Quantitative Assay of Intrinsic Factor Activity by Urinary Excretion of Radioactive Vitamin B₁₂

A technic for assaying consecutively up to 18 intrinsic factor preparations with one pernicious anemia patient is described. The use of repeated flushing doses appeared to have little effect on the respsonse in the urinary excretion test; nine patients were given up to 38 flushing doses of 1 mg. of nonradioactive B₁₂ over a period of 44 days and showed essentially the same response to a standard intrinsic factor preparation at the beginning and end of this period.

The excretion of vitamin B₁₂ in the test increased with increasing amounts of intrinsic factor in each of four different preparations, indicating that large amounts of intrinsic factor did not inhibit absorption. There was no relationship between the urinary excretion values obtained with radioactive B₁₂ alone and the values obtained with radioactive B₁₂ plus standard intrinsic factor in a series of tests on 127 pernicious anemia patients. It appears that total rather than net excretion values are to be preferred in evaluating preparations. Since it has been shown that pernicious anemia patients given intrinsic factor absorb more than 0.5 µg. of radioactive vitamin B₁₂, an oral dose of 2.0 µg. of B₁₂ which allows an absorption of more than 0.5 µg. is preferred in order to extend the range of intrinsic factor test doses that can be assayed.