Haematology & Oncology

B12 and Folate Deficiency

B12 or folate deficiency impairs DNA synthesis and can cause megaloblastic cytopenias; B12 deficiency additionally threatens the nervous system, so it must be recognised and replaced promptly, especially before or alongside folic acid.

Definition

Vitamin B12 and folate deficiency impair DNA synthesis and can cause megaloblastic cytopenias. Vitamin B12 deficiency can additionally cause neurological, cognitive, psychiatric or visual problems, sometimes without anaemia or macrocytosis.

First principles

B12 and folate deficiency share a megaloblastic mechanism

Both vitamins support one-carbon metabolism and normal DNA synthesis. When either is deficient, nuclear maturation lags behind cytoplasmic growth in marrow precursors, producing macro-ovalocytes, hypersegmented neutrophils and sometimes pancytopenia. Macrocytosis is a useful clue but is not required: vitamin B12 deficiency can occur without anaemia or a raised MCV.

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