What is the differential diagnosis and work-up for macrocytic anemia?

Differential Diagnosis and Work-up for Macrocytic Anemia

The initial evaluation of macrocytic anemia (MCV >100 fL) should include testing for vitamin B12 and folate deficiencies, liver function, thyroid function, alcohol use assessment, medication review, and peripheral blood smear examination to distinguish between megaloblastic and non-megaloblastic causes. 1, 2

Classification and Differential Diagnosis

Megaloblastic Causes

• Vitamin B12 deficiency
  • Pernicious anemia
  • Malabsorption (celiac disease, Crohn's disease, gastric surgery)
  • Dietary deficiency (strict vegans)
  • Bacterial overgrowth
• Folate deficiency
  • Dietary deficiency
  • Malabsorption
  • Increased requirements (pregnancy, hemolysis)
  • Medications (anticonvulsants, methotrexate)
• Medications affecting DNA synthesis
  • Chemotherapeutic agents
  • Hydroxyurea
  • Azathioprine

Non-Megaloblastic Causes

• Alcohol use disorder
• Liver disease
• Hypothyroidism
• Myelodysplastic syndrome (MDS)
• Hemolysis or hemorrhage (reticulocytosis)
• Medications (antiretrovirals, immunosuppressants)
• Aplastic anemia
• Multiple myeloma
• Inherited disorders of DNA synthesis (rare) 3, 4, 5

Diagnostic Work-up Algorithm

Step 1: Initial Laboratory Testing

• Complete Blood Count (CBC) with peripheral smear
• Reticulocyte count
• Vitamin B12 and folate levels
• Liver function tests
• Thyroid function tests (TSH, free T4)
• Basic metabolic panel

Step 2: Peripheral Smear Interpretation

• Megaloblastic pattern:

  • Macro-ovalocytes
  • Hypersegmented neutrophils (>5 lobes)
  • Suggests vitamin B12 or folate deficiency

• Non-megaloblastic pattern:

  • Round macrocytes
  • Normal neutrophil segmentation
  • Suggests liver disease, alcohol use, medications, or MDS 2, 5

Step 3: Reticulocyte Count Interpretation

• Elevated reticulocyte count:

  • Suggests hemolysis or recent hemorrhage
  • Reticulocytes are larger than mature RBCs, causing macrocytosis

• Normal or low reticulocyte count:

  • Suggests vitamin deficiency, bone marrow disorder, medication effect, or hypothyroidism 6, 4

Step 4: Further Testing Based on Initial Results

• If B12 deficiency is suspected:

  • Methylmalonic acid and homocysteine levels (both elevated in B12 deficiency)
  • Anti-intrinsic factor antibodies and anti-parietal cell antibodies (for pernicious anemia)
  • Upper endoscopy (if malabsorption suspected)

• If folate deficiency is suspected:

  • Homocysteine level (elevated in folate deficiency)
  • RBC folate level (more reliable than serum folate)

• If myelodysplastic syndrome is suspected:

  • Bone marrow aspiration and biopsy with cytogenetic analysis
  • Especially important with unexplained cytopenias in other cell lines 1, 4

Common Pitfalls and Caveats

• Mixed anemias: Patients may have both macrocytic and microcytic RBC populations, masking macrocytosis by producing a normal MCV. Review the RBC distribution width (RDW) and peripheral smear.

• Functional B12 deficiency: Normal serum B12 levels don't exclude deficiency; methylmalonic acid and homocysteine may be needed for confirmation.

• Overlooking medications: Many commonly prescribed medications can cause macrocytosis, including anticonvulsants, methotrexate, and antiretrovirals.

• Accepting anemia without thorough investigation: Particularly in elderly patients, this can miss serious underlying pathology like MDS or malignancy. 1

• Failing to evaluate for other nutrient deficiencies: Patients with one deficiency often have others, particularly in malabsorptive conditions.

• Overlooking alcohol use: Even moderate alcohol consumption can cause macrocytosis without anemia, often preceding the development of anemia. 3, 4

When macrocytic anemia is accompanied by abnormalities in other cell lines (leukopenia, thrombocytopenia), prompt hematology consultation is warranted as this raises suspicion for myelodysplastic syndrome, particularly in older adults. 3, 5