Megaloblastic Anemia 

Updated: Jul 17, 2021
Author: Srikanth Nagalla, MD, MS, FACP; Chief Editor: Emmanuel C Besa, MD 


Practice Essentials

Megaloblasts are large nucleated red blood cells. (See the image below.) Vitamin B12 deficiency (eg, pernicious anemia), folic acid deficiency, and certain medications are the most common causes of megaloblastic anemia, a macrocytic anemia. Although patients may be virtually asymptomatic, since the anemia develops insidiously, those with severe anemia may experience weakness and cardiopulmonary, gastrointestinal, mental, and neurologic signs and symptoms. 

Megaloblastic anemia. View of red blood cells Megaloblastic anemia. View of red blood cells

See 21 Hidden Clues to Diagnosing Nutritional Deficiencies, a Critical Images slideshow, to help identify clues to conditions associated with malnutrition.

The objectives of this article are to review the pathophysiology, clinical presentation, diagnosis, and management of megaloblastic anemias. An overview of the physiology and biochemistry of vitamin B12 and folate under normal and pathological conditions is included.

Go to Pediatric Megaloblastic Anemia, Anemia, Chronic Anemia, Myelophthisic Anemia, Hemolytic Anemia, and Sideroblastic Anemias for complete information on these topics.


Megaloblastosis describes a heterogeneous group of disorders that share common morphologic characteristics: large cells with an arrest in nuclear maturation. Nuclear maturation is immature relative to cytoplasmic maturity. Hence, these cells, which can be seen in bone marrow aspirates and in peripheral smears, have been called megaloblasts. These abnormalities are due to impaired DNA synthesis and, to a lesser extent, RNA and protein synthesis.

Megaloblastic changes are most apparent in rapidly dividing cells such as blood cells and gastrointestinal cells.[1, 2, 3]  In addition to large nucleated red blood cells (megaloblasts), hypersegmented neutrophils can be seen on peripheral smears, and giant bands occur in bone marrow.

The common feature in megaloblastosis is a defect in DNA synthesis in rapidly dividing cells. To a lesser extent, RNA and protein synthesis are impaired. Unbalanced cell growth and impaired cell division occur since nuclear maturation is arrested. More mature RBC precursors are destroyed in the bone marrow prior to entering the blood stream (intramedullary hemolysis).[1, 3]

The most common causes of megaloblastosis are vitamin B12 and folate deficiencies, medications, and direct interference of DNA synthesis by HIV infections and myelodysplastic disorders.

Vitamin B12 (cobalamin) and folate biochemistry

Vitamin B12 differs from other water-soluble vitamins in that it is stored in the liver. In addition, vitamin B12 has to be protected during its passage through the gastrointestinal tract to the distal ileum, the site of B12 absorption.  The primary sources of cobalamin (Clb), a cobalt-containing vitamin, are meat, fish, and dairy products and not vegetables and fruit. Cyano - Clb, the form used in supplements,  is not a natural form but is an in vitro artifact. However, cyano-Clbis readily converted into biologically active forms in humans and other mammals. 5’-Deoxyladenosyl-Clb, methyl-Clb, are the active forms of cobalamin.

Clb is a cofactor for only 2 enzymes in mammals, methionine synthase and L-methylmalonyl-CoA mutase. Methyl-Clb is the cofactor for methionine synthase, and 5’-deoxyladenosyl-Clb is the cofactor for L-methylmalonyl-CoA mutase.

Methionine synthase that requires cofactor methyl-Clb is important for one carbon transfer and is a key enzyme in the methionine cycle. This enzyme is needed to convert homocysteine to methionine involving the transfer of a methyl group. Tetrahydrofolate is a cofactor in this reaction. Methionine, in turn, is required for the synthesis of S-adenosylmethionine (SAM), a methyl group donor used in many biological methylation reactions, including the methylation of sites in DNA and RNA. Diminished activity of methionine synthase or decreased tetrahydrofolate can cause defective DNA maturation and megaloblastic changes. Diminished methionine synthase leads to the “folate trap” in which 5-methyl-THF accumulates and cannot serve as a methyl donor and cannot be converted to the THF needed for methionine synthesis (ie, biological dead end).

L-methylmalonyl-CoA mutase requires cofactor 5-deoxyadenosylcobalamin and catalyzes the conversion of L-methylmalonyl-CoA to succinyl-CoA, a key component of the tricarboxylic acid cycle. This biochemical reaction is important for the production of energy from fats and proteins. Succinyl CoA is also required for the synthesis of hemoglobin, the oxygen carrying pigment in red blood cells. The substrate of methylmalonyl-CoA mutase, methylmalonyl-CoA, is derived from propionyl-CoA from the catabolism of valine, threonine, methionine, thymine, cholesterol, and odd-chain fatty acids.

The mechanisms for patchy demyelination and other neurological consequences of cobalamin deficiency are not well-understood. They appear to be independent and different from those responsible for the development of megaloblastic morphology and anemia. Several theories have been developed for the genesis of cobalamin neuropathy, such as the following[4] :

  • Reduced SAM and resultant abnormal methylation may be responsible. Methylation reactions are needed for myelin maintenance and synthesis.

  • Elevated methylmalonic acid (MMA) may be responsible. Cobalamin deficiency leads to reduced cofactor 5-deoxyadenosylcobalamin that is instrumental in an increase in MMA. Increased MMA is associated with the production of abnormal odd chain and branched chain fatty acids with subsequent abnormal myelination

  • Cobalamin deficiency impacts a network of cytokines and growth factors that can be neurotrophic and others neurotoxic. These factors might play a role in cobalamin related neuropathy.[5]

The sources of folates are ubiquitous, and folate is found in vegetables, fruits, and animal protein. Dietary folic is usually conjugated, polyglutamate folates, and are converted to dihydrofolic acid so they can be absorbed. Dihydrofolate is processed to tetrahydrofolate that participates along with methyl-Clb in the synthesis of methionine. Tetrahydrofolate is conjugated to glutamate to function intracellularly.

Cobalamin transport and uptake

The uptake of cobalamin is complex. Dietary cobalamin binds nonspecifically to dietary proteins. Cobalamin is released from food during gastric digestion at a low pH. The released cobalamin then binds to and is protected by R-proteins. R-proteins have a high affinity for binding cobalamin at a low pH. As cobalamin–R-protein complexes enter the duodenum, cobalamin is released from R-proteins because of the alkaline environment (R-proteins have a low affinity at an alkaline pH) and the presence of pancreatic enzymes. Cobalamin released from R-proteins is free to bind to intrinsic factor (IF), which has a high affinity for binding cobalamin at an alkaline pH.

IF is produced in the gastric fundus and cardia. The role of IF is to stabilize cobalamin and transport it to the terminal ileum. Cobalamin-IF complexes are processed by a receptor, cubulin, in the terminal ileum, and cobalamin is released and absorbed.

The absorbed cobalamin is bound to transcobalamin II (TC II). TC II transports cobalamin to cells that internalize and use cobalamin for DNA synthesis. Transcobalamin I (TC I) might be involved in cobalamin storage and is elevated in leukocytes in patients with chronic myelogenous leukemia.

Storage of cobalamin and folate

Cobalamin is the only water-soluble vitamin stored in the body. About 3 mg of cobalamin are stored, of which 1 mg is stored in the liver. Hence, it takes 3-5 years to develop a vitamin B12 deficiency after a total gastrectomy. In contrast, significant amounts of folate are not stored. Clinical evidence of folate deficiency can occur within a month after folate intake is stopped.

Enterohepatic cycle for cobalamin

Several micrograms of cobalamin are secreted daily in bile and then reabsorbed in the terminal ileum. This enterohepatic cycle can stabilize the daily availability of cobalamin when dietary intake is low.

Uptake of folates

Physiologic folate absorption and transport is receptor mediated. There is no equivalent of IF to stabilize and transport ingested folate. Uptake occurs in the jejunum and throughout the small intestine.


Major causes for cobalamin deficiency

Cobalamin deficiency may result from the following:

  • Atrophy or loss of gastric mucosa (eg, pernicious anemia, gastrectomy, ingestion of caustic material, hypochlorhydria, histamine 2 [H2] blockers)
  • Functionally abnormal intrinsic factor (IF)
  • Inadequate dietary intake (ie, vegetarian diet)
  • Inadequate proteolysis of dietary cobalamin
  • Insufficient pancreatic protease (eg, chronic pancreatitis, Zollinger-Ellison syndrome [ZES])
  • Bacterial overgrowth in intestine (eg, blind loop, diverticula) - Bacteria compete with the body for cobalamin
  • Diphyllobothrium latum (fish tapeworm) - Competes with the body for cobalamin
  • Disorders of ileal mucosa (eg, resection, ileitis, sprue, lymphoma, amyloidosis, absent IF-cobalamin receptor, Imerslünd-Grasbeck syndrome, ZES, transcobalamin II [TCII] deficiency, use of certain drugs)
  • Disorders of plasma transport of cobalamin (eg, TCII deficiency, R-binder deficiency)
  • Dysfunctional uptake and use of cobalamin by cells (eg, defects in cellular deoxyadenosylcobalamin [AdoCbl] and methylcobalamin [MeCbl] synthesis)

Pernicious anemia

Pernicious anemia, the best-known cause for cobalamin deficiency, results from autoimmune destruction of gastric parietal cells and subsequent reduction in intrinsic factor (IF) production. Significant amounts of cobalamin are not absorbed in the absence of IF. The term “pernicious anemia” is an anachronism—it dates from the era when treatment had not yet been discovered, and the disease was fatal—but it remains in use.

An increased incidence of pernicious anemia in families suggests a hereditary component to the disease. Patients with pernicious anemia have an increased incidence of autoimmune disorders and thyroid disease, suggesting that the disease has an immunologic component. For example, pernicious anemia may occur together with autoimmune thyroid disease, type 1A diabetes mellitus, alopecia, vitiligo, and chronic atrophic gastritis in type III polyglandular autoimmune (PGA) syndrome—one of a rare group of disorders also known as autoimmune polyendocrine syndromes (APS) and polyglandular failure syndromes. Type III PGA occurs in adults.[6]

Children who develop cobalamin deficiency usually have a hereditary disorder, and the etiology of their cobalamin deficiency is different from the etiology observed in classic pernicious anemia. Congenital pernicious anemia is a hereditary disorder in which an absence of IF occurs without gastric atrophy.

Dietary cobalamin deficiency rarely causes megaloblastic anemia, except in strict vegetarians who avoid meat, eggs, and dairy products. In addition, elderly persons may develop megaloblastic anemia as a result of atrophic gastritis and achlorhydria, which impair the release of cobalamins bound to food and, hence, the availability of cobalamin.[7]

Recommended dietary allowances for vitamin B12 are as follows[8] :

  • Birth to 6 months: 0.4 mcg
  • 7–12 months: 0.5 mcg
  • 1–3 years: 0.9 mcg
  • 4–8 years: 1.2 mcg
  • 9–13 years: 1.8 mcg
  • 14+ years: 2.4 mcg
  • Pregnancy: 2.6 mcg
  • Lactation: 2.8 mcg

In pancreatic insufficiency, the alkaline environment in the small intestine is insufficient for release of cobalamin from R-proteins and binding to intrinsic factor. In the Zollinger-Ellison syndrome, the acid environment also prevents binding of cobalamin to intrinsic factor. In both conditions, the diminished binding to intrinsic factor interferes with cobalamin absorption.

Disorders of the terminal ileum can result in cobalamin deficiency, because the terminal ileum is the site of uptake of cobalamin-IF complexes. Such disorders include tropical sprue, inflammatory bowel disease, lymphoma, and ileal resection. Tropical sprue is more severe than nontropical sprue (celiac disease) and can be associated with both cobalamin and folate deficiencies. It takes several years for cobalamin deficiency to develop after the onset of these disorders because of the time required to deplete cobalamin reserves.

In the Imerslund-Grasbeck syndrome, there is autoimmune destruction of the ileal receptor, cubulin, for the uptake of cobalamin bound to intrinsic factor.

Blind loop syndrome can result in cobalamin deficiency. Bacterial colonization can occur in intestines deformed by strictures, surgical blind loops, scleroderma, inflammatory bowel disease, or amyloidosis. Bacteria then compete with the host for cobalamin.

The fish tapeworm Diphyllobothrium latum can compete with the host for ingested cobalamin. Diphyllobothriasis most often occurs in people in northern latitudes who eat raw or pickled fish.

Nitrous oxide exposure can cause megaloblastosis by oxidative inactivation of cobalamin. Prolonged exposure to nitrous oxide can lead to severe mental and neurological disorders.

The details of hereditary disorders are beyond the scope of this review, but information can be found in other references.[1, 3]

A partial list of medications that can cause cobalamin deficiency includes the following[1, 9, 10] :

  • Purine analogs (6-mercaptopurine, 6-thioguanine, acyclovir)
  • Pyrimidine analogues (5-fluorouracil, 5-azacytidine, zidovudine)
  • Ribonucleotide reductase inhibitors (hydroxyurea, cytarabine arabinoside)
  • Drugs that affect cobalamin metabolism ( p-aminosalicylic acid, phenformin, metformin)

Major causes for folate deficiency

The daily requirement for adults is about 0.4 mg/d. Storage is limited, and folate deficiency develops about 3-4 weeks after the cessation of folate intake.

Folate content in foods and the preparation of foods are major causes for folate deficiency, especially in elderly persons. Folates are very thermolabile. Therefore, excessive heating can lead to inactivation, especially when foods are excessively diluted in water. In the United States, most people obtain sufficient folate from fortified foods. However, alternative diets may contain little folate.

Increased demand can result in deficiency. There is an increased need for folate in hemolysis, pregnancy, lactation, rapid growth, hyperalimentation, renal dialysis, psoriasis, and exfoliative dermatitis.

Intestinal disorders that impede folate absorption include tropical sprue, nontropical sprue (celiac disease or gluten sensitivity), amyloidosis, and inflammatory bowel disease.

With alcoholism, the bioavailability of folate and folate-dependent biochemical reactions can be impaired.

A partial list of medications that can cause folate deficiency includes phenytoin, metformin, phenobarbital, dihydrofolate reductase inhibitors (trimethoprim, pyrimethamine), methotrexate and other antifolates, sulfonamides (competitive inhibitors of 4-aminobenzoic acid), and valproic acid.

The details of hereditary disorders that cause folate deficiency are beyond the scope of this review, but information can be found in other references).[1, 3, 11, 12]

Other causes for megaloblastosis

Megaloblastosis in HIV infection and myelodysplastic disorders is due to a direct effect on DNA synthesis in hematopoietic and other cells.


United States statistics

Faulty preparation of foods and folate deficiency during pregnancy are the most common causes of megaloblastic anemias. Pernicious anemia is less common. About 1 in 7,500 people in the United States develops pernicious anemia each year. However, current folate administration during pregnancy and vitamin supplementation in elderly persons has decreased the incidence of megaloblastosis.

International statistics

The frequency of megaloblastosis is highest in countries in which malnutrition is rampant and routine vitamin supplementation for elderly individuals and pregnant women is not available.


Pernicious anemia and folate deficiencies usually occur in individuals older than 40 years, and the prevalence increases in older populations. Pernicious anemia is diagnosed in about 1% of people older than 60 years. The incidence is slightly higher in women than in men.

The incidence of pernicious anemia is reported to be higher in Sweden, Denmark, and the United Kingdom than in other developed countries.


The prognosis is favorable if the etiology of megaloblastosis has been identified and appropriate treatment has been instituted. However, patients are at risk for hypokalemia and anemia-related cardiac complications during therapy for cobalamin deficiency.

Folate deficiency during pregnancy can lead to neural tube defects and other developmental disorders in the fetus. However, folate in prenatal vitamins given during pregnancy has reduced these morbidities.[13, 14]




A patient’s history might reveal manifestations of anemia and neurological abnormalities. Since anemia develops insidiously, patients may be virtually asymptomatic. However, those with severe anemia may experience weakness and cardiopulmonary impairment. A lemon-color complexion occurs due to intramedullary hemolysis. Some patients can have gastrointestinal signs and symptoms such as loss of appetite, weight loss, nausea, and constipation. Patients may have a sore tongue and canker sores.

A spectrum of mental changes, from a change in personality to psychosis, as well as peripheral neuropathy, can occur in both folate and cobalamin deficiencies. Peripheral neuropathy presents as numbness, pain, tingling, and burning in a patient’s hands and feet. Patients may report loss of sensation and that they feel like they are wearing a thin stocking or glove.

Unsteady gait, abnormal proprioception, and loss of balance occur in subacute combined spinal cord degeneration. Some patients with cobalamin deficiency can present primarily with neurological impairment and are not anemic. Neurologic symptoms may range from mild to severe. Cobalamin deficiency should be considered even if a patient has minimal neurologic symptoms and is not anemic.

History findings to help identify a cobalamin deficiency are as follows:

  • Evidence for achlorhydria such as abdominal discomfort, reflux, early satiety, and abdominal bloating: This condition can impair cobalamin absorption.

  • Pernicious anemia: These patients may have signs of other autoimmune disorders such as thyroid disorders, type I diabetes, or Addison disease.

  • Family history, HLA (HLA A2, A3, B7, B12), and type A blood (Scandinavians and African Americans)

  • History of a gastrectomy

  • Conditions that affect the terminal ileum (site of cobalamin absorption), such as inflammatory bowel disease, sprue, or ileal resection

  • Conditions in which cobalamin is competitively consumed: History of abdominal surgery might suggest a blind loop syndrome. Exposure to raw fish might suggest Diphyllobothrium latum (fish tapeworm) infestation.[15]

  • Zollinger-Ellison syndrome or pancreatic insufficiency: There is impaired binding of cobalamin to intrinsic factor.

  • Strict vegetarian with no consumption of eggs and dairy products

  • A history of folate administration without vitamin B-12 therapy: This should alert one to the possibility of the progression of neuropsychiatric complications in a patient who is not anemic.

  • A history of megaloblastosis since childhood: This would suggest a hereditary cause of cobalamin deficiency.

History findings to help identify folate deficiency are as follows:

  • Poor nutrition, alternative diets, and excessive heating and dilution of foods

  • Chronic alcoholism

  • Conditions that interfere with folate absorption, including inflammatory bowel disease, sprue or gluten sensitivity, and amyloidosis

  • Conditions that increase folate consumption, such as pregnancy, lactation, hemolytic anemia, hyperthyroidism, and exfoliative dermatitis

  • Hyperalimentation and hemodialysis

  • Medications that affect folate (see the list in Etiology)

  • Hereditary disorder: A lifelong history of megaloblastosis or folate deficiency would suggest a hereditary disorder as the cause.

Physical Examination

Evidence of anemia: Patients may be asymptomatic if the anemia had developed gradually and was compensated. In severe anemia, patients may have dyspnea, tachycardia, and cardiopulmonary distress.

Patients may have a lemon-yellow hue due to the combination of anemia and an increased indirect bilirubin level. The source of the bilirubin is intramedullary hemolysis.

Glossitis, characterized by a smooth tongue due to loss of papillae, occurs in persons with cobalamin deficiency.

Dermatologic signs include hyperpigmentation of the skin and abnormal pigmentation of hair due to increased melanin synthesis. Infantile vitamin B-12 deficiency is common in underdeveloped countries and is associated with hyperpigmentation of the dorsa of hands and feet as well as generalized hyperpigmentation.[16]

.A wide range of mental changes, from irritability to psychosis, as well a peripheral neuropathy, can occur in both folate and cobalamin deficiencies.

Subacute combined neurologic degeneration occurs in cobalamin deficiency. Patients present with abnormal gait, loss of balance, speech impairment, and loss of proprioceptive and vibratory senses. Blindness due to optic atrophy may occur.

Abdominal scars may suggest a blind loop syndrome due to gastric surgery or a lack of ileal absorption of cobalamin in a patient who had an ileal resection.

Patients with nontropical and tropical sprue may have signs of malabsorption, such as weight loss, abdominal distention, diarrhea, and steatorrhea. These patients often have metabolic bone disease or bleeding resulting from to deficiencies in vitamin K–dependent factors.

Patients who have megaloblastosis as a result of HIV infection or myelodysplastic syndromes usually have signs of these disorders.

Children with inborn errors associated with folate and cobalamin deficiencies may have signs of these hereditary disorders .



Diagnostic Considerations

Conditions that can cause a megaloblastic anemia include the following:

  • Vitamin B12 insufficiency
  • Pernicious anemia
  • Lack of absorption of B12 complexes in the terminal ileum (eg, from small bowel bacterial overgrowth, pancreatic exocrine insufficiency, tapeworm, familial factors, drugs, ileal bypass, ileal enteritis, celiac disease)
  • Folic acid deficiency
  • Thiamine-responsive megaloblastic anemia syndrome (TRMA)
  • Inherited defects of cobalamin transport and metabolism

By definition, pernicious anemia refers specifically to vitamin B12 deficiency resulting from a lack of production of intrinsic factor (IF) by parietal cells in the stomach. This can be due to autoantibodies directed against the parietal cells, removal of parietal cells by bariatric surgery or gastrectomy, or destruction of parietal cells by gastric disease or alcohol abuse.

Pernicious anemia may rarely be associated with liver disease (eg, primary biliary cholangitis, autoimmune hepatitis, interferon-treated hepatitis C). Yan et al report two cases of pernicious anemia in patients with cryptogenic cirrhosis, in both of whom the neuropsychiatric symptoms of pernicious anemia were initially attributed to hepatic encephalopathy.[16]

Vitamin B12 absorption is a complex process, and other causes of vitamin B12 deficiency exist. Pernicious anemia must be differentiated from other disorders that interfere with the absorption and metabolism of vitamin B12 and produce cobalamin deficiency, with the development of a macrocytic anemia and neurologic complications. It is very important to distinguish between vitamin B12 and folate deficiencies since the treatment of the former with folate can lead to the progression of neurological impairment. See Overview/Etiology.

Other potential causes of macrocytosis (eg, liver disease, hypothyroidism, copper deficiency, hemolytic anemia) should be considered in the differential diagnosis. Copper deficiency can present as normocytic, microcytic, or macrocytic anemia. Patients with copper deficiency presenting with macrocytic anemia and myeloneuropathy could be misdiagnosed as having vitamin B12 deficiency. 

Occasionally, the morphologic changes in hematopoietic cells of patients with megaloblastic anemia are extremely bizarre and can be misinterpreted as neoplasia, such as acute leukemia or myelodysplasia.

Thiamine-responsive megaloblastic anemia syndrome

TRMA is an autosomal recessive disorder characterized by megaloblastic anemia, progressive sensorineural hearing loss, and diabetes mellitus. Onset of megaloblastic anemia occurs between infancy and adolescence. Vitamin B12 and folic acid levels are normal. On bone marrow examination, affected individuals have megaloblastic changes with erythroblasts often containing iron-filled mitochondria (ringed sideroblasts). Molecular genetic testing will show biallelic pathogenic variants in SLC19A2.[17]  

Uncommonly, variable ocular anomalies may be present in TRMA. One case report describes symmetric bull's eye maculopathy and other ocular findings consistent with cone-rod degeneration.[18]

The anemia in TRMA is corrected with pharmacologic doses (50-100 mg/day) of thiamine (vitamin B1). However, the red cells remain macrocytic.[19]

Inherited defects of cobalamin transport and metabolism

Three hereditary disorders affect absorption and transport of cobalamin, and another seven alter cellular use and coenzyme production. Clinical onset of these disorders usually occurs in infancy and childhood.

The three disorders of absorption and transport are transcobalamin II (TCII) deficiency, IF deficiency, and IF receptor deficiency. These defects produce developmental delay and a megaloblastic anemia, which can be alleviated with pharmacologic doses of cobalamin. Serum cobalamin values are decreased in the two IF abnormalities but may be within the reference range in TCII deficiency.[20]

The seven abnormalities of cellular use, commonly denoted by letters A through G, can be detected by the presence or absence of methylmalonic aciduria and homocystinuria. The presence of only methylmalonic aciduria indicates a block in conversion of methylmalonic CoA to succinyl CoA, caused by either a genetic deficit in the methylmalonyl CoA mutase that catalyzes the reaction or a defect in synthesis of its CoA cobalamin (cobalamin A and cobalamin B deficiency).

The presence of only homocystinuria results either from poor binding of cobalamin to methionine synthase (cobalamin E deficiency) or from producing methylcobalamin from cobalamin and S adenosylmethionine (cobalamin G deficiency). This results in a reduction in methionine synthesis, with pronounced homocystinemia and homocystinuria.

Methylmalonic aciduria and homocystinuria occur when the metabolic defect impairs reduction of cobalamin III to cobalamin II (cobalamin C, cobalamin D, and cobalamin F deficiency). This reaction is essential for formation of both methylmalonic acid and homocystinuria.

Early detection of these rare disorders is important because most patients respond favorably to large doses of cobalamin. However, some of these disorders are less responsive than others, and delayed diagnosis and treatment are less efficacious.

Differential Diagnoses



Approach Considerations

The anemia should be characterized. Pancytopenia and systemic impairment should be evaluated. The etiology of megaloblastosis should be identified.

Initial Studies

Initial workup for megaloblastic anemia should include the following assays:

  • Complete blood count (CBC)
  • Red blood cell (RBC) indices
  • Peripheral blood smear
  • Reticulocyte count
  • Lactate dehydrogenase (LDH) 
  • Indirect bilirubin
  • Iron and ferritin
  • Serum cobalamin
  • Serum folate, and possibly RBC folate 

The LDH level is usually markedly increased in severe megaloblastic anemia. Reticulocyte counts are inappropriately low, representing lack of production of RBCs due to massive intramedullary hemolysis. These findings are characteristics of ineffective hematopoiesis that occurs in megaloblastic anemia as well as in other disorders such as thalassemia major.

Peripheralsmear morphology

A peripheral smear may reveal macroovalocytes, a characteristic of megaloblastosis (see the image below). They should be distinguished from macrocytes that are not oval, which can occur in liver disease and hypothyroidism. Polychromatophilic macrocytes, reticulocytes, and immature RBCs can be seen in hemolytic anemia and disorders associated with increased RBC production.

Peripheral smear of blood from a patient with pern Peripheral smear of blood from a patient with pernicious anemia. Macrocytes are observed, and some of the red blood cells show ovalocytosis. A 6-lobed polymorphonuclear leukocyte is present.

Single and multiple Howell-Jolly bodies, nuclear fragment, may be seen in RBCs. Cabot rings, remnants of mitotic spindles, may also be present in RBCs.

Nucleated RBCs and megaloblasts can be seen.

Smears may reveal hypersegmented neutrophils if at least 5% neutrophils have 5 or more lobes. Normal neutrophils contain 3-4 lobes.

Macrocytosis due to cobalamin or folate deficiencies may be masked in patients with iron deficiency. However, hypersegmented neutrophils can persist in iron deficiency.

Bone marrow aspiration

Bone marrow aspiration is usually not needed to make the diagnosis of vitamin B-12 deficiency. However, it can help rule out myelodysplasia and assess iron stores. The bone marrow is hypercellular with erythroid hyperplasia. Erythroid precursors have megaloblastic features being larger than normoblastic cells. In addition, nuclear maturation is immature relative to cytoplasmic maturation. Megaloblastic changes are most prominent in more mature RBC precursors. Giant bands, neutrophil precursers, can be present. Megakaryocytes may be large and hyperlobulated .

Bone marrow megaloblastic changes are reversed within 12 hours after treatment with cobalamin or folate, and bone marrow morphology appears to be normal within 2-3 days. Therefore, bone marrow aspiration, if necessary, should be performed as soon as possible and preferably before therapy.

Primary Tests for B-12 and Folate Deficiencies

Serum B-12 (cobalamin)

Reference range: 200-900 pg/mL

  • Borderline: 180-250 pg/mL
  • Associated with anemia and neuropathy: < 180 mg/L
  • Diagnostic of B-12 deficiency: < 150 mg/L

The serum cobalamin level can be normal in the following circumstances:

  • In some inborn areas of cobalamin deficiency
  • Transcobalamin II (TC II) deficiency
  • Cobalamin deficiency due to nitrous oxide

Serum cobalamin levels may be low in the following circumstances:

  • Pregnancy
  • Oral contraceptives
  • Transcobalamin I (TC I) deficiency
  • Severe folic acid deficiency
  • Patients taking large doses of ascorbic acid

Serum folate

Folate reference range in adults: 2-20 ng/mL

  • Folate deficiency likely (overlap with normal): < 2.5 ng/mL 

The following should be considered:

  • Effect of diet: A single meal may falsely elevate serum folate levels to normal. Hence, blood should be drawn prior to transfusions, meals, and therapy to achieve accurate results.
  • Hemolysis: Might cause false positive results

RBC folate

Reference range for adults: >140 ng/mL

  • Not affected by diet and reflects tissue stores (folate content is established early in RBC development)
  • Affected by hemolysis
  • Low in severe B-12 deficiency
  • Test is intricate and expensive

Serum for folate and cobalamin should be frozen and stored prior to meals or therapy if the tests cannot be performed within a reasonable timeframe.

Lab tests to confirm and distinguish B-12 and folate deficiencies

Serum homocysteine and methylmalonic acid (MMA) levels are helpful confirmatory tests for cobalamin and folate deficiencies. Both are increased in cobalamine deficiency. Homocysteine but not MMA is increased in folate deficiency. Homocysteine and MMA levels should be used if the clinical presentation and serum vitamin B-12 and folate levels are ambiguous.

The MMA level can be increased in the following circumstances:

  • End-stage renal disease
  • Inborn error of methylmalonic acid metabolism

Serum homocysteine can be increased in the following circumstances:

  • Homocystinuria
  • Hyperhomocysteinemia

  • MTHFR C677T

Serum homocysteine can be decreased in the following circumstances:

  • A high rate of conversion back to methionine
  • Low production

  • A high rate of conversion of into sulfite/sulfate etc

Intrinsic factor(IF)blocking and parietal cell and antibodies

IF antibodies, type 1 and type 2, occur in 50% of patients with pernicious anemia and are specific for this disorder. Therefore, they can be used to confirm the diagnosis of pernicious anemia. Parietal cell antibody occurs in 90% of patients with pernicious anemia but can also occur in thyroid disease and other autoimmune disorders. Therefore, parietal cell antibodies are not specific for pernicious anemia.

A valuable test that is no longer available (Schilling test)

The value of a Schilling test (a radiometric test) is that it can confirm B-12 deficiency, can be done after patient has been given B-12 therapy, and can distinguish between pernicious anemia and failure in transport or ileal uptake. Unfortunately, the test is no longer available at most hospitals. The 3 parts to the Schilling test are as follows:

  1. First, radioactive cyanocobalamin is given orally and its urinary secretion is measured to estimate cobalamin uptake. Low urinary secretion suggests pernicious anemia, a failure in intestinal transport, defective uptake of cobalamin in the terminal ileum, or a blind loop syndrome.

  2. The second part is performed in the same manner, except that IF is given orally along with radioactive cyanocobalamin. If IF restores cobalamin uptake, the patient most likely has pernicious anemia. If not, an abnormality in cobalamin intestinal transport, defective absorption in the terminal ileum, or a blind loop syndrome might be responsible for the deficiency.

  3. In the third phase, the patient is treated with antibiotics before the administration of radioactive cyanocobalamin. If antibiotics restore cobalamin uptake, the patient most likely has a blind loop syndrome.

Diagnostic Therapeutic Trial

If the results of the evaluation are ambiguous, a clinical trial of the effects of cobalamin therapy may be indicated. However, a clinical trial of folate is contraindicated if cobalamin deficiency has not been ruled out. The administration of folate to patients with cobalamin deficiencies may precipitate or worsen neurologic impairment.

Other Studies

Baseline iron studies and serum ferritin should be obtained since they may predict the need for iron therapy since iron stores can be consumed during cobalamin or folate therapy.

Radiographic imaging of the upper and lower gastrointestinal tract may be useful for detecting abnormalities that could cause a blind loop syndrome. These procedures also may detect defects in the terminal ileum that might interfere with cobalamin absorption.

Other tests that may be considered include the following:

  • With cobalamin deficiency, evaluate and rule out autoimmune disorders, Zollinger-Ellison syndrome, pancreatic insufficiency, fish tapeworm infestation, Imerslund-Grasbeck syndrome, Crohn disease, or ileal scarring.

  • With folate deficiency, evaluate evidence for malnutrition and alcoholism, sprue, chronic hemolysis, and exfoliative dermatitis.

  • Megaloblastic anemia



Approach Considerations

Once drug-induced megaloblastic changes and myelodysplasia-related megaloblastosis have been ruled out, most patients are treated with cobalamin or folate. Since megaloblastic anemias usually develop gradually, many patients adjust to low hemoglobin levels and do not require transfusions. Transfusion therapy should be restricted to patients with severe, uncompensated, and life-threatening anemia.

Go to Anemia, Chronic Anemia, Myelophthisic Anemia, Hemolytic Anemia, and Sideroblastic Anemias for complete information on these topics.

Cobalamin Therapy

Cobalamin (1000 µg) should be given intramuscularly daily for 2 weeks, then weekly until the hematocrit value is normal, and then monthly for life. A dose of 1000 µg is large, but it may be required in some patients. The reader should be aware that several other protocols for cobalamin therapy have been recommended. It is important to emphasize that patients with mental and neurological impairment due to cobalamin deficiency should be treated more aggressively.

Oral cobalamin (1000-2000 µg) also can be administered. Oral cobalamin is less expensive and is better tolerated by patients. A wide range of doses and schedules have been recommended. For example, a randomized controlled trial in 40 vegans and vegetarians with marginal vitamin B12 deficiency found that adequate vitamin B12 levels could be achieved with cobalamin given in either a sublingual dosage of 50 μg/day (350 μg/week) or 2000 μg/week in a single oral dose.[21]

Patients receiving oral dosages should be monitored for the desired response, since absorption can be variable and may be insufficient in some patients. Note the following[22] :

  • Oral cobalamin is indicated in patients with hemophilia to avoid bleeding from intramuscular injections.
  • Intramuscular cobalamin and not oral cobalamin should be used to treat patients with cobalamin-related neurological disorders.
  • One advantage of parenteral over oral cobalamin is that all abnormalities in cobalamin absorption are bypassed.
  • It may be practical to administer parenteral cobalamin initially and then switch to oral cobalamin.
  • Proton pump inhibitors, H2-receptor antagonists, and metformin may reduce serum vitamin B-12 concentrations by inhibiting the absorption of vitamin B12. [23]

Patients who have undergone either a total or partial gastrectomy should be started on replacement therapy after the surgery to prevent the development of megaloblastosis. Oral vitamin B12 supplementation is effective and safe in patients who underwent total gastrectomy and should be considered the preferential form of supplementation.[24]

Folate Therapy

Folate should be administered orally. If this is difficult, comparable doses can be administered parenterally.[25, 26] In addition, the patient should consume a folate-enriched diet. The dosage range for folate is 1 to 5 mg daily; 1 mg/d is the usual dosage for adults with megaloblastic anemia, while a higher dosage is indicated in hemolysis, malabsorption, alcoholism, and exfoliative dermatitis. However, there is no harm in giving the higher dosage of folate.

Folate should be administered prophylactically during pregnancy, lactation, and the perinatal period. Folate is also indicated in patients with chronic hemolytic anemias, psoriasis and exfoliative dermatitis, and during extensive renal dialysis. Folate therapy has been recommended in patients with hyperhomocysteinemia who are at risk for thromboembolic complications.[27]

Fortification of foods and folic acid supplements have been recommended to reduce the risk of pancreatic, cervical, and colon cancers. Folic acid supplements are indicated in patients with kidney failure. Folate supplementation is indicated in elderly persons. However, opponents of fortification and supplementation are concerned that giving folate-fortified foods to persons with unrecognized cobalamin deficiencies will increase the frequency of cobalamin-induced neuropsychiatric disorders.

The dosage and protocol for folic acid therapy and supplementation in the various disorders mentioned above are summarized in a communication from the Mayo Clinic.[28]


Folate therapy should not be instituted in a patient with megaloblastic anemia if cobalamin deficiency has not been definitively ruled out. The danger is that folic acid will improve the anemia but not the neurological complications of cobalamin deficiency, and the neurological disorder will worsen. Both cobalamin and folate should be given if cobalamin deficiency has not been ruled out.

Monitoring Response to Therapy

Although patients may feel better as soon as therapy is started, improvements must be monitored. Laboratory tests to order include the following:

  • Complete blood cell count
  • Reticulocyte count
  • Lactate dehydrogenase (LDH) level
  • Indirect bilirubin
  • Hemoglobin level
  • Serum potassium level
  • Serum ferritin

Elevated levels of LDH and indirect bilirubin should fall rapidly. A prolonged elevation of the LDH level indicates a failure of therapy, development of iron deficiency, or an error in diagnosis.

Reticulocytosis should be evident within 3-5 days and peaks in 4-10 days. Leukocyte and platelets counts are usually restored to normal within days after therapy has been started, but hypersegmented neutrophils may persist for 10-14 days.

The hemoglobin should rise approximately 1 g/dL each week. This rise is valuable for monitoring a complete response. If the hemoglobin does not rise appropriately and is not normal within 2 months, other causes of anemia, such as iron deficiency, should be considered.

Serum potassium levels can fall during therapy for severe cobalamin or folate deficiency, which can lead to sudden death. Therefore, potassium should be monitored and supplements may be indicated.

Iron deficiency can occur in the course of treatment due to the consumption of iron stores for red blood cell production. The development of iron deficiency can impede the response to cobalamin or folate therapy. Iron therapy may be indicated.

Treatment of Other Related Conditions

Other related conditions, if present, should be addressed as follows:

  • Blind loop syndrome should be treated with antibiotics.
  • Patients with transcobalamin II (TCII) deficiency may require higher doses of cobalamin.
  • Tropical sprue should be treated with both cobalamin and folate.
  • Acute megaloblastic anemias due to nitrous oxide exposure can be treated with folate and cobalamin.
  • Fish tapeworm infection, pancreatitis, Zollinger-Ellison syndrome, and inborn errors should be treated with appropriate measures.

Dietary Measures

Patients with folate or cobalamin deficiency should receive dietary education on the choice of foods and instructions on how to prepare foods.

Patients should have diets rich with folic acid. Examples of such foods include asparagus, broccoli, spinach, lettuce, lemons, bananas, melons, liver, and mushrooms. To prevent loss of folate, foods should not be cooked excessively and should not be diluted in large amounts of water. To prevent cobalamin deficiency, vegetarians should include dairy products and eggs in their meals. Patients should know that goat milk contains little folate.


A hematologist should be consulted to assist with the diagnosis and management of patients with megaloblastic anemias.

A neurologist should be consulted for patients with potential neurological complications of cobalamin and folate deficiencies.

A gastroenterologist should be consulted since an endoscopy may be indicated to rule out atrophic gastritis and to evaluate patients for gastric carcinoma with periodic endoscopies.

A pediatrician with expertise in inborn errors should be consulPernicious anaemia (PA) is associated with increased gastric cancer risk,ted to help treat children with hereditary megaloblastosis.

Long-Term Monitoring

Patients should be monitored for recurrence of megaloblastosis by periodic testing for hemoglobin levels and, if necessary, LDH and indirect bilirubin levels.

Patients with pernicious anemia should periodically undergo endoscopic screening, as they are at increased risk for gastric cancer. In addition, a systematic review found that the incidence of biliary tract cancers and hematological malignancies (multiple myeloma, Hodgkin lymphoma, non-Hodgkin lymphoma, and leukemia) was also increased in these patients.[29]



Medication Summary

Pharmacotherapy involves the administration of vitamin B12, folate, or both, as indicated. The goals of pharmacotherapy are to correct vitamin deficiencies, to prevent complications, and to reduce morbidity. For information on iron supplements, see Iron Deficiency Anemia.


Class Summary

Cyanocobalamin (vitamin B12) is used to correct vitamin B12 deficiency and folic acid is used to treat folic acid deficiencies. Cyanocobalamin does not naturally occur. It is an in vitro artifact that is readily converted to active forms of cobalamin in humans and mammals.

There are several forms of cyanocobalamin available for treatment of B12 deficiencies. Only intramuscular and oral cyanocobalamin are recommended. However, the effectiveness of oral cyanocobalamin in managing cobalamin-related neurological disorders has not been proven. Hence, oral cyanocobalamin is not recommended for subacute combined system degeneration and other cobalamin-related neurological disorders.[22]

Intravenous cobalamin is not recommended because (1) it does not accumulate since most of a dose is excreted in urine and (2) intravenous cobalamin can raise blood pressure. Intranasal cobalamin is not recommended because (1) absorption is inconsistent and not predictable and (2) it is expensive.[30, 31]

Cobalamin has been associated with allergic reactions. It is not clear whether the reactions are due to preservatives in intramuscular preparations or whether it is due to cobalamin in both parenteral and oral preparations.[31]

Cobalamin therapy for patients with Leber hereditary neuropathy, especially during early phases of the disorder, may cause blindness and is contraindicated.[31]

Cyanocobalamin (Calo-Mist, Ener-B, Nascobal)

Cyanocobalamin is most commonly given to patients. It is an in vitro artifact and is not an active form of the vitamin. However, it is converted to active forms.

Folic acid (Folvite)

Folic acid is an essential cofactor for enzymes used in DNA synthesis.

Electrolyte Supplements

Class Summary

Serum potassium levels can fall during therapy for severe cobalamin or folate deficiency and can lead to sudden death. Therefore, potassium supplements may be indicated.

Potassium chloride (K-Tab, Klor-Con, microK, Epiklor)

Potassium is essential for transmission of nerve impulses, contraction of cardiac muscle, maintenance of intracellular tonicity, skeletal and smooth muscles, and maintenance of normal renal function. Gradual potassium depletion occurs via renal excretion, through GI loss, or because of low intake.

Potassium depletion sufficient to cause 1 mEq/L drop in serum potassium requires a loss of about 100-200 mEq of potassium from body stores.


Questions & Answers


What is megaloblastic anemia?

What is the pathophysiology of megaloblastic anemia?

What are the roles of vitamin B-12 (cobalamin) and folate in the pathogenesis of megaloblastic anemia?

What theories have been developed for the genesis of cobalamin neuropathy relative to megaloblastic anemia?

What is the role of cobalamin uptake in the pathogenesis of megaloblastic anemia?

What is the role of cobalamin and folate storage in the pathogenesis of megaloblastic anemia?

What is the role of the enterohepatic cycle of cobalamin in the pathogenesis of megaloblastic anemia?

What is the role of folate uptake in the pathogenesis of megaloblastic anemia?

What is the role of cobalamin deficiency in the etiology of megaloblastic anemia?

What is the role of folate deficiency in the etiology of megaloblastic anemia?

What causes megaloblastic anemia in patients with HIV infection or myelodysplastic disorders?

What is the prevalence of megaloblastic anemia in the US?

What is the global prevalence of megaloblastic anemia?

Which patient groups are at highest risk for megaloblastic anemia?

What is the prognosis of megaloblastic anemia?


Which clinical history findings are characteristic of megaloblastic anemia?

Which clinical history findings are characteristic of cobalamin deficiency in megaloblastic anemia?

Which clinical history findings are characteristic of folate deficiency in megaloblastic anemia?

Which physical findings are characteristic of megaloblastic anemia?


Which conditions should be included in the differential diagnoses of megaloblastic anemia?

What are the differential diagnoses for Megaloblastic Anemia?


Which studies are performed in the workup of megaloblastic anemia?

Which studies are performed in the initial workup of megaloblastic anemia?

What is the role of peripheral smear morphology in the diagnosis of megaloblastic anemia?

What is the role of bone marrow aspiration in the diagnosis of megaloblastic anemia?

Which serum B-12 (cobalamin) level findings are characteristic of megaloblastic anemia?

Which serum folate level findings are characteristic of megaloblastic anemia?

Which red blood cell (RBC) folate level findings are characteristic of megaloblastic anemia?

What is the role of serum homocysteine and methylmalonic acid (MMA) measurement in the evaluation of megaloblastic anemia?

What is the role of intrinsic factor (IF) antibody testing in the diagnosis of megaloblastic anemia?

What is the role of Schilling test in the diagnosis of megaloblastic anemia?

When is a cobalamin therapy given to confirm the diagnosis of megaloblastic anemia?

What is the role of iron studies in the diagnosis of megaloblastic anemia?

What is the role of imaging in the diagnosis of megaloblastic anemia?

Which tests should be considered in patients with cobalamin deficiency in megaloblastic anemia?

Which tests should be considered in patients with folate deficiency in megaloblastic anemia?


How is megaloblastic anemia treated?

What is the role of cobalamin therapy in the treatment of megaloblastic anemia?

How is cobalamin therapy administered for the treatment of megaloblastic anemia?

What is the role of folate therapy in the treatment of megaloblastic anemia?

How is response to therapy for megaloblastic anemia monitored?

How are the comorbidities of megaloblastic anemia treated?

Which dietary modifications are used in the treatment of megaloblastic anemia?

Which specialist consultations are beneficial to patients with megaloblastic anemia?

What is included in long-term monitoring following the treatment of megaloblastic anemia?


What is the role of drug treatment for megaloblastic anemia?

Which medications in the drug class Electrolyte Supplements are used in the treatment of Megaloblastic Anemia?

Which medications in the drug class Vitamins are used in the treatment of Megaloblastic Anemia?