The Schilling Test for Vitamin B2 Absorption

The absorption of vitamin B12 can be determined by the Schilling test. An oral dose of [57Co] or [58Co]vitamin B12 is given with a parenteral flushing dose of 1 mg of nonradioactive vitamin, and the urinary excretion of radioactivity is followed as an index of absorption of the oral material. Normal subjects excrete 16% to 45% of the radioactivity over 24 hours, whereas patients lacking intrinsic factor or with antiintrinsic factor antibodies excrete less than 5%.

The test can be repeated, giving intrinsic factor orally together with the radioactive vitamin B12 - if the impaired absorption was because of a simple lack of intrinsic factor, and not to antiintrinsic factor antibodies in saliva or gastric juice, then a normal amount of the radioactive material should be absorbed and excreted.

A modified technique permits determination of the absorption in the presence and absence of exogenous intrinsic factor at the same time, by giving intrinsic factor-[57Co]vitamin B12 complex and free [58Co]vitamin B12 together, and measuring the relative amounts of each isotope excreted in the urine.

Atrophic gastritis will cause decreased secretion of gastric acid before there is any impairment of intrinsic factor secretion. This means that the absorption of crystalline vitamin B12, as used in the Schilling test, is normal but the absorption of protein-bound vitamin B12 from foods will be impaired (Section 10.7.1), and the Schilling test will give a false-negative result.

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