Vitamin B12 Blood Test, Unsaturated Binding Capacity - NOT CURRENTLY AVAILABLE

Differential diagnosis of polycythemia vera from secondary/relative polycythemias; evaluate macrocytic/megaloblastic anemia; diagnose congenital absence of transcobalamin II or cobalophilin (transcobalamin I and III) Serum transport of vitamin B12 is accomplished by normally occurring proteins termed transcobalamins including I (an α-globulin), II (a β-globulin), and III (a group of transport factors − “R-type” binders or binder III − found also in some tissues, saliva, milk, and tears). The term “R-type” refers to binding protein with “rapid” mobility on electrophoresis. A family of immunologically identical proteins is known, not all of which have the initially described rapid mobility. They are known also as cobalophilins. Transcobalamin I is the major B12 transport protein and bears immunologic identity to granulocyte cobalophilin. Isoelectric focusing has shown that the cobalophilins are a microheterogeneous group of plasma binding proteins. Stenman has reviewed this subject in detail.1 Cobalophilin is increased in diseases characterized by excess granulocyte production, reactive leukocytosis, chronic myelogenous leukemia, and other myeloproliferative states, in particular polycythemia vera. UBBC levels were increased in over two-thirds of cases with polycythemia vera, while nearly 90% of secondary/relative polycythemia patients had normal levels.2 Very high levels have been reported in some patients with hepatoma.3,4 The transcobalamins are normally about 25% saturated with vitamin B12.