5, 4
• Hepatomegaly or lymphadenopathy 3
• Weight loss, bleeding, or bruising 3

When to Consult Hospitalist/Hematology

Immediate consultation is warranted if:

• White blood cell count >100,000/mm³ (medical emergency due to risk of brain infarction and hemorrhage) 3
• Concurrent abnormalities in red blood cells or platelets suggesting bone marrow disorder 3
• Splenomegaly, hepatomegaly, or lymphadenopathy on examination 3
• Weight loss, bleeding, or bruising 3

Routine consultation after repeat labs if:

• B12 remains >1,000 pg/mL on second measurement 1
• Leukocytosis persists and cannot be explained by lithium alone 3
• Any new cytopenias develop 3

Special Considerations for Psychiatric Patients

B12 deficiency (not elevation) can cause psychiatric symptoms:

• Severe depression, mania, psychosis, and cognitive impairment can occur with B12 deficiency 6, 7
• However, your patient has elevated B12, which is a different clinical scenario 1
• Post-bariatric surgery patients and those with autoimmune conditions may have altered B12 metabolism 2

Lithium-specific considerations:

• Lithium commonly causes leukocytosis as a benign side effect 3
• This may be the sole explanation for the elevated white count if it's mild and chronic 3
• However, don't assume lithium is the cause without ruling out serious pathology 3

Common Pitfalls to Avoid

• Don't assume elevated B12 is benign just because the patient takes supplements - persistently elevated levels warrant malignancy workup 1
• Don't attribute leukocytosis solely to lithium without evaluating the complete blood count and differential 3
• Don't delay hematology referral if white count is extremely elevated (>100,000/mm³) - this is a medical emergency 3
• Don't confuse B12 elevation with B12 deficiency - metformin causes deficiency, not elevation 2

Practical Algorithm

1. Next week labs: CBC with differential, repeat B12, consider active B12 (holotranscobalamin) 2, 3
2. If B12 >1,000 pg/mL on repeat AND leukocytosis persists: Consult hospitalist/hematology for malignancy workup 1
3. If WBC >100,000/mm³: Emergency hematology consultation 3
4. If splenomegaly, hepatomegaly, lymphadenopathy, or constitutional symptoms: Urgent hematology referral 3
5. If B12 normalizes and leukocytosis is mild/stable: Likely lithium effect, continue monitoring 3