Intended for healthcare professionals

Practice Lesson of the week

Diagnosing vitamin B-12 deficiency on the basis of serum B-12 assay

BMJ 2006; 333 doi: (Published 17 August 2006) Cite this as: BMJ 2006;333:385
  1. Vinod Devalia, consultant haematologist (
  1. 1 Princess of Wales Hospital, Bridgend CF31 1RQ
  • Accepted 7 March 2006

Serum vitamin B-12 concentrations are measured to assess the presence of its deficiency in patients presenting with haematolological, neurological, and neuropsychiatric abnormalities. Replacement therapy is instituted promptly, particularly to prevent irreversible neurological and cognitive dysfunction.

I present two cases with paradoxical vitamin B-12 results, which highlight the fallacies of the serum vitamin B-12 assay and emphasise the importance of taking into account the overall clinical picture before prejudging the significance of the vitamin B-12 assay result.

Case reports

Case 1

A 59 year old white woman was seen urgently for assessment of a macrocytic anaemia. She had normal serum B-12 concentrations, confirmed on three occasions. She complained of progressively increasing lethargy, palpitations, and buzzing in the ears over about three months. She had a good, well balanced diet and was not a vegetarian. Apart from thyroxine, she was taking no regular medication. She said her father had had pernicious anaemia. On clinical examination the only clinically significant findings were a mild glossitis and pallor. A full blood count showed a substantial macrocytic anaemia and a mild reduction of the white cell count (figure). The blood film showed mild oval macrocytosis, occasional nucleated red cells, and some hypersegmented neutrophils. An urgent bone marrow examination showed megaloblastic haemopoiesis.

Embedded Image

Laboratory results and bone marrow appearances for the cases presented. Case 1 shows a normal serum vitamin B-12 concentration with biochemical features of B-12 deficiency (raised homocysteine and methylmalonic acid). The haematology indices (macrocytic anaemia) and bone marrow appearances are consistent with megaloblastic anaemia caused by classic pernicious anaemia, confirmed by a positive anti-intrinsic factor antibody. Case 2 shows a markedly reduced serum vitamin B-12 concentration but largely …

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