Vitamin B12 deficiency

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  • Vitamin B12, also known as cobalamin, cyanocobalamin
  • Ingested in dietary form
    • Binds to intrinsic factor, a protein secreted by gastric parietal cells
    • B12-intrinsic factor complex then absorbed in the terminal ileum
    • Stored in liver, takes up to 3 years to deplete hepatic stores after ceasing intake
  • Causes of deficiency:
    • Decreased intake (e.g. strict vegan diet)
    • Competition in gut (e.g. blind loop syndrome, fish tapeworm)
    • Intrinsic factor deficiency
      • Pernicious anemia (associated with atrophic gastritis, IgA deficiency, polyglandular endocrine failure)
      • Post-gastrectomy
    • Decreased ileal absorption (e.g. post ileectomy, Crohn's disease)
    • Pancreatic insufficiency
    • H. pylori infection

Clinical Features

Differential Diagnosis


RBC Loss

RBC consumption (Destruction/hemolytic)

Impaired Production (Hypochromic/microcytic)

  • Iron deficiency
  • Anemia of chronic disease
  • Thalassemia
  • Sideroblastic anemia

Aplastic/myelodysplastic (normocytic)

  • Marrow failure
  • Chemicals (e.g. ETOH)
  • Radiation
  • Infection (HIV, parvo)

Megaloblastic (macrocytic)

Altered mental status

Diffuse brain dysfunction

Primary CNS disease or trauma


General Psychiatric


  • CBC, peripheral smear, reticulocyte count
    • Macrocytic anemia (MCV>100 fL), usually moderate to severe
    • High RDW
    • Low reticulocyte count
    • Hypersegmented neutrophils
    • If severe, low WBC and low platelets
  • Serum B12
    • Normal: > 210 pg/mL
    • Deficiency: <170 pg/mL
    • Symptomatic patients usually have levels <100 pg/mL
  • LDH, bilirubin may be elevated
  • Evaluate for underlying causes and complications


  • Vitamin B12
    • Normal absorption: 1000 mcg/day PO
    • Malabsorption: 100-1000 mcg IV/IM or deep SC injection daily for 6 or 7 days
    • Nasal spray: 500 mcg intranasally in one nostril weekly
  • Folic acid: 1mg daily
  • Transfusion rarely needed


See Also

Vitamin deficiencies

External Links