Lab Workup for Macrocytic Anemia
Begin with a complete blood count (CBC) with mean corpuscular volume (MCV), reticulocyte count, and peripheral blood smear to differentiate megaloblastic from nonmegaloblastic causes and assess bone marrow response. 1, 2
Initial Essential Tests
The minimum workup for macrocytic anemia (MCV >100 fL) should include:
- CBC with differential and MCV to confirm macrocytosis 1
- Reticulocyte count (corrected for anemia as reticulocyte index) to distinguish between decreased RBC production versus increased destruction/loss 1, 2
- Peripheral blood smear to identify megaloblastic features (macro-ovalocytes and hypersegmented neutrophils) versus nonmegaloblastic morphology 3, 4, 5
Algorithmic Approach Based on Peripheral Smear
If Megaloblastic Features Present (Macro-ovalocytes, Hypersegmented Neutrophils)
Order vitamin B12 and red blood cell folate levels immediately, as these are the most common causes of megaloblastic macrocytic anemia 1, 3, 4:
- Serum vitamin B12 level - deficiency is a primary cause of megaloblastic anemia 1, 3
- Red blood cell folate level (preferred over serum folate) - folate deficiency causes identical megaloblastic changes 1, 3, 4
Critical pitfall: Neutrophil hypersegmentation (≥5% of neutrophils with ≥5 lobes or any neutrophil with ≥6 lobes) is one of the most sensitive and specific signs of megaloblastic anemia 3. Do not miss this on smear review.
If Nonmegaloblastic Features Present
Check reticulocyte index to guide further testing 1, 2, 3:
Elevated Reticulocyte Index (RI >2.0)
Indicates increased RBC production, suggesting hemolysis or hemorrhage 1, 2, 3:
- Hemolysis workup: lactate dehydrogenase (LDH), haptoglobin, indirect bilirubin, direct antiglobulin test (Coombs), peripheral smear for schistocytes/spherocytes 1, 2
- Hemorrhage evaluation: stool guaiac, assess for clinical bleeding (melena, hematochezia, menorrhagia) 1, 2
Low or Normal Reticulocyte Index (RI <2.0)
Indicates decreased RBC production, requiring evaluation for 1, 2, 4:
- Thyroid function tests (TSH) - hypothyroidism is a common nonmegaloblastic cause 1, 3, 4
- Liver function tests (AST, ALT, bilirubin, albumin) - liver disease causes nonmegaloblastic macrocytosis 1, 3, 4
- Medication review - thiopurines (azathioprine, 6-mercaptopurine), hydroxyurea, methotrexate, anticonvulsants cause macrocytosis 1, 4
- Alcohol history - chronic alcohol use is the most common cause of nonmegaloblastic macrocytic anemia 1, 3, 4
Extended Workup for Unclear Cases
If the cause remains unclear after initial testing, obtain 1, 2:
- Soluble transferrin receptor and percentage of hypochromic red cells to evaluate for combined iron deficiency 1, 2
- Red cell distribution width (RDW) - elevated RDW suggests coexisting microcytosis and macrocytosis that neutralize each other 1, 2
- Bone marrow aspirate and biopsy with hematology consultation, particularly if myelodysplastic syndrome (MDS) is suspected in elderly patients with cytopenias 1, 2, 5
Critical Pitfalls to Avoid
Do not give folic acid empirically without checking B12 levels first - doses of folic acid >0.1 mg daily may correct the anemia but allow irreversible neurologic damage from B12 deficiency to progress 6, 7. This is particularly dangerous in pernicious anemia, where subacute combined degeneration of the spinal cord can become permanent if B12 deficiency persists beyond 3 months 6.
Combined deficiencies can mask each other - coexisting iron deficiency and B12/folate deficiency may result in a falsely normal MCV, with only an elevated RDW as a clue 1, 2. Always check iron studies (ferritin, transferrin saturation) even in macrocytic anemia 1.
Inflammatory conditions falsely elevate ferritin - use transferrin saturation <15% as the primary indicator of iron deficiency when inflammation is present 1, 2.
Additional Considerations for Specific Populations
In patients with inflammatory bowel disease, serum ferritin <30 μg/L indicates iron deficiency without inflammation, but ferritin up to 100 μg/L may still represent iron deficiency when inflammation is present 1.
In cancer patients receiving chemotherapy, evaluate for myelosuppression as a cause of macrocytosis, and consider bone marrow biopsy if MDS is suspected 1, 5.