Immunoassay of gastric intrinsic factor and the titration of antibody to intrinsic factor.

A modification of the charcoal method for the detection and titration of antibody to intrinsic factor in serum and for the assay of intrinsic factor in human gastric juice is described. The antibody was detected in a titre greater than 5 ng units/ml in 52% of the sera of 162 patients with Addisonian pernicious anaemia. False positive results may occur if the serum is withdrawn within 24 hr of the parenteral administration of normal therapeutic doses of vitamin B. In the analysis of human gastric juice, the charcoal method of immunoassay clearly distinguishes between intrinsic factor and other vitamin B binding substances. A good correlation was observed between the gastric acid secretion, the ability to absorb oral radioactive vitamin B in the Schilling test, and the assay of intrinsic factor in the gastric juice. Intrinsic factor as an antigen is stable in acid and alkali media but is destroyed by incubation at 56°C for 30 min. A secretion of less than 100 ng units intrinsic factor in the gastric juice during the post-histamine hour is not compatible with adequate vitamin B absorption and is indicative of Addisonian pernicious anaemia. A good correlation was observed between the immunoassay and the biological activity of preparations of hog intrinsic factor. To correct the absorption of oral vitamin B in the Schilling test in patients with pernicious anaemia 300–600 ng units hog intrinsic factor are required. It is suggested that hog intrinsic factor preparations should be defined in terms of nanogram units intrinsic factor per milligram.