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PA15.1-3 | Megaloblastic Anaemia — Aetiology, Blood Picture & Diagnostic Framework — Part 2

Bone Marrow Findings in Megaloblastic Anaemia

Bone Marrow Findings in Megaloblastic Anaemia

Bone marrow examination is rarely needed for diagnosis today (B12/folate assays are definitive), but the morphology is classic and examination-essential.

Macroscopic (aspirate): Hypercellular marrow — all three lineages expanded but abnormal.

Erythroid lineage — megaloblastic change:
• Megaloblasts — erythroid precursors that are larger than normal normoblasts. The defining feature is nuclear-cytoplasmic (N:C) asynchrony: cytoplasm haemoglobinises normally (matures on schedule) but the nucleus remains large, open-faced, and immature (lacy, fine chromatin with visible nucleoli) because DNA synthesis is stalled. Normal normoblasts show the reverse — nucleus condenses ahead of cytoplasm.
• As megaloblasts attempt to divide, their large nuclei fragment → giant bands and metamyelocytes (see myeloid below).

Myeloid lineage:
• Giant metamyelocytes and band forms — horseshoe-shaped giant cells; these are the precursors of the hypersegmented neutrophils seen in the peripheral blood. Pathognomonic of megaloblastic haemopoiesis.

Megakaryocytes:
• Hypersegmented, large, multilobulated megakaryocytes — mirror the same DNA synthesis defect.

Summary mnemonic — MARROW TRIAD: Megaloblasts (N:C asynchrony) | Giant metamyelocytes | Hypersegmented megakaryocytes.

Differentiating B12 from Folate Deficiency — The Diagnostic Algorithm

Diagnostic Algorithm: Differentiating B12 from Folate Deficiency

The peripheral smear and bone marrow cannot distinguish B12 from folate deficiency — both produce identical megaloblastic morphology. You need specific assays.

Step 1 — Serum assays:

Test	B12 deficiency	Folate deficiency
Serum B12	↓ (<200 pg/mL)	Normal or ↑
Serum folate	Normal	↓ (<3 ng/mL)
RBC folate	Normal or ↓	↓↓ (more reliable than serum folate)

RBC folate reflects folate status over the prior 3 months (lifetime of the RBC) — more stable than serum folate, which fluctuates with a single meal.

Step 2 — Metabolite assays (when serum B12/folate equivocal):

Metabolite	B12 deficiency	Folate deficiency
Methylmalonic acid (MMA)	↑↑	Normal
Homocysteine	↑↑	↑↑

Methylmalonic acid (MMA) accumulates specifically when B12 is low — B12 (as adenosylcobalamin) is required to convert methylmalonyl-CoA → succinyl-CoA. Folate has no role in this pathway. An elevated MMA in the right clinical context confirms B12 deficiency even when serum B12 is borderline.

Homocysteine is elevated in BOTH deficiencies (B12 as cofactor for methionine synthase; folate as 5-methyl-THF donor). It is sensitive but not specific for B12.

Decision rule: Raised MMA + raised homocysteine = B12 deficiency. Raised homocysteine alone (MMA normal) = folate deficiency.

Step 3 — Aetiological workup for B12 deficiency:
• Anti-parietal cell antibody, anti-intrinsic factor antibody → pernicious anaemia
• Schilling test (historical) — two-stage oral radiolabelled B12 absorption test; now largely replaced by antibody assays and gastroscopy

Bone Marrow Morphology in Megaloblastic Anaemia

Megaloblastic vs Non-Megaloblastic Macrocytes: Peripheral Smear Comparison

Non-Megaloblastic Macrocytosis — Key Differentiating Features

Non-Megaloblastic Macrocytosis: Differential Diagnosis and Key Features

When the MCV is raised but smear shows round macrocytes without hypersegmented neutrophils, and B12/folate are normal, consider:

Cause	Mechanism	Clue
Alcohol (chronic)	Membrane lipid effect; also nutritional folate deficiency coexists	History, round macrocytes, GGT ↑, transaminases ↑
Liver disease	↑ Membrane cholesterol → acanthocytes + round macrocytes	LFTs, target cells, may also have thrombocytopenia
Hypothyroidism	Reduced erythropoietin + altered metabolism	TSH ↑, T4 ↓, bradycardia, cold intolerance
Reticulocytosis	Young reticulocytes are 20–30% larger than mature RBCs	High reticulocyte count; look for haemolysis/haemorrhage
MDS	Dyserythropoiesis; clonal disease	Dysplastic features in all lineages; ring sideroblasts; BM shows dysplasia

Key distinction: None of the above produce hypersegmented neutrophils or macro-ovalocytes — those are the megaloblastic signature and are absent in non-megaloblastic macrocytosis.

Schilling test (historical note): The two-stage Schilling test used radiolabelled oral B12 to locate the site of B12 malabsorption (gastric/IF problem vs intestinal). Stage 1: B12 alone (low urinary excretion = malabsorption). Stage 2: B12 + IF (corrects if pernicious anaemia). Now largely replaced by anti-IF antibodies, endoscopy, and metabolite assays — but still appears in exam questions.