Sensitive test for antibody type I to intrinsic factors.

The disturbing effect of vitamin B binding to test serum in assay of type I antibody to intrinsic factor was reduced by pre-treatment of the serum. The B-binding sites of serum proteins were blocked by adding unlabelled cyanocobalamin, the unbound excess of which was adsorbed to albumin-coated charcoal. Using an earlier described sensitive small-scale gel filtration technique for antibody assay, it was possible to decrease the smallest amount of antibody demonstrable from 2 to 0·5 ng B units per ml of serum by the pre-treatment of serum. This four-fold increase in sensitivity implied that the positive results rose from 60% to 76% in a material of twenty-five pernicious-anaemia patients. No positive results were obtained in thirty-four control subjects or in a serum pool from twenty blood donors. The pre-treatment, including charcoal adsorption of unbound B, also eliminated the risk of false-positive results because of recent B injection.