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Vitamin B12 - Deficiency Test

The vitamin B12 radioisotopic assay of competitive binding is a quantitative analysis of serum levels of vitamin B12 (also called cyanocobalamin, antipernicious anemia factor, or extrinsic factor). This test is usually performed concurrently with measurement of serum folic acid levels.

A water-soluble vitamin containing cobalt, vitamin B12 is essential to hematopoiesis, deoxyribonucleic acid synthesis and growth, myelin synthesis, and central nervous system (CNS) integrity. This vitamin is found almost exclusively in animal products, such as meat, shellfish, milk, and eggs. (See Cobalt: Critical trace element.)

COBALT: CRITICAL TRACE ELEMENT

A trace element found mainly in the liver, cobalt is an essential component of vitamin B12 and therefore is a critical factor in hematopoiesis. A balanced diet supplies sufficient cobalt to maintain hematopoiesis, primarily through foods containing vitamin B12.

However, excessive ingestion of cobalt may have toxic effects. Toxicity has occurred, for example, in individuals who consumed large quantities of beer containing cobalt as a stabilizer, resulting in heart failure from cardiomyopathy. Since quantitative analysis of cobalt alone is difficult because of the minute amount found in the body, cobalt is often measured by bioassay as part of vitamin B12.

Normal cobalt reference values range from 0.007 to 6 ug/dl.

Purpose

• To aid differential diagnosis of megaloblastic anemia, which may be due to a deficiency of vitamin B12 or folic acid
• To aid differential diagnosis of CNS disorders that are affecting peripheral and spinal myelinated nerves

Patient preparation

• Explain to the patient that this test determines the amount of vitamin B12 in the blood.
• Instruct the patient to observe an overnight fast before the test.
• Tell him that this test requires a blood sample. Explain who will perform the venipuncture and when.
• Reassure him that although he may feel some discomfort from the needle puncture and the pressure of the tourniquet, collecting the sample takes only a few minutes.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 7-ml royal-blue-top tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.
• Remove diet restrictions.

Precautions

• Handle the sample gently to prevent hemolysis, and send it to the laboratory immediately.

Reference values

Normally, serum vitamin B12 values range from 190 to 900 ng/L.

Abnormal findings

Decreased serum levels may indicate inadequate dietary intake, especially if the patient is a strict vegetarian. Low levels are also associated with malabsorption syndromes (such as celiac disease), isolated malabsorption of vitamin B12, hypermetabolic states (such as hyperthyroidism), pregnancy, and CNS damage (posterolateral sclerosis or funicular degeneration, for example).

Elevated levels of serum vitamin B12 may result from excessive dietary intake; hepatic disease, such as cirrhosis or acute or chronic hepatitis; and myeloproliferative disorders such as myelocytic leukemia.

Interfering factors

• Failure to observe pretest restrictions
• Hemolysis due to rough handling of the sample
• Neomycin, metformin, anticonvulsants, and ethanol (possible decrease)
• Oral contraceptives (increase)