A modification of charcoal adsorption immunoassay of gastric intrinsic factor.

serum containing intrinsic factor antibody. The antibody serum itselfbinds a small amount ofvitamin B12. This is ignored by Gottlieb, Lau, Wasserman, and Herbert (1956). R0dbro, Christiansen, and Schwartz (1965) and Irvine, Cullen, Scarth, and Simpson (1968) use normal serum during the determination of total vitamin B12-binding capacity of the gastric juice to 'balance' the antibody serum. However, unless great care is taken in choosing antibody and normal sera, it is unlikely that the vitamin B12-binding capacity of the two would balance in the assay. In the method of Ardeman and Chanarin (1963) additional determinations of vitamin B12-binding capacity of normal and antibody serum are carried out. These are subtracted from the binding capacity of gastric juice + normal serum and gastric juice + antibody serum, respectively. In our opinion these steps involving additional radioactivity counts add to the error of the method. To circumvent these difficulties we have devised a modified assay in which the contribution to vitamin B12-binding by the antibody serum is 'balanced' by the same serum.