Patent ID: 7344849

Claim:
An assay method for determination of holo-transcobalamin II (holo-TC II) in a body fluid sample having a holo-TC II concentration, said method being capable of detecting holo-TC II in said body fluid sample at a concentration at least as low as 35 pM, said method comprising: contacting said body fluid sample with a specific binding ligand which is an immobilized or immobilizable specific binding antibody or antibody fragment specific for transcobalamin II (TCII) and having an affinity constant of at least 10 9 M −1 and a cross reactivity with haptocorrin of less than 1%, to form bound TC II, separating a ligand bound fraction containing substantially all of said specific binding ligand and the bound TC II from a non-ligand bound fraction containing substantially none of said specific binding ligand, dissociating bound cobalamin from the holo-TC II in the bound fraction into a reduced volume to provide a cobalamin containing sample having a concentration of released cobalamin which is at least 3 times greater than the concentration of holo-TC II in said body fluid sample, and determining said holo-TC II content in said body sample by determining the concentration of the released cobalamin in the cobalamin containing sample.