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PA15.1-3 | Megaloblastic Anaemia — Aetiology, Blood Picture & Diagnostic Framework — Part 1

The First Fork: Megaloblastic vs Non-Megaloblastic Macrocytosis

Classification of Macrocytic Anemias: Megaloblastic vs Non-Megaloblastic

Not every large red cell is megaloblastic. The term megaloblastic macrocytosis is reserved specifically for macrocytosis caused by impaired DNA synthesis, producing a distinct morphological signature. Non-megaloblastic macrocytosis produces large cells by a different mechanism (see below).

Megaloblastic causes (impaired DNA synthesis)
• B12 deficiency
• Folate deficiency
• Drugs that block DNA synthesis (methotrexate, hydroxyurea, anticonvulsants — discussed in detail below)
• Inborn errors of B12/folate metabolism (rare)

Non-megaloblastic causes (impaired DNA synthesis is NOT the mechanism)
• Alcohol — direct membrane effect on RBC precursors; also round macrocytes, not oval
• Liver disease — excess cholesterol in RBC membrane → target cells + macrocytosis
• Hypothyroidism — reduced erythropoietin + altered lipid metabolism
• Reticulocytosis — young reticulocytes are larger than mature RBCs; seen in haemolysis/haemorrhage
• Myelodysplastic syndrome (MDS) — dyserythropoiesis, not B12/folate block

Distinguishing clue on the smear: In megaloblastic anaemia the macrocytes are macro-ovalocytes (oval, large cells). In non-megaloblastic causes they are round macrocytes. Hypersegmented neutrophils are the other gold-standard feature — present only in megaloblastic disease.

Aetiology of Megaloblastic Anaemia

Aetiology of Megaloblastic Anaemia

I. Vitamin B12 Deficiency

1. Inadequate intake: Strict vegetarians/vegans (most common cause in India) — B12 is absent from plant foods
2. Malabsorption — gastric:
• Pernicious anaemia (PA) — autoimmune destruction of gastric parietal cells → absent intrinsic factor (IF) → B12 not absorbed in terminal ileum; anti-parietal cell antibodies in 90%, anti-IF antibodies (more specific) in 60%
• Post-gastrectomy — loss of parietal cells
3. Malabsorption — intestinal:
• Terminal ileal disease or resection (Crohn's disease, TB ileitis)
• Blind loop syndrome — bacterial overgrowth consumes B12
• Fish tapeworm (Diphyllobothrium latum) — rare in India
4. Increased demand: Pregnancy, hyperthyroidism

II. Folate Deficiency

1. Inadequate intake: Poverty, food faddism, hyperemesis, old age — folate stores last only 3–4 months (cf. B12 stores last 3–4 years)
2. Malabsorption: Coeliac disease, tropical sprue, extensive small bowel resection
3. Increased demand: Pregnancy (commonest cause), haemolytic anaemia, exfoliative dermatitis, dialysis
4. Impaired utilisation: Methotrexate (see Drug section below)

III. Drug-Induced Megaloblastic Anaemia

• Methotrexate (MTX) — dihydrofolate reductase (DHFR) inhibitor: blocks conversion of dihydrofolate → tetrahydrofolate; mimics folate deficiency; responds to folinic acid (leucovorin rescue), not folic acid
• Hydroxyurea — ribonucleotide reductase inhibitor; blocks deoxyribonucleotide synthesis directly; used in chronic myeloid leukaemia and sickle cell disease
• Anticonvulsants (phenytoin, phenobarbitone, primidone) — impair folate absorption and increase folate catabolism
• Trimethoprim (weak DHFR inhibitor), pyrimethamine, co-trimoxazole
• Azathioprine, 6-MP, 5-FU — purine/pyrimidine synthesis inhibitors; cause megaloblastic change via nucleotide pool depletion

IV. Inborn Errors of B12 and Folate Metabolism (Rare)

Transcobalamin II deficiency, hereditary folate malabsorption, methylenetetrahydrofolate reductase (MTHFR) mutations — typically present in infancy or early childhood with severe megaloblastic anaemia unresponsive to diet alone.

Full Blood Picture in Megaloblastic Anaemia

Full Blood Picture in Megaloblastic Anaemia

MCV elevation:
MCV is the entry point. In megaloblastic anaemia MCV is typically markedly elevated — often 110–130 fL, sometimes exceeding 140 fL in severe cases (normal 80–100 fL). Values of 100–110 fL suggest early or mixed deficiency.

Peripheral smear findings (the diagnostic signature):

1. Macro-ovalocytes — large, oval (egg-shaped) red cells; the shape distinguishes them from the round macrocytes of non-megaloblastic causes. Prominent anisocytosis and poikilocytosis accompany them.

1. Hypersegmented neutrophils — the most specific finding. A neutrophil normally has 2–4 lobes. Define the 5-lobe rule: the presence of ≥1 neutrophil with 5 or more lobes (or ≥5% of neutrophils with ≥5 lobes, or any neutrophil with ≥6 lobes) is pathognomonic of megaloblastic anaemia. Hypersegmentation precedes anaemia and persists after treatment begins — it is an early and late marker.

1. Pancytopenia in severe cases — DNA synthesis failure affects all marrow lineages. Red cell count ↓ (anaemia), WBC ↓ (leucopenia), platelets ↓ (thrombocytopenia). This combination in a macrocytic patient virtually clinches megaloblastic disease.

Biochemical markers (CBC supplementary):

• Serum LDH (lactate dehydrogenase) — markedly raised: Intramedullary destruction of abnormal megaloblasts (ineffective erythropoiesis) releases massive amounts of LDH. Values may exceed 5× upper limit of normal.
• Indirect (unconjugated) bilirubin — mildly raised: Haem released from destroyed megaloblasts is catabolised to bilirubin. Jaundice is typically mild (lemon-yellow tinge — classic description).
• Serum iron — raised: Reduced erythroid consumption of iron (cells die in marrow before leaving).
• Reticulocyte count — low (inappropriately low): The marrow is ineffective — cells die before maturation. A low reticulocyte count with severe anaemia is the hallmark of ineffective erythropoiesis. Contrast haemolytic anaemia: high reticulocyte count.

IMPORTANT NOTE: The LDH + indirect bilirubin + low reticulocyte triad = ineffective erythropoiesis — the key pathophysiological concept linking the morphology to the biochemistry.

Morphological Features of Megaloblastic Anemia

Megaloblastic vs Normal Bone Marrow: Nuclear-Cytoplasmic Asynchrony