eMedicine Specialties > Hematology > Red Blood Cells and Disorders

Megaloblastic Anemia: Differential Diagnoses & Workup

Author: Paul Schick, MD, Emeritus Professor, Department of Internal Medicine, Thomas Jefferson University Medical College; Research Professor, Department of Internal Medicine, Drexel University College of Medicine; Adjunct Professor of Medicine, Lankenau Hospital, Wynnewood, PA
Contributor Information and Disclosures

Updated: Aug 26, 2009

Differential Diagnoses

Macrocytosis

Other Problems to Be Considered

Occasionally, the morphological changes in megaloblasts and other cells may be extremely bizarre; these changes have been misinterpreted as neoplasia, acute leukemia, or myelodysplasia.

Workup

Laboratory Studies

• A CBC count, RBC indices, platelet count, differential count, reticulocyte count, and microscopic examination of the peripheral blood smear should be performed.
  • A typical patient with megaloblastic anemia presents with macrocytic anemia with thrombocytopenia and a decreased reticulocyte count. The mean cell volume can range from 100-150 fL or greater.
  • Hypersegmented neutrophils can be observed on the peripheral smear and represent an early phase of megaloblastosis in persons with nutritional megaloblastic anemias. Hypersegmented neutrophils contain 5 or more lobes, while normal neutrophils contain 3-4 lobes.
  • Macrocytes are oval and have been called macroovalocytes. In persons with severe anemia, macrocytes with nuclear remnants and erythrocytes with megaloblastic nuclei can be present in the peripheral blood. Macrocytes can be found in the peripheral blood in patients with liver disease or hemolytic anemia (because of an increase in reticulocytes) and usually do not have oval features. However, macroovalocytes are characteristic of megaloblastic anemias.
  • In general, the profoundness of megaloblastic changes is proportional to the severity of the anemia.
  • In some cases of megaloblastosis, no anemia is present despite overt neuropsychiatric disease. One cause of this disparity is the administration of folic acid to patients with cobalamin deficiency. This therapy partially corrects the anemia, but the neuropathy is not affected and progresses.
  • Macrocytosis due to cobalamin or folate deficiencies may be masked in patients with microcytic anemias because of thalassemia or iron deficiency. However, hypersegmentation of neutrophils may persist. Transfusion therapy or infections may modify the expression of megaloblastosis.
• LDH and indirect bilirubin assays should be ordered, and results are expected to be high because of intramedullary destruction of megaloblastic red cell precursors. LDH fraction 1 (LDH₁) and LDH fraction 2 (LDH₂) are elevated, with LDH₁ being greater than LDH₂. The LDH level is often extremely high, and, following therapy, the fall in the LDH level is an excellent indication of response to or failure of therapy. Increased LDH and indirect bilirubin levels along with a decreased reticulocyte count suggest ineffective hemopoiesis in which intramedullary hemolysis is occurring.
• Serum iron and ferritin assays should be ordered initially and during the treatment of megaloblastic anemias. These parameters may be high. Increased iron turnover occurs in persons with untreated megaloblastosis. However, serum iron and ferritin levels may also decrease because patients respond to therapy and consume iron stores for the production of new RBCs. If iron stores are depleted, patients have an incomplete response to cobalamin or folate therapy.
• Tests for the diagnosis of cobalamin deficiency are described as follows:
  • The most important test is measuring the serum cobalamin level. In a typical clinical presentation of megaloblastic anemia, a low serum cobalamin level and a full response to cobalamin may be sufficient to establish a diagnosis. A Schilling test can be performed in patients who have been treated with cobalamin and folate. This test can be used to diagnose cobalamin deficiency and to distinguish between pernicious anemia and ileal malabsorption.
  • Serum for cobalamin levels should be drawn before transfusions or vitamin B-12 therapy. If the test cannot be performed within a reasonable time frame, serum should be frozen to preserve it for testing so that therapy can be started. Serum cobalamin levels are usually low in patients with anemia due to cobalamin deficiency. However, exceptions to this rule exist.
  • Cobalamin levels may be falsely high in patients with megaloblastosis due to nitrous oxide, TCII deficiency, inborn errors in cobalamin metabolism, and myeloproliferative disorders. On the other hand, serum cobalamin levels can be falsely low with normal tissue levels in some patients with folate or iron deficiency, vegetarians, individuals on high doses of ascorbic acid, pregnant women, and persons with transcobalamin I (TCI) deficiency.
  • Serum samples for folate levels should also be obtained and, if necessary, frozen prior to therapy in patients with possible cobalamin deficiency because patients with folate deficiency can have reduced cobalamin levels.
  • A Schilling test is a radiometric test of cobalamin absorption. The test is given in 3 parts, as follows:
    • In the first part of the test, radioactive cyanocobalamin is given orally. Unlabeled cyanocobalamin is given intramuscularly to inhibit the uptake of radioactive cobalamin by the liver. Next, the urinary secretion of radioactive cobalamin is measured to estimate whether the orally administered cobalamin has been taken up. Low secretion suggests either pernicious anemia or an abnormality in the terminal ileum that prevented the uptake of IF-cobalamin complexes.
    • The second part of the test is performed in the same manner, except that IF is given orally along with radioactive cyanocobalamin. If IF restores the uptake of ingested radioactive cyanocobalamin, the patient most likely has pernicious anemia. However, if IF does not restore uptake, then an abnormality in the terminal ileum is most likely present.
    • A third phase can be performed in which the patient is treated with antibiotics prior to administering radioactive cyanocobalamin. If antibiotics restore cobalamin absorption from the gastrointestinal tract, the patient most likely has a blind loop syndrome.
    • The main difficulty with the Schilling test is inadequate collection of urine samples in patients who are either noncompliant or in renal failure.
    • The results of the Schilling test may indicate cobalamin malabsorption in patients who have severe and long-standing folate deficiencies. This is because of the effect of severe folate deficiency on the ileal mucosa that leads to a decrease in cobalamin uptake in the terminal ileum. Treating patients with severe folate deficiency with both cobalamin and folate for a month may be advisable to restore the ileal mucosa before performing a Schilling test.
  • A  protein-bound absorption test (also known as food-cobalamin absorption test) should be performed if food-cobalamin malabsorption is suggested. In this disorder, IF is present, but cobalamin bound to r-binder is not released and thus cannot bind to IF. Results of a standard Schilling test are normal in persons with this disorder. However, if the Schilling test is modified by using in vivo cyanocobalamin-radiolabeled food or in vitro cyanocobalamin-radiolabeled chicken serum or eggs instead of free radiolabeled cyanocobalamin, the Schilling test result will be abnormal. The results of the modified Schilling test can help detect the failure of the release of cobalamin bound to foods.
  • Methylmalonic aciduria is another test. Urinary excretion is a reliable index of cobalamin deficiency, provided the patient does not have renal failure.
  • Serum methylmalonic acid and homocysteine test results are elevated in more than 90% of patients with cobalamin deficiencies.
  • Antiparietal cell antibodies are rarely ordered in current practice. Of patients with pernicious anemia, 90% are positive for these antibodies. However, antiparietal cell antibodies are also present in patients with thyroid disease and other autoimmune disorders.
  • Anti-IF antibodies (type I and II) are highly specific for pernicious anemia. However, tests for these are rarely ordered to diagnose or treat patients with megaloblastosis.
• Tests for folate deficiency
  • Serum folate is the earliest indicator of folate deficiency. Serum samples should be collected prior to therapy or transfusions. If necessary, serum can be frozen until the laboratory can perform the test. Folate levels respond rapidly to changes in dietary folate. A low folate level reflects dietary intake during the previous 2-3 days. Conversely, a single meal with normal folate content can restore serum folate levels to normal.
  • The RBC folate level is usually low in patients with folate deficiency. Folate is incorporated into erythrocytes when they are formed, and folate levels do not fluctuate with changes in diet during the lifespan of the RBC. The RBC folate level may not be low in persons with rapidly developing acute folate deficiency. Another limitation of this test is that RBC folate levels are low in more than 50% of patients with cobalamin deficiency, and this test cannot be used to distinguish between these disorders.

Imaging Studies

• Abdominal x-ray films, upper and lower GI series, and CT scans may be useful for detecting and evaluating blind loop syndromes, strictures, and other gastrointestinal tract abnormalities that may cause a blind loop syndrome.

Other Tests

• Cobalamin deficiency - Detection and evaluation of autoimmune disorders, regional ileitis, fish tapeworm infection, Zollinger-Ellison syndrome, pancreatitis, and myeloproliferative disorders
• Folate deficiency - Detect and evaluate pregnancy, malnutrition, and other complications of sprue, chronic hemolysis, and exfoliative dermatitis
• Tests relevant for the diagnosis and evaluation of inborn errors that cause or are associated with cobalamin or folate deficiency

Procedures

• Bone marrow aspiration and biopsy results are useful to confirm the diagnosis, to rule out myelodysplasia, and to assess the iron stores. Marrow is cellular with erythroid hyperplasia. Megaloblastic RBC precursors are abundant, and giant metamyelocytes are present. Iron stores may vary from high to low. The bone marrow begins to convert from megaloblastic to normoblastic within 12 hours, and normalization is complete within 2-3 days. Therefore, bone marrow aspiration should be performed as soon as possible and preferably before therapy if the procedure is considered useful for the patient's treatment.

Histologic Findings

Bone marrow is hypercellular. An increase in erythropoietic activity is reflected by a decreased or reversed myeloid-to-erythroid ratio. Erythroid precursors have megaloblastic features in that they are larger than normoblastic cells and they have immature nuclear development. Cytoplasmic maturation is normal, but nuclear remnants, Howell-Jolly bodies, may be present in the cytoplasm. Giant bands (neutrophils) may be present. Megakaryocytes may be large and hyperlobulated. Iron stores vary from being increased before therapy to decreased if iron is consumed during therapy for megaloblastosis. Bone marrow studies should be performed before therapy because therapy may restore normoblastic erythropoiesis rapidly.

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