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Introduction ⬇

Vitamin B12 (VB12)

VB12, also known as the antipernicious anemia factor, is necessary for the production of RBCs. It is obtained only from ingestion of animal protein and requires an intrinsic factor for absorption. Both VB12 and folic acid depend on a normally functioning intestinal mucosa for their absorption and are important for the production of RBCs. Levels of VB12 and folate are usually tested in conjunction with one another because the diagnosis of macrocytic anemia requires measurement of both.

This determination is used in the differential diagnosis of anemia and conditions marked by high turnover of myeloid cells, as in the leukemias. When binding capacity is measured, it is the unsaturated fraction that is determined. The measurement of unsaturated VB12–binding capacity (UBBC) is valuable in distinguishing between untreated polycythemia vera and other conditions in which there is an elevated Hct.

Normal Findings ⬆ ⬇

Adults: 200–800 pg/mL or 147–590 pmol/L

Older adults (60–90 years): 110–770 pg/mL (81–568 pmol/L)

Newborns: 160–1300 pg/mL or 118–959 pmol/L

UBBC: 600–1400 pg/mL or 443–1033 pmol/L

Procedure ⬆ ⬇

  1. Obtain a fasting venous blood sample of at least 2 mL, using a red-topped tube or serum separator tube (SST). Label the specimen with the patient’s name, date and time of collection, and test(s) ordered.

  2. Obtain the specimen before an injection of VB12 is administered and before a Schilling test is done.

Clinical Implications ⬆ ⬇

  1. Decreased VB12 (<200 pg/mL or <147 pmol/L) is associated with:

    1. Pernicious anemia (megaloblastic anemia)

    2. Malabsorption syndromes and inflammatory bowel disease

    3. Fish tapeworm infestation

    4. Primary hypothyroidism

    5. Loss of gastric mucosa, as in gastrectomy and resection

    6. Zollinger-Ellison syndrome

    7. Blind loop syndromes (bacterial overgrowth)

    8. Vegetarian diets (dietary insufficiency)

    9. Folic acid deficiency

    10. Iron deficiency may be present in some patients (e.g., gastrectomy)

  2. Increased VB12 (>800 pg/mL or >590 pmol/L) is associated with:

    1. Chronic granulocytic leukemia, lymphatic and monocytic leukemia

    2. CKD

    3. Liver disease (hepatitis, cirrhosis)

    4. Some cases of cancer, especially with liver metastasis

    5. Polycythemia vera

    6. Congestive heart failure

    7. Diabetes

    8. Obesity

    9. COPD

  3. Increased UBBC is found in:

    1. Sixty percent of cases of polycythemia vera. (This test is normal in secondary relative polycythemia, aiding in the differential diagnosis of these two states.)

    2. Reactive leukocytosis (leukemoid reaction)

    3. CML

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and procedure.

  2. Alert patient that overnight fasting from food is necessary. Water is permitted.

  3. Withhold VB12 injection or supplements before the blood is drawn.

  4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Have the patient to resume normal activities and diet.

  2. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings; explain the need for possible follow-up testing and treatment. Monitor for anemia, leukemia, or polycythemia.

  3. Follow guidelines in Chapter 1 for safe, effective, informed posttest care. see Appendix C for more information on vitamin testing.

Clinical Alert

  1. Persons who have recently received therapeutic or diagnostic doses of radionuclides will have unreliable results.

  2. The Schilling test is used to confirm pernicious anemia and to determine whether VB12 deficiency is caused by malabsorption.

  3. see Appendix C for more information on nutritional status of VB12.

Interfering Factors ⬆

The following result in increased VB12 values:

  1. Pregnancy

  2. Blood transfusion

  3. Older persons

  4. High vitamin C and A doses

  5. Smoking

  6. Drugs capable of interfering with VB12 absorption (see Appendix E)