Insolubilized pure human intrinsic factor used for quantitation of cobalamins in serum

The use of insolubilized pure human intrinsic factor as binding protein in an isotope dilution assay for quantitation of serum cobalamins is described. The reference interval was 200-600 pmol/l. The coefficients of variation were 0.05 to 0.13 and the equilibrium constant for association to cyanocobalamin was 2 x 10(11) 1/mol. The major advantage of the presented method is that intrinsic factor is kept saturated with cobalamin and is thereby stable until just prior to use.