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Clinical Review

ABC of clinical haematology: Macrocytic anaemias

BMJ 1997; 314 doi: (Published 08 February 1997) Cite this as: BMJ 1997;314:430
  1. Victor Hoffbrand,
  2. Drew Provan


    Macrocytosis is a rise in the mean cell volume of the red cells above the normal range (in adults 80-95 fl (femtolitres). It is detected with a blood count, in which the mean cell volume, as well as other red cell indices, is measured. The mean cell volume is lower in children than in adults, with a normal mean of 70 fl at age 1 year, rising by about 1 fl each year until adult volumes are reached at puberty.

    Megaloblastic bone marrow is exemplified by developing red blood cells that are larger than normal, with nuclei more immature than their cytoplasm. The underlying mechanism is defective DNA synthesis

    Causes of megaloblastic anaemia


    Vitamin B12 deficiency–Veganism, poor quality diet

    Folate deficiency–Poor quality diet, old age, poverty, synthetic diet without added folic acid, goats' milk


    Gastric causes of vitamin B12 deficiency–Pernicious anaemia, congenital intrinsic factor deficiency, gastrectomy

    Intestinal causes of vitamin B12 deficiency–Stagnant loop, congenital selective malabsorption, ileal resection

    Intestinal causes of folate deficiency–gluten induced enteropathy, tropical sprue

    Increased cell turnover

    Folate deficiency–Pregnancy, prematurity, chronic haemolytic anaemia (such as sickle cell anaemia), inflammatory and malignant diseases

    Renal loss

    Folate deficiency–Congestive cardiac failure, dialysis


    Folate deficiency–Anticonvulsants, sulphasalazine

    Defects of vitamin B12 metabolism–for example, transcobalamin II deficiency, nitrous oxide anaesthesia–or of folate metabolism (such as methotrexate treatment), or rare inherited defects of DNA synthesis may all cause megaloblastic anaemia

    The causes of macrocytosis fall into two groups: (a) deficiency of vitamin B12 (cobalamin) or folate (or rarely abnormalities of their metabolism) in which the bone marrow is megaloblastic and (b) other causes, in which the bone marrow is usually normoblastic. In this article the two groups are considered separately, and then the reader is taken through the steps to diagnose the cause of macrocytosis and its management.

    Deficiency of vitamin B12 or folate

    Vitamin B12 deficiency

    The body's requirement for vitamin B12 is about l μg daily. This is amply supplied by a normal Western diet (vitamin B12content 10-30 μg daily) but not by a strict vegan diet, which excludes all animal produce (including milk, eggs, and cheese). Absorption of vitamin B12 is through the ileum, facilitated by intrinsic factor, which is secreted by the parietal cells of the stomach. Absorption is limited to 2-3 μg daily.

    In Britain, vitamin B12 deficiency is usually due to pernicious anaemia, which now accounts for up to 80% of all cases of megaloblastic anaemia. The incidence of the disease is 1:10 000 in northern Europe, and the disease occurs in all races. The underlying mechanism is an autoimmune gastritis that results in achlorhydria and the absence of intrinsic factor. The incidence of pernicious anaemia peaks at age 60; the condition has a female:male incidence of 1.6:1.0 and is more common in those with early greying, blue eyes, and blood group A, and in those with a family history of the disease or of diseases that may be associated with it–for example, vitiligo, myxoedema, Hashimoto's disease, Addison's disease of adrenal, and hypoparathyroidism.

    Other causes of macrocytosis*

    • Liver disease

    • Hypothyroidism

    • Reticulocytosis

    • Aplastic anaemia

    • Red cell aplasia

    • Myelodysplasia

    • Cytotoxic drugs Paraproteinaemia (such as myeloma)

    • Pregnancy

    • Neonatal period

    *These are usually associated with a normoblastic marrow

    Other causes of vitamin B12 deficiency are infrequent in Britain. Veganism is an unusual cause of severe deficiency as most vegetarians and vegans include some vitamin B12 in their diet. Moreover, unlike in pernicious anaemia, the enterohepatic circulation for vitamin B12 is intact in vegans, so vitamin B12 stores are conserved. Gastric resection and intestinal causes of malabsorption of vitamin B12–for example, ileal resection or the intestinal stagnant loop syndrome–are less common now that abdominal tuberculosis is infrequent and H2-antagonists have been introduced for treating peptic ulceration, thus reducing the need for gastrectomy.


    Patient with vitiligo on neck and back.

    Folate deficiency

    The daily requirement for folate is 100-200 μg, and a normal mixed diet contains about 200-300 μg. Natural folates are largely in the polyglutamate form, and these are absorbed through the upper small intestine after deconjugation and conversion to the monoglutamate 5-methyl tetrahydrofolate.


    Patient with coeliac disease: underweight and low stature.

    Body stores are sufficient for only about four months. Folate deficiency may arise because of inadequate dietary intake, malabsorption (especially gluten induced enteropathy), or excessive use as proliferating cells degrade folate. Deficiency in pregnancy may be due partly to inadequate diet, partly to transfer of folate to the fetus, and partly to increased folate degradation.

    Consequences of vitamin B12 or folate deficiencies

    Megaloblastic anaemia–Clinical features include pallor and jaundice. The onset is gradual, and a severely anaemic patient may present in congestive heart failure or only when an infection supervenes. The blood film shows oval macrocytes and hypersegmented neutrophil nuclei (with six lobes). In severe cases, the white cell count and platelet count also fall (pancytopenia). The bone marrow shows characteristic megaloblastic erythroblasts and giant metamyelocytes (early granulocyte precursors). Biochemically, there is an increase in plasma of unconjugated bilirubin and serum lactic dehydrogenase, with, in severe cases, an absence of haptoglobins and presence in urine of haemosiderin. These changes, including jaundice, are due to increased destruction of red cell precursors in the marrow (ineffective erythropoiesis).


    Blood film in vitamin B12 deficiency showing macrocytic red cells and hypersegmented neutrophil.

    Vitamin B12 neuropathy–A minority of patients with vitamin B12 deficiency develop a neuropathy due to symmetrical damage to the peripheral nerves and posterior and lateral columns of the spinal cord, the legs being more affected than the arms. Psychiatric abnormalities and visual disturbance may also occur. Men are more commonly affected than women. The neuropathy may occur in the absence of anaemia. Psychiatric changes and at most a mild peripheral neuropathy may be ascribed to folate deficiency.

    Neural tube defects–Folic acid supplements in pregnancy have been shown to reduce the incidence of neural tube defects (spina bifida, encephalocoele and anencephaly) in the fetus and may also reduce the incidence of cleft palate and hare lip. No clear relation exists between the incidence of these defects and folate deficiency in the mother, although the lower the maternal red cell folate (and serum vitamin B12) concentrations even within the normal range, the more likely neural tube defects are to occur in the fetus. One underlying mechanism seems to be a genetic defect in folate metabolism.


    Glossitis due to vitamin B12 deficiency.

    Gonadal dysfunction–Deficiency of either vitamin B12 or folate may cause sterility, which is reversible with appropriate vitamin supplementation.

    Epithelial cell changes–Glossitis and other epithelial surfaces may show cytological abnormalities.

    Cardiovascular disease– Raised serum homocysteine concentrations have been associated with arterial obstruction and venous thrombosis.

    Other causes of macrocytosis

    The most common cause of macrocytosis in Britain is alcohol. Fairly small quantities of alcohol–for example, two gin and tonics or half a bottle of wine a day–especially in women, may cause a rise of mean cell volume to >100 fl, typically without anaemia or any detectable change in liver function.

    The mechanism for the rise in mean cell volume is uncertain. In liver disease the volume may rise due to excessive lipid deposition on red cell membranes, and the rise is particularly pronounced in liver disease caused by alcohol. A modest rise in mean cell volume is found in severe thyroid deficiency.

    In other causes of macrocytosis, other haematological abnormalities are usually present–in myelodysplasia (a frequent cause of macrocytosis in elderly people) there are usually quantitative or qualitative changes in the white cells and platelets in the blood. In aplastic anaemia, pancytopenia is present; pure red cell aplasia may also cause macrocytosis. Changes in plasma proteins–presence of a paraprotein (as in myeloma)–may cause a rise in mean cell volume without macrocytes being present in the blood film. Physiological causes of macrocytosis are pregnancy and the neonatal period. Drugs that affect DNA synthesis–for example, hydroxyurea and azathioprine–can cause macrocytosis with or without megaloblastic changes. Finally, a rare, benign familial type of macrocytosis has been described.

    Investigations which may be needed in patients with macrocytosis

    • Serum B12 assay

    • Serum and red cell folate assays

    • Liver and thyroid function

    • Reticulocyte count

    • Serum protein electrophoresis

    • For vitamin B12 deficiency: serum parietal cell and intrinsic factor antibodies, radioactive vitamin B12 absorption with and without intrinsic factor (Schilling test), possibly serum gastrin concentration

    • Consider bone marrow examination for megaloblastic changes suggestive of vitamin B12 or folate deficiency, or alternative diagnoses–for example, myelodysplasia, aplastic anaemia, myeloma

    • Endoscopy–gastric biopsy (vitamin B12 deficiency), duodenal biopsy (folate deficiency)


    Biochemical assays

    The most widely used screening tests for the deficiencies are the serum B12 and folate assays. A low serum concentration implies a deficiency, but a subnormal serum concentration may occur in the absence of pronounced body deficiency–for example, in pregnancy (vitamin B12) and with very recent poor dietary intake (folate).

    Red cell folate can also be used to screen for folate deficiency; a low concentration usually implies appreciable depletion of body folate, but the concentration also falls in severe vitamin B12 deficiency, so it is more difficult to interpret the significance of a low red cell than serum folate concentration in patients with megaloblastic anaemia. Moreover, if the patient has received a recent blood transfusion the red cell folate concentration will partly reflect the folate concentration of the transfused red cells.

    Results of absorption tests of radioactive vitamin B12

    View this table:

    Specialist investigations

    Assays of serum homocysteine (raised in vitamin B12 or folate deficiency) or methylmalonic acid (raised in vitamin B12 deficiency) are used in some specialised laboratories.


    For patients with vitamin B12 or folate deficiency it is important to establish the underlying cause. In pernicious anaemia, intrinsic factor antibodies are present in plasma in 50% and parietal cell antibodies in 90% of patients.

    Other investigations

    Radioactive vitamin B12 absorption studies–for example, Schilling test–show impaired absorption of the vitamin in pernicious anaemia; this can be corrected by giving intrinsic factor. In patients with an intestinal lesion, however, absorption of vitamin B12 cannot be corrected with intrinsic factor.

    Endoscopy should be performed to confirm atrophic gastritis and exclude gastric carcinoma or gastric polyps, which are two to three times more common in patients with pernicious anaemia than in age and sex matched controls.

    A bone marrow examination is usually performed to confirm megaloblastic anaemia. It is also required for the diagnosis of myelodysplasia, aplastic anaemia, myeloma, or other marrow disorders associated with macrocytosis.


    Bone marrow appearances in megaloblastic anaemia: developing red cells are larger than normal, with nuclei that are immature relative to their cytoplasm (nuclear:cytoplasmic asynchrony).

    If folate deficiency is diagnosed it is important to assess dietary folate intake and to exclude gluten induced enteropathy by endoscopy and duodenal biopsy. The deficiency is common in patients with diseases of increased cell turnover who also have a poor diet.


    Preventing folate deficiency in pregnancy

    • As prophylaxis against folate deficiency in pregnancy, daily doses of folic acid 400 μg are usual

    • Larger doses are not recommended as they could mask megaloblastic anaemia due to vitamin B12 deficiency and thus allow B12 neuropathy to develop

    • As neural tube defects occur by the 28th day of pregnancy, it is advisable for a woman's daily folate intake to be increased by 400 μg/day at the time of conception

    • Whether this can be achieved by increased consumption of foods with a high folate content–for example, liver, green vegetables, and cereals–or whether women have to take additional folic acid or eat foods deliberately fortified with added folate is the subject of current discussion

    • The US Food and Drugs Administration announced in 1996 that specified grain products (including most enriched breads, flours, cornmeal, rice, noodles, and macaroni) will be required to be fortified with folic acid to levels ranging from 0.43 mg to 1.5 mg per pound (453 g) of product

    • For mothers who have already had an infant with a neural tube defect, larger doses of folic acid–for example, 5 mg daily–are recommended before and during subsequent pregnancy

    Vitamin B12 deficiency is treated initially by giving the patient six injections of hydroxocobalamin l mg at intervals of about three to four days, followed by four such injections a year for life. For patients undergoing total gastrectomy or ileal resection it is sensible to start the maintenance injections from the time of operation. For vegans, less frequent injections–for example, one or two a year–may be sufficient, and the patient should be advised to eat foods to which vitamin B12 has been added, such as bread.

    Folate deficiency is treated with folic acid, usually 5 mg daily orally for four months, which is continued only if the underlying cause cannot be corrected. As prophylaxis against folate deficiency in patients with a severe haemolytic anaemia–such as sickle cell anaemia–5 mg folic acid once weekly is probably sufficient. Vitamin B12 deficiency must be excluded in all patients starting folic acid treatment at these doses as such treatment may correct the anaemia in vitamin B12 deficiency but allow neurological disease to develop.


    Victor Hoffbrand is professor of haematology at the Royal Free Hospital, London.

    The illustration of the bone marrow is reproduced with permission from Clinical Haematology (AV Hoffbrand, J Pettit), 2nd ed, London: Mosby International, 1994.

    The ABC of clinical haematology is edited by Drew Provan, consultant haematologist and honorary senior lecturer at the Southampton University Hospitals NHS Trust, and Andrew Henson, clinical research fellow, university department of primary care, Royal South Hants Hospital, Southampton.

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