Paper I
Question
Laboratory diagnosis of megaloblastic anaemia
Answer
Megaloblastic anaemia results from impaired DNA synthesis due to vitamin B12 and/or folate deficiency, causing nuclear-cytoplasmic maturation asynchrony.
Peripheral blood findings
- Macrocytic anaemia — raised MCV (>100 fL)
- Oval macrocytes on peripheral smear
- Hypersegmented neutrophils (>5 lobes) — a hallmark finding
- Anisopoikilocytosis; occasionally leukopenia and thrombocytopenia in severe cases
Bone marrow findings
- Hypercellular marrow with megaloblastic erythroid hyperplasia
- Nuclear-cytoplasmic asynchrony — nuclei remain immature/open (“megaloblastic”) while cytoplasm matures normally (haemoglobinized)
- Giant metamyelocytes and band forms in the myeloid series
- Ineffective erythropoiesis — increased intramedullary destruction of erythroid precursors
Biochemical investigations
- Serum vitamin B12 assay — low in B12 deficiency
- Serum/red cell folate assay — low in folate deficiency
- Serum LDH — elevated (ineffective erythropoiesis, intramedullary haemolysis)
- Indirect bilirubin — mildly elevated
- Serum methylmalonic acid and homocysteine — elevated in B12 deficiency (methylmalonic acid specifically elevated only in B12, not folate deficiency)
- Schilling test (historical) — to differentiate causes of B12 deficiency (pernicious anaemia vs. malabsorption)
Additional tests
- Anti-intrinsic factor and anti-parietal cell antibodies — for pernicious anaemia
- Upper GI endoscopy — to assess for atrophic gastritis