Megaloblastic Anemia

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
B₁₂ and folate levels – use to differentiate single and combined defect
- Folate levels
  - Low folate suggests folate deficiency
  - Serum RBC folate – acceptable alternative test to serum folate testing
  - Suggest concurrent testing with B₁₂
- B12 levels
  - B₁₂ <100 pg/mL – B₁₂ deficiency confirmed; consider evaluation for pernicious anemia
  - B₁₂ 100-400 pg/mL – order methylmalonic acid (MMA)
    - If MMA elevated – B₁₂ deficiency likely; consider pernicious anemia
    - If MMA normal – not pernicious anemia
  - B₁₂ >400 pg/mL – B₁₂ deficiency unlikely; folate deficiency likely
    - If clinical suspicion of B₁₂ deficiency remains – order MMA and check homocysteine levels
      - If MMA and homocysteine elevated – B₁₂ deficiency confirmed
- Normal B₁₂ and folate levels – consider bone marrow biopsy
Parietal cell antibody (PCA) and intrinsic factor (IF)-blocking antibody testing (see Megaloblastic Anemia Testing algorithm)
- PCA is not as helpful as IF-blocking test because of lack of 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
- Alcoholism
- Medication-induced macrocytosis
- Liver disease-associated macrocytosis
- Hypothyroidism
- Infiltrative disorders of the bone marrow
- Nitrous oxide abuse
- Splenectomy-induced macrocytosis
Megaloblastic etiologies
- See information in Clinical Background

Megaloblastic anemias are a group of macrocytic anemias in which the bone marrow shows megaloblastic erythropoieses.

Epidemiology

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

Definitions

Anemia
- Men – HgB <13 g/dL
- Women – HgB <12 g/dL
Macrocytosis – mean corpuscular volume >100 fL

Etiology

Vitamin B₁₂ (cobalamin) deficiency
- Malabsorption
  - Absence of intrinsic factor (IF)
- Achlorhydria
  - Most common in the elderly and patients on acid suppression
- Postgastrectomy syndrome
- Intestinal stasis due to anatomic lesions
- Ileal abnormalities
  - Tropical sprue
  - Inherited 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

Clinical Presentation

Symptoms
- Often based on the presence of anemia
- Pale skin, anorexia, sore tongue, numbness, paresthesias
- >50% present without anemia and have few symptoms
Pernicious anemia – most common cause of B₁₂ deficiency
- Absence of IF
  - Autoimmune destruction of parietal cell antibodies (PCA) is most common etiology
  - 80% have PCA; ≥50% have IF-blocking antibodies
- Most common in patients of northern European descent
- Increased incidence in patients with other autoimmune diseases such as Graves disease, vitiligo, hypoparathyroidism, Addison disease, Hashimoto disease

Treatment

Oral – folate replacement
Parenteral or intranasal – B₁₂ replacement (B₁₂ cannot be replaced orally)
- In pernicious anemia, lifelong replacement is necessary

Tests generally appear in the order most useful for common clinical situations

Test name: CBC with Platelet Count

ARUP #: 0040002

Methodology: Automated Cell Count

Use:

Screen for anemia and assess for macrocytosis

Test name: Vitamin B₁₂ & Folate

ARUP #: 0070160

Methodology: Quantitative Chemiluminescent Immunoassay

Use:

First-line test to differentiate between single and combined defect in macrocytic anemia

Test name: Vitamin B₁₂ with Reflex to Methylmalonic Acid, Serum (Vitamin B₁₂ Deficiency)

ARUP #: 0055662

Methodology: Quantitative Chemiluminescent Immunoassay/Quantitative High Performance Liquid Chromatography-Tandem Mass Spectrometry

Use:

Use in workup of megaloblastic anemia when B₁₂ is 100-400 pg/mL

Limitations:

Renal failure may increase serum methylmalonic acid (MMA) concentrations in patients who do not have vitamin B₁₂ deficiency

Test name: Folate, RBC

ARUP #: 0070385

Methodology: Quantitative Chemiluminescent Immunoassay

Use:

Include in differential workup of patients with megaloblastic anemia

Acceptable alternative to serum folate

Test name: Intrinsic Factor Blocking Antibody

ARUP #: 0070210

Methodology: Qualitative Enzyme-Linked Immunosorbent Assay

Use:

Include in differential workup of patients with megaloblastic anemia and suspicion of pernicious anemia

Test name: Parietal Cell Antibody, IgG

ARUP #: 0050596

Methodology: Qualitative Indirect Fluorescent Antibody

Use:

Include in differential workup of patients with megaloblastic anemia and suspicion of pernicious anemia

Limitations:

Although 93% of patients with pernicious anemia will be positive for parietal cell antibody, the fact that such antibodies are found with increased frequency in unaffected family members, as well as in patients with other autoimmune diseases, suggests these antibodies do not cause disease by themselves

Test name: Gastrin

ARUP #: 0070075

Methodology: Quantitative Chemiluminescent Immunoassay

Use:

Include in differential workup of patients with megaloblastic anemia and suspicion of pernicious anemia in the face of negative antibody tests

Test name: Methylmalonic Acid, Serum or Plasma (Vitamin B₁₂ Deficiency)

ARUP #: 0099431

Methodology: Quantitative Liquid Chromatography-Tandem Mass Spectrometry

Use:

Evaluate patients with megaloblastic anemia and normal B₁₂ levels but neurological symptoms present

Test name: Homocysteine, Total

ARUP #: 0099869

Methodology: Quantitative Enzymatic

Use:

Evaluate patients with megaloblastic anemia and normal B₁₂ levels but neurological symptoms present

Confirm functional deficiency of vitamin B₁₂ or folate

False elevations of plasma or serum homocysteine may occur if the plasma or serum is not promptly separated from the cells at the time of collection

Additional Tests Available

Test name: Vitamin B₁₂

ARUP #: 0070150

Methodology: Quantitative Chemiluminescent Immunoassay

Comments:

Preferred test is B₁₂ reflex to MMA

Test name: Folate, Serum

ARUP #: 0070070

Methodology: Quantitative Chemiluminescent Immunoassay

Comments:

Preferred test is B₁₂ & folate panel

Test name: Vitamin B12 Binding Capacity

ARUP #: 0070260

Methodology: Quantitative Radioimmunoassay

Comments:

General

ARUP Institute for Clinical and Experimental Pathology®

Comprehensive Review: September 2011
Last Update: August 2012

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