vitamin B12 (Cyanocobalamin and methylmalonic acid [MMA])
المؤلف:
Kathleen Deska Pagana, Timothy J. Pagana, Theresa Noel Pagana.
المصدر:
Mosbys diagnostic and laboratory test reference
الجزء والصفحة:
15th edition , p969-970
2025-10-06
363
Type of test Blood; urine
Normal findings
Vitamin B12 : 160-950 pg/mL or 118-701 pmol/L (SI units)
MMA: < 3.6 μmol/mmol creatinine
Test explanation and related physiology
Vitamin B12 is necessary for conversion of the inactive form of folate to the active form. This function is most notable in the formation and function of RBCs. Vitamin B12 deficiency, like folic acid deficiency, causes anemia. The RBCs formed in light of these deficiencies consist of large megaloblastic RBCs. These RBCs cannot conform to the size of small capillaries. Instead, they fracture and hemolyze. The shortened life span ultimately leads to anemia.
In the stomach, gastric acid detaches vitamin B12 from its binding proteins. Intrinsic factor (IF), which is necessary for vita min B12 absorption in the terminal ileum, is made in the stomach mucosa. Without IF, vitamin B12 cannot be absorbed. Deficiency of IF is the most common cause of vitamin B12 deficiency (pernicious anemia). The next most common cause of vitamin B12 deficiency is lack of gastric acid to separate the ingested vitamin B12 from its binding proteins. This is common in patients who have had gastric surgery. A third cause of vitamin B12 deficiency is malabsorption caused by diseases of the small terminal ileum. Vitamin B12 deficiency is common in poorly nourished elderly people and in vegetarians.
Serum B12 is a measurement of recent B12 ingestion. More prolonged B12 deficiency is better and more easily measured by urinary methylmalonic acid (MMA) measurement. Elevated serum MMA levels and urinary excretion of MMA are direct measures of tissue vitamin B12 activity. The active form of B12 is essential in the intracellular conversion of L-methylmalonyl coenzyme A (MMA CoA) to succinyl CoA. Without B12 , MMA CoA metabolism is diverted to make large quantities of MMA. MMA is then excreted by the kidneys. MMA testing is the most sensitive test for B12 deficiency.
Procedure and patient care
• See inside front cover for Routine Blood Testing.
• Fasting: verify with laboratory
• Blood tube commonly used: red
* Instruct the patient not to consume alcoholic beverages before the test. Check the time period with the physician or laboratory.
• Draw the specimen before starting vitamin B12 therapy.
Abnormal findings
Increased levels
- Leukemia
- Polycythemia vera
- Severe liver dysfunction
- Myeloproliferative disease
Decreased levels
- Pernicious anemia
- Malabsorption syndromes
- Inflammatory bowel disease
- Intestinal worm infestation
- Atrophic gastritis
- Zollinger-Ellison syndrome
- Large proximal gastrectomy
- Resection of terminal ileum
- Achlorhydria
- Pregnancy
- Vitamin C deficiency
- Folic acid deficiency
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