Vitamin B12 is transported throughout the body by proteins called transcobalamins. These vitamin transport proteins are found in the blood, tissues, tears,milk, and saliva. The major transport protein for B12 is Transcobalamin I (TCI). This protein transports about 80-90% of vitamin B12 and is transported from the peripheral tissues to the liver. Transcobalamin II (TCII) binds a smaller amount of Vitamins (10-25%) in blood, but is responsible for most of the unsaturated B12 binding capacity(UBBC). TCI and TCII can be indicative of certain disease states.
TCII bound vitamin B12 is decreased in untreated vitamin B12 deficiency. UBBC will continue to rise as the vitamin B12 deficiency progresses. TCI is shown to be increased in myeloproliferative disorders, polycythemia vera, myelofibrosis, chronic myelogenousLeukemia, hepatoma, and metastatic Cancer. TCII is increased in myeloproliferative disorders, liver disease, inflammatory disorders, and Gaucher disease. UBBC can alsobe increased in Pregnancy and with the use of contraceptive Hormones. TCII can be decreased in some shortly after birth presenting with vomiting, weakness, pancytopenia,and megaloblastic Anemia, but is very rare. TCI deficiency shows no signs of vitamin B12 deficiency and is very rare.
Testing for vitamin B12 binding capacity is performed on a blood sample drawn from a vein in the patient’s arm. Additional tests may include Vitamin B12, Folate, Methylmalonic acid (MMA) serum or urine, CBC with differential, Helicobacter pylori,Homocysteine, Erythropoetin, Iron, Iron saturation, and Hemochromatosis.
Reference ranges and specimen collection vary from test method and laboratories performing this test. To properly evaluate your test results, consult with the ordering physician or healthcare provider. If you would like to learn more about this test click herefor further information or you can research one of the references listed below.
Br Med J (Clin Res Ed). 1982 September 25; 285(6345): 840-842. [PMID:-6288165]
Lab Invest. 1988 Mar;58(3):332-7. [PMID:-3347009]
Blood. 1975 Feb;45(2):287-93. [PMID:-1054610]
David S. Jacobs. Laboratory Test Handbook. Hudson, OH, Lexi-Comp Inc (1996); Page 1316.