Megaloblastic Anemia

Macrocytosis refers to a blood condition in which red blood cells are larger than normal; the condition can occur with or without associated anemia. Macrocytic anemia can be considered as nonmegaloblastic or megaloblastic. Megaloblastic anemias are a group of macrocytic anemias in which the bone marrow shows megaloblastic erythropoieses. Laboratory testing may include a CBC, peripheral blood smear, and reticulocyte count.

Quick Answers for Clinicians

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Indications for Testing

Anemia with macrocytosis

Laboratory Testing

  • CBC
    • Blood smear – may view macro-ovalocytes, anisocytosis, and hypersegmented nuclei in white blood cells
    • Hemoglobin and hematocrit – if anemia not present, evaluate for nonmegaloblastic causes of macrocytosis
  • Reticulocyte count – usually low; if elevated, proceed with hemolytic evaluation
  • Vitamin B12 and folate levels
    •  Folate levels
      • Do not order red blood cell folate level testing
      • Not necessary testing for most patients due to folate supplementation in U.S. foods (Gudgeon, 2015; Gilfix, 2014)
    • Vitamin B12 levels
      • Vitamin B12 <100 pg/mL – vitamin B12 deficiency confirmed; consider evaluation for pernicious anemia
      • Vitamin B12 100-400 pg/mL – order methylmalonic acid (MMA)
        • MMA elevated – vitamin B12 deficiency likely; consider pernicious anemia
        • MMA normal – not pernicious anemia
      • Vitamin B12 >400 pg/mL – vitamin B12 deficiency unlikely
        • If clinical suspicion of vitamin B12 deficiency remains – order MMA and check homocysteine levels
          • MMA and homocysteine elevated – vitamin B12 deficiency confirmed
    • Normal vitamin B12 – consider other causes of megaloblastic anemia or perform bone marrow biopsy
  • Parietal cell antibody (PCA) and intrinsic factor (IF)-blocking antibody testing (see Megaloblastic Anemia Testing algorithm)
    • PCA – not as helpful as IF-blocking test; lacks specificity
    • IF positive – pernicious anemia confirmed
    • IF negative – order PCA
      • PCA positive – pernicious anemia confirmed
      • PCA negative – order gastrin (serum)
        • Gastrin <100 pg/mL – not pernicious anemia
        • Gastrin >100 pg/mL – pernicious anemia (indirect confirmation)

Differential Diagnosis

  • Nonmegaloblastic etiologies
  • Megaloblastic etiologies – see information in Background



  • Prevalence – macrocytosis occurs in 2-4% of the population
  • Age – usually occurs in older adults


  • Anemia
    • Male – hemoglobin (Hb) <13 g/dL
    • Female – Hb <12 g/dL
  • Macrocytosis – mean corpuscular volume >100 fL


  • Vitamin B12 (cobalamin) deficiency
    • Malabsorption
    • Absence of intrinsic factor (IF) (pernicious anemia)
    • Achlorhydria – most common in the elderly and patients on acid suppression
    • Postgastrectomy syndrome
    • Intestinal stasis due to anatomic lesions
    • Ileal abnormalities
      • Tropical sprue
      • Inherited vitamin B12 disorders
  • Folic acid deficiency
    • Malabsorption, inadequate intake
    • Drugs (anticonvulsants, 5-fluorouracil, methotrexate)
    • Metabolic disorders
    • Inherited disorders of folate
  • Combined deficiencies of folic acid and cobalamin are not uncommon


  • Pernicious anemia
    • Most common cause of vitamin B12 deficiency
    • Absence of IF
      • Autoimmune destruction of parietal cell antibodies (PCA) is most common etiology
      • 80% have PCA; ≥50% have IF-blocking antibodies

Clinical Presentation

  • Often present with anemia
    • Pale skin
    • Anorexia
    • Sore tongue
    • Numbness
    • Paresthesias
  • >50% present without anemia and have few symptoms
  • Pernicious anemia – increased incidence of other autoimmune diseases such as Graves disease, vitiligo, hypoparathyroidism, Addison disease, Hashimoto disease

ARUP Lab Tests

Screen for anemia and assess for macrocytosis

Assess erythropoiesis in hematologic conditions

Aid in detection of vitamin B12 and folate deficiency in individuals with macrocytic or unexplained anemia, or unexplained neurologic disease

Preferred reflex test for detection of vitamin B12 deficiency in individuals with macrocytic or unexplained anemia, or unexplained neurologic disease

Reflex pattern: if vitamin B12 is <300 pg/mL, methylmalonic acid, serum will be added

Aid in detection of folate deficiency

Confirm pernicious anemia as etiology of megaloblastic anemia

Recommended for evaluation of pernicious anemia or immune-mediated deficiency of vitamin B12 with or without megaloblastic anemia

Negative results do not rule out pernicious anemia

Aid in diagnosis of carcinoid and gastrinoma tumors

Evaluate vitamin B12 deficiency in individuals with macrocytic or unexplained anemia, or unexplained neurologic disease in patients with megaloblastic anemia and normal vitamin B12 levels with neurological symptoms present and suspicion for vitamin B12 deficiency

Preferred test is vitamin B12 test that reflexes to serum methylmalonic acid (MMA)

Acceptable screening test for disorders of methionine metabolism (congenital hyperhomocysteinemia)

Not recommended for risk assessment of cardiovascular disease or venous thromboembolism

Related Tests

Aid in detection of vitamin B12 deficiency in individuals with macrocytic or unexplained anemia, or unexplained neurologic disease

Aid in detection of folate deficiency

Preferred test is vitamin B12 and folate panel

Medical Experts



Elizabeth L. Frank, PhD, DABCC
Professor of Clinical Pathology, University of Utah
Medical Director, Analytic Biochemistry, Calculi and Manual Chemistry; Co-Medical Director, Mass Spectrometry, ARUP Laboratories


Additional Resources
Resources from the ARUP Institute for Clinical and Experimental Pathology®