title: Dual-isotope separation technique for radioassay of cobalamin.
abstract: Cobalamin is assayed by a dual-isotope separation method using sodium [125I]iothalamate as a marker. Two systems are used: one in which the incompletely-separated bound fraction is counted and compared with the single-isotope method in which the bound fraction is separated by washing (Phadebas radiosorbent assay); and one in which an aliquot of the free fraction is counted. In the dual-isotope method counting bound fractions, about 97% of the supernatant is removed by pouring from silicone fluid separators. The results for serum samples obtained using dual- and single-isotope methods were similar (between run coefficients of variation 5--7%). Experimental errors were smaller in the dual-isotope method. A factor in the kit standards, presumably the absence of proteins, was found to affect the separation technique, resulting in relatively large experimental errors for standards in the single-isotope method. Washing the solid phase in the single-isotope method apparently resulted in a loss of bound isotope. In the dual-isotope method counting free fractions reasonable precision was obtained (coefficient of variation of serum samples 6.6%) even though only about 56% of the supernatant (free fraction) was counted.
mesh_terms: D006801:Humans; D007457:Iodine Radioisotopes; D007483:Iothalamic Acid; D011852:Radioallergosorbent Test / Q000379:methods; D011933:Reagent Kits, Diagnostic; D014805:Vitamin B 12 / Q000097:blood*
chemical_list: D007457:Iodine Radioisotopes; D011933:Reagent Kits, Diagnostic; D007483:Iothalamic Acid; D014805:Vitamin B 12