Macrocytic and megaloblastic anemias

01Definition and overview

Megaloblastic anaemia is an anaemia in which DNA synthesis in the marrow is too slow for the cells' growth. The erythroblasts become large, with immature nuclei and normal cytoplasm (megaloblasts), many die before leaving the marrow, and the red cells that reach the blood are large and oval. Because every dividing cell is affected, the white cells, the platelets and the lining of the mouth and gut suffer too. Almost all cases are due to a lack of vitamin B12 (cobalamin) or folate, or to drugs that interfere with DNA synthesis.

Anaemia is a haemoglobin below the normal for age and sex (by the WHO 2024 cut-offs, below 130 g/L in men and below 120 g/L in non-pregnant women), and it is macrocytic when the mean cell volume (MCV) is above about 100 fL. Its general symptoms, signs and classification are covered in iron deficiency anaemia.

A large MCV has two families of causes, which a film and two blood tests usually separate:

02Etiology and pathophysiology

Vitamin B12

of B12 stored, mainly in the liver: enough for 2 years or more

2–3mgof B12 stored, mainly in the liver: enough for 2 years or more

lost, so a deficiency takes years to develop

1–2µg a daylost, so a deficiency takes years to develop

of a large oral dose absorbed passively, without intrinsic factor

1per centof a large oral dose absorbed passively, without intrinsic factor

Vitamin B12 is made only by microorganisms; people get it from meat, fish, eggs and dairy, and plants contain none unless fortified. Its absorption takes several steps, and each can fail:

1. 1

   Stomach

   Acid and pepsin free B12 from food proteins, and it binds haptocorrin from saliva. The parietal cells of the stomach body secrete intrinsic factor along with acid.
2. 2

   Duodenum

   Pancreatic enzymes digest haptocorrin, and the freed B12 binds intrinsic factor.
3. 3

   Terminal ileum

   The intrinsic factor–B12 complex binds its receptor (cubilin, with amnionless) on the last part of the ileum and is taken in.
4. 4

   Blood

   B12 travels on transcobalamin, which delivers it to cells (holotranscobalamin, the active B12, is only 10 to 30 per cent of the total); most plasma B12 sits on haptocorrin, which most cells cannot take up. Bile carries B12 back to the gut, where most is reabsorbed.

Folate

Folate comes from green leafy vegetables, pulses, liver and fortified foods, and cooking destroys much of it. It is absorbed in the duodenum and jejunum, after food polyglutamates are cut to monoglutamates, and circulates as methyltetrahydrofolate. The body holds only about 10 mg, enough for about 4 months on a deficient diet, and less when demand is high.

How the DNA synthesis fails

1. 1

   Folate makes thymidine

   A folate coenzyme, methylene-tetrahydrofolate, gives the methyl group that turns deoxyuridine into thymidine, a building block of DNA. Without it, DNA synthesis slows and uracil is wrongly built into DNA.
2. 2

   B12 recycles folate

   Methionine synthase, which needs B12, takes the methyl group from methyltetrahydrofolate to make methionine from homocysteine, freeing tetrahydrofolate for reuse. Without B12, folate is trapped in its methyl form (the methylfolate trap), so B12 deficiency starves DNA synthesis of folate.
3. 3

   Nucleus lags behind cytoplasm

   RNA and protein are still made, so the cytoplasm matures while the nucleus does not: large cells with open, immature chromatin (megaloblasts), and giant metamyelocytes in the white cell line.
4. 4

   Cells die in the marrow

   Many megaloblasts die before release (ineffective erythropoiesis), which raises the bilirubin and the LDH; the marrow is full, yet the blood is short of cells.

B12 has a second job, as the coenzyme of methylmalonyl-CoA mutase. When it fails, methylmalonic acid (MMA) builds up: a raised MMA points to B12 rather than folate deficiency (kidney failure raises it too), while a raised homocysteine occurs in both deficiencies.

The damage to the spinal cord and nerves is a feature of B12 deficiency, not of folate deficiency. The mechanism is not fully known; a failure of methylation that myelin depends on, and abnormal fatty acids made from the excess methylmalonyl-CoA, are thought to demyelinate the dorsal columns and corticospinal tracts of the spinal cord, the peripheral nerves, the optic nerves and the brain. Large doses of folic acid can bypass the trap and correct the anaemia, but not the nerve damage.

Causes of vitamin B12 deficiency

Autoimmune gastritis destroys the acid-secreting mucosa of the stomach body and fundus, sparing the antrum. Without acid, the antral G cells make more gastrin, which drives enterochromaffin-like cell hyperplasia and can give rise to type 1 gastric neuroendocrine tumours; the risk of gastric adenocarcinoma is also raised, from two- to about sevenfold in different studies. Many patients have iron deficiency first, because iron absorption needs acid, and B12 deficiency later, as the stores run out over years. It goes with other autoimmune diseases: thyroid disease, type 1 diabetes, Addison's disease and vitiligo. UK guidance now speaks of autoimmune gastritis rather than pernicious anaemia, because the life-threatening anaemia the old name describes has become rare.

Metformin lowers B12 in up to 1 in 10 users, more with higher doses and longer use. In the UK, B12 is checked in people taking it who develop anaemia or neuropathy, and periodically in those with other risk factors; metformin is continued.

Causes of folate deficiency

03Clinical features

The anaemia

The anaemia comes on slowly and is often severe before it is noticed, because the body adapts. Patients may have a lemon-yellow tint, the pallor of anaemia with the mild jaundice of ineffective erythropoiesis. A red, smooth, sore tongue (glossitis), angular cheilitis and mouth ulcers reflect the failing turnover of epithelia; there may be weight loss, diarrhoea and darker skin pigmentation. Severe anaemia in an older person can present as heart failure.

The nervous system in B12 deficiency

Subacute combined degeneration of the cord damages the dorsal columns and lateral corticospinal tracts together, usually with a peripheral neuropathy:

• symmetrical tingling and numbness that start in the feet, then the hands;
• early loss of vibration and position sense, with a sensory ataxia: unsteadiness in the dark and a positive Romberg test;
• spastic weakness of the legs with extensor plantars, often with absent ankle jerks (from the neuropathy) but brisk knee jerks (from the cord);
• untreated, paraplegia and loss of bladder control.

Beyond the cord: memory loss and cognitive decline, which can mimic dementia; depression, irritability and psychosis; optic neuropathy, with blurred vision and a central scotoma; and in infants, developmental delay and regression.

Folate deficiency

Folate deficiency causes the same megaloblastic anaemia and glossitis, but not the neurological syndrome in adults. In pregnancy, too little folate around conception raises the risk of neural tube defects (spina bifida, anencephaly) in the baby.

04Diagnosis and management

The blood and the marrow

• Full blood count: a macrocytic anaemia, often with an MCV above 110 fL; low white cells and platelets in severe cases (pancytopenia); a low reticulocyte count.
• Blood film: oval macrocytes and red cells of varied shape; hypersegmented neutrophils (six or more lobes, or more than 5 per cent with five), an early sign.
• Signs of ineffective erythropoiesis: a raised LDH, often very high, a raised unconjugated bilirubin and a low haptoglobin, which can look like haemolysis. Severe deficiency can even mimic a thrombotic microangiopathy (red cell fragments, low platelets, very high LDH), but the reticulocytes are low.
• Bone marrow: not needed when the picture is typical and a deficiency is proven. It is examined when the cause is unclear, to exclude myelodysplasia or leukaemia: a full marrow of megaloblasts with giant metamyelocytes.

Vitamin B12 tests

The UK guidance on B12 deficiency (2024) tests anyone with at least one common symptom or sign (an abnormal blood count, cognitive change, neuropathy or ataxia, glossitis, visual change, unexplained tiredness) and at least one risk factor, and does not rule deficiency out because there is no anaemia or macrocytosis. Blood is taken before any B12 is given.

• Total B12 is the usual first test; active B12 is used in pregnancy, when total B12 falls normally. The combined pill can lower total B12 without a deficiency, people of Black ethnicity may have a higher reference range, and supplements can raise the level without fully treating a deficiency. Some assays give falsely normal results in pernicious anaemia, so a convincing picture with a normal B12 is not dismissed. Laboratories set their own thresholds where they differ.
• Methylmalonic acid (MMA) rises in B12 deficiency, and also in kidney failure; it is the second-line test for an indeterminate result. Homocysteine rises in both B12 and folate deficiency. After nitrous oxide, homocysteine or MMA is the first test, because the serum B12 can be normal.

Folate tests

Serum folate is the first test; a level below about 3 µg/L (7 nmol/L) suggests deficiency. It reflects recent intake and can rise after a single meal. Red cell folate reflects the stores over the cells' life, but it is also low in B12 deficiency, because cells cannot hold folate without B12.

Finding the cause

• Anti-intrinsic factor antibodies: highly specific for pernicious anaemia, but found in only about 40 to 60 per cent of patients, so a negative result does not rule out autoimmune gastritis. Parietal cell antibodies are found in about 90 per cent but also in many healthy older people.
• If autoimmune gastritis is still suspected: parietal cell antibodies, a raised gastrin, or a gastroscopy with biopsies of the stomach body; an oral B12 absorption test is available in some places. The Schilling test is obsolete.
• Coeliac serology when the cause is still unknown, and in folate deficiency without an obvious cause.
• History: diet, alcohol, drugs (metformin, acid suppressants), nitrous oxide, gut surgery, bowel symptoms, autoimmune disease.
• Thyroid function, since autoimmune thyroid disease often accompanies pernicious anaemia and hypothyroidism itself raises the MCV. European consensus also suggests looking for Helicobacter pylori in unexplained B12 deficiency.

Treating vitamin B12 deficiency

In suspected megaloblastic anaemia with neurological symptoms, B12 is started at once after blood is taken, without waiting for the results. Most patients are treated with intramuscular hydroxocobalamin; UK practice follows the BNF and the product information:

Hydroxocobalamin is preferred in the UK because it stays in the body longer; cyanocobalamin, used in many countries, is given more often. Oral B12 at high doses works through passive absorption, even without intrinsic factor, and suits some causes:

What to expect and watch for:

• Many patients feel better within days, though symptoms can take up to 3 months to improve and may worsen briefly at first. The reticulocytes peak after about a week, and the haemoglobin then rises steadily.
• Potassium can fall in the first days of treating a severe anaemia, as the new cells take it up: it is checked and replaced. Iron deficiency often appears as the marrow recovers and needs iron.
• Nerve damage improves over 6 to 12 months; long-standing damage to the cord may not recover. Follow-up is at 3 months (at 1 month in pregnancy and breastfeeding); people on injections are not retested, but asked about symptoms.
• Transfusion is avoided unless the anaemia is life-threatening, because the circulation of an older patient with chronic anaemia overloads easily.

Treating folate deficiency

• Folic acid 5 mg daily for about 4 months, and longer while the cause persists; higher doses in malabsorption.
• Prevention: 5 mg daily to weekly in chronic haemolytic anaemia and on dialysis. Before conception and for the first 12 weeks of pregnancy, 400 µg daily prevents neural tube defects. In UK guidance, 5 mg is offered at higher risk: a neural tube defect in either partner, the family or a previous pregnancy; diabetes; sickle cell disease or thalassaemia; or antiepileptic drugs. A raised BMI alone no longer calls for the higher dose.
• Folinic acid (calcium folinate) bypasses dihydrofolate reductase and is used as rescue after high-dose methotrexate.
• Fortifying flour with folic acid prevents neural tube defects: the USA and Canada have done so since 1998, and the UK is fortifying non-wholemeal wheat flour from the end of 2026.

Macrocytosis without megaloblasts

A raised MCV with normal B12 and folate has other causes, most of them common:

05Epidemiology

Vitamin B12 deficiency becomes commoner with age: in the UK it affects about 5 per cent of people aged 65 to 74 and more than 10 per cent of those aged 75 or over, many with only mild or no symptoms. Autoimmune gastritis affects up to about 1 to 2 per cent of people, 2 to 3 per cent of women and of those over 60, at least twice as many women as men, and is found in all ethnic groups. Deficiency is common in vegans who take no supplements and in long-term users of metformin and acid suppressants, and recreational nitrous oxide has made subacute combined degeneration a disease of young adults too. In low-income countries, and wherever animal food is scarce, too little B12 in the diet is the main cause.

Folate deficiency has become uncommon where flour is fortified, and remains common with poor diets, alcohol misuse, pregnancy without supplements and chronic haemolysis.

06Prognosis

Pernicious anaemia was named for being fatal, until in 1926 a diet rich in liver was shown to cure it, and B12 was isolated in 1948. With treatment, the blood count recovers completely. Nerve damage is the lasting harm: the longer it has been present and the deeper the deficiency, the less it recovers, so early diagnosis matters most. Some patients still have tiredness or poor memory despite injections; they may be given them more often, and other causes are looked for.

Autoimmune gastritis needs lifelong B12 and carries a higher risk of gastric neuroendocrine tumours, and probably of adenocarcinoma. UK guidance considers gastroscopy when upper gastrointestinal symptoms appear or worsen, while several gastroenterology societies suggest endoscopic surveillance every 3 to 5 years, on weak evidence. Folate deficiency has an excellent outlook once the cause is dealt with.