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

Abstract

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. 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 unremarkable biochemical features (normal homocysteine concentration and slightly raised methylmalonic acid) of vitamin B-12 deficiency. The mild anaemia and reduced mean cell volume are consistent with a β thalassaemia trait, and the bone marrow appearances are normal The most likely diagnosis, in the face of normal serum vitamin B-12 concentration, would have been a myelodysplastic syndrome needing urgent blood transfusion and further haematology assessment. However, in view of her history of thyroid disease and family history of pernicious anaemia, an underlying functional vitamin B-12 deficiency could not be entirely discounted. A blood sample for autoantibody screen, anti-intrinsic factor antibody, and serum homocysteine and methylmalonic acid were taken and she was started immediately on replacement vitamin B-12 therapy pending these results. She had a prompt response, with her full blood count returning to normal within eight weeks of therapy (haemoglobin 126 g/l; mean cell volume 87.5 fl) without the need for blood transfusion. The results of serum homocysteine and methylmalonic acid were substantially raised and consistent with functional vitamin B-12 deficiency. The anti-intrinsic factor antibody was positive, confirming the diagnosis of pernicious anaemia. Cytogenetic analysis of her bone marrow was normal. Case 2 A 60 year old white woman with β thalassaemia trait had her serum vitamin B-12 concentration checked as she mentioned that she was a vegan. She had no symptoms of note and gave a history of avoiding meat and milk products for the past eight years. She had no family history of pernicious anaemia or other autoimmune conditions. She was not taking any medications. Her serum vitamin B-12 concentration was markedly reduced (figure)—confirmed on three occasions—with a haemoglobin concentration at the lower end of the normal range, consistent with her thalassaemia trait. On examination, there were no clinical features of note. Her anti-intrinsic factor antibody was negative and her serum ferritin concentration was within normal limits. Conventionally, she would have been given a course of vitamin B-12 injections immediately, and subsequently a Schilling test (or equivalent vitamin B-12 absorption study) would have been performed. In view of her general wellbeing, a bone marrow examination was done to evaluate any effect on erythropoiesis as a marker for functional vitamin B-12 deficiency. The bone marrow showed normal erythropoiesis, her plasma homocysteine was normal, and her methylmalonic acid was raised sufficiently to indicate possible subclinical deficiency.1