What is the initial workup and treatment for a patient with macrocytic (large cell) anemia?

Macrocytic Anemia Workup

Begin with a CBC to confirm macrocytosis (MCV >100 fL), then immediately obtain a peripheral blood smear, reticulocyte count, vitamin B12, folate, and TSH to differentiate between megaloblastic and non-megaloblastic causes. 1, 2

Initial Laboratory Assessment

Essential First-Line Tests

• CBC with indices: Confirms MCV >100 fL and identifies other cytopenias 1, 2
• Peripheral blood smear: Critical to distinguish megaloblastic (macro-ovalocytes, hypersegmented neutrophils with ≥5 lobes) from non-megaloblastic causes 1, 3, 4
• Reticulocyte count/index: Differentiates between production defects (low reticulocyte index <2.0) versus hemolysis/hemorrhage (elevated reticulocyte index) 1, 2, 3
• Vitamin B12 level: Most common cause of megaloblastic anemia; however, be aware that automated analyzers may give falsely normal results in the presence of anti-intrinsic factor antibodies 5, 3, 4
• Folate level: Serum folate <10 nmol/L (4.4 μg/L) or RBC folate <305 nmol/L (<140 mg/L) indicates deficiency 1
• TSH (and free T4 if TSH abnormal): Hypothyroidism is a common non-megaloblastic cause 1, 3

Additional Targeted Tests Based on Clinical Context

• Liver function tests: Evaluate for chronic liver disease as a non-megaloblastic cause 1, 6, 3
• Creatinine: Assess for renal insufficiency contributing to anemia 1
• Methylmalonic acid (MMA): If B12 level is 150-400 pmol/L and clinical suspicion remains high; MMA >271 nmol/L supports B12 deficiency even with "normal" B12 1, 5

Diagnostic Algorithm by Peripheral Smear Findings

Megaloblastic Pattern (Macro-ovalocytes + Hypersegmented Neutrophils)

Most likely causes: Vitamin B12 or folate deficiency 1, 3, 4

If Vitamin B12 Deficiency Confirmed (<150 pmol/L or <203 ng/L):

• Anti-intrinsic factor antibodies: Most specific test for pernicious anemia 5, 7
• Anti-parietal cell antibodies: Less specific but supportive if positive 7
• Consider trial of mecobalamin/cyanocobalamin even if B12 appears "normal" when morphology is classic for megaloblastic anemia, as false-normal results occur 5

If Folate Deficiency:

• Investigate underlying cause: malabsorption, dietary insufficiency, increased demand (pregnancy), or medications (methotrexate, phenytoin) 1

Critical Pitfall to Avoid:

Never give folic acid >0.1 mg/day without first ruling out B12 deficiency, as folic acid can correct the anemia but allow irreversible neurologic damage from B12 deficiency to progress 8

Non-Megaloblastic Pattern (Normal Smear or Absent Megaloblastic Features)

Evaluate reticulocyte count to guide next steps 1, 3

Elevated Reticulocyte Count (Reticulocyte Index >2.0):

• Hemolysis workup: LDH, haptoglobin, indirect bilirubin, direct antiglobulin test 1
• Assess for acute blood loss: History, stool guaiac, imaging as indicated 1

Low/Normal Reticulocyte Count (Reticulocyte Index <2.0):

• Alcohol use history: Most common cause of non-megaloblastic macrocytosis 6, 3, 4
• Medication review: Hydroxyurea, azathioprine, zidovudine, chemotherapy agents 1
• Liver disease markers: AST, ALT, bilirubin, albumin 6, 3
• Hypothyroidism: TSH already obtained in initial workup 1, 3
• Bone marrow aspiration and biopsy with cytogenetics: Required if myelodysplastic syndrome (MDS) suspected, particularly in elderly patients with unexplained macrocytosis, other cytopenias, or dysplastic features on smear 1, 6

When to Perform Bone Marrow Examination

Obtain bone marrow aspiration and biopsy with cytogenetics when: 1, 6

• Unexplained macrocytosis persists after excluding common causes
• Multiple cytopenias present
• Peripheral smear shows dysplastic features beyond megaloblastic changes
• Suspected MDS or myeloproliferative disorder
• Megaloblastic changes present but B12/folate levels are normal and patient fails empiric treatment 5

Common Diagnostic Pitfalls

• Relying solely on serum B12 levels: Automated analyzers can produce false-normal or false-elevated results in pernicious anemia due to anti-intrinsic factor antibodies; if morphology is classic, proceed with trial treatment 5
• Missing mixed deficiencies: Concurrent iron deficiency can normalize MCV in B12/folate deficiency; check RDW (elevated suggests mixed picture) and iron studies 2
• Overlooking medication causes: Many drugs cause macrocytosis without anemia (e.g., thiopurines, antiretrovirals) 1
• Failing to assess for pernicious anemia: Patients require lifelong monthly B12 injections and have 3-times increased risk of gastric carcinoma requiring surveillance 8
• Treating with folic acid before excluding B12 deficiency: This masks anemia while allowing irreversible spinal cord degeneration 8

Special Considerations

• Inflammatory states: Ferritin 30-100 μg/L with inflammation may indicate coexisting iron deficiency requiring treatment before full response to B12/folate therapy 1, 2
• Pregnancy and lactation: B12 requirements increase to 4 mcg daily; deficiency in vegetarian mothers can cause severe deficiency in breastfed infants even without maternal symptoms 8
• Chronic kidney disease (GFR <30 mL/min/1.73 m²): Check hemoglobin every 3 months; if <12 g/dL (women) or <13 g/dL (men), perform complete anemia workup including iron studies before attributing to renal disease 1