Megaloblastic anemias are a group of macrocytic anemias caused by impaired DNA synthesis due to deficiency of vitamin B12 (cobalamin) or folate. The impaired nuclear maturation affects all rapidly dividing cells, most notably the bone marrow, but also epithelial surfaces. Recognition through characteristic CBC parameters and peripheral blood smear findings is critical for timely treatment.
Vitamin B12 Metabolism
Vitamin B12 (cobalamin) is a water-soluble vitamin found exclusively in animal products (meat, fish, eggs, dairy). The recommended daily intake is 2.4 µg. Absorption requires: gastric acid and pepsin to release B12 from food proteins, binding to haptocorrin (R-binder) in the stomach, pancreatic protease digestion in the duodenum releasing B12, and binding to intrinsic factor (IF) produced by gastric parietal cells. The IF-B12 complex is absorbed in the terminal ileum via cubilin receptor-mediated endocytosis. B12 is transported in plasma bound to transcobalamin II (holotranscobalamin, the metabolically active fraction). Total body stores (2–5 mg) are primarily hepatic, sufficient for 3–5 years, explaining the long latency of deficiency after dietary or absorption changes.
Folate Metabolism
Folate (vitamin B9) is present in green leafy vegetables, legumes, fruits, and fortified grains. The recommended daily intake is 400 µg (600 µg in pregnancy). Folate is absorbed in the proximal jejunum and converted to its active form, tetrahydrofolate (THF), which is essential for one-carbon transfer reactions including DNA synthesis (thymidylate synthesis) and amino acid metabolism. Unlike B12, body stores are limited (5–20 mg, sufficient for 2–4 months), so deficiency develops more rapidly. Folate is heat-labile and destroyed by prolonged cooking.
Causes of B12 Deficiency
Pernicious anemia is the most common cause of clinically significant B12 deficiency, an autoimmune condition with antibodies against gastric parietal cells (anti-H⁺/K⁺-ATPase, 90% of patients) and/or intrinsic factor (50–70% of patients). It causes atrophic gastritis, achlorhydria, and loss of IF production, leading to malabsorption. Other causes include gastrectomy, gastric bypass surgery, Crohn disease affecting the terminal ileum, ileal resection, pancreatic insufficiency, bacterial overgrowth (competition for B12), Diphyllobothrium latum (fish tapeworm) infestation, chronic proton pump inhibitor use (reduced gastric acid), dietary deficiency (vegan/vegetarian, uncommon but important), and nitrous oxide abuse (inactivates B12 via cobalt oxidation). Aging reduces gastric acid production and B12 absorption.
Causes of Folate Deficiency
Dietary deficiency (inadequate intake) is the most common cause, especially in the elderly, alcohol use disorder, fad diets, and low-resource settings. Increased requirements occur in pregnancy (neural tube defects), lactation, hemolytic anemias (chronic RBC production), and exfoliative skin disorders. Malabsorption occurs in celiac disease, tropical sprue, Crohn disease, and short bowel syndrome. Drugs: methotrexate (dihydrofolate reductase inhibitor), trimethoprim, phenytoin, sulfasalazine, and oral contraceptives can interfere with folate metabolism or absorption. Alcohol inhibits folate absorption and accelerates its excretion.
Laboratory Findings
The CBC shows macrocytic anemia (MCV typically 100–140 fL), often with moderate to severe anemia (Hb 6–10 g/dL). RDW is elevated due to anisocytosis (mixed macro-ovalocytes and normal cells). The reticulocyte count is low (RPI < 2), indicating ineffective erythropoiesis. The peripheral blood smear is diagnostic: macro-ovalocytes (large, oval RBCs), hypersegmented neutrophils (≥ 5 lobes, some with 6+ lobes), and in severe cases, Howell-Jolly bodies, basophilic stippling, and Cabot rings. Pancytopenia (low WBC and platelets in addition to anemia) may occur in advanced cases. The white blood cell differential may show hypersegmented neutrophils, and giant metamyelocytes may be seen.
Vitamin B12 and Folate Assays
Serum B12 is measured by immunoassay. A level < 200 pg/mL (< 148 pmol/L) is consistent with deficiency, but the test has limited sensitivity and specificity. Levels between 200–400 pg/mL are borderline and require reflex testing. Serum folate (< 4 ng/mL) indicates deficiency, but red blood cell folate (< 150 ng/mL) is a more reliable indicator of chronic folate status. Methylmalonic acid (MMA) is the most sensitive and specific marker for B12 deficiency: elevated serum MMA (> 0.4 µmol/L) indicates cellular B12 deficiency. Homocysteine is elevated in both B12 (> 15 µmol/L) and folate deficiency. MMA and homocysteine are the gold standard tests for confirming B12 deficiency when serum B12 is borderline.
Bone Marrow Findings
Bone marrow aspiration shows classic megaloblastic changes: erythroid hyperplasia with nuclear-cytoplasmic dyssynchrony (mature hemoglobinized cytoplasm with immature, open “washed-wool” nuclear chromatin), giant metamyelocytes and band forms (large WBC precursors), and large megakaryocytes with hypersegmented nuclei. These changes reverse rapidly after appropriate vitamin therapy.
Treatment
B12 deficiency: intramuscular hydroxocobalamin (cyanocobalamin) 1000 µg daily for 1 week, then weekly for 4 weeks, then monthly for life in pernicious anemia. High-dose oral B12 (1000 µg daily) is also effective for many patients with mild to moderate deficiency, as 1% is absorbed by passive diffusion. Folate deficiency: oral folic acid 1–5 mg daily. Before treating with folate alone, B12 deficiency must be excluded — folate therapy can correct the hematologic abnormalities of B12 deficiency while allowing neurologic deterioration to progress (subacute combined degeneration of the spinal cord). Response monitoring: reticulocytosis (peak at 5–10 days), rising hemoglobin, and normalization of MCV over 4–8 weeks. Hypersegmented neutrophils disappear within 2 weeks.
