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Macrocytosis and Macrocytic Anaemia

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Megaloblastic macrocytic anaemia

This is seen on a blood film with Large RBCs and Hypersegmented neutrophils. It's main 2 causes are:

  • B12 deficiency

  • Folate deficiency


This type of macrocytic anaemia is characterised by a failure to synthesise adequate amounts of DNA.


Vit B12 deficiency

This is absorbed in the Terminal Ileum, but needs Intrinsic Factor (IF) from gastric parietal cells for it to occur.

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Pernicious Anaemia is the autoimmune attack of parietal cells by autoantibodies against IF, which is needed for B12 absorption.


If this co-exists with a folate deficiency, B12 should be replaced first to prevent subacute combined degeneration of the spinal cord (Remember B comes before F). This is because folate metabolism requires B12, therefore further depleting this vitamin.


Folate deficiency

This is absorbed in the Jejunum.

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Non-megaloblastic macrocytic anaemia

  • Alcohol

  • Liver disease

  • Hypothyroidism

  • Reticulocytosis - ↑Reticulocytes to try and replace the lost RBCs in haemolytic anaemia or haemorrhage

  • Myelodysplastic syndromes - Type of blood cancer with a lack of healthy blood cells



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