Schilling test: physiologic basis for and use as a diagnostic test.

With the advent of binding assays for vitamin B12 in blood, the Schilling test, which involves administration of radioactive B12 to a patient and subsequent urine collection for 24 to 48 h, fell into disuse in many laboratories. However, the test is still the only way to actually measure whether vitamin B12 is being absorbed through the terminal ilium. By administering radioactive vitamin B12, along with a preparation of intrinsic factor (IF), lack of functional IF may also be demonstrated. The Schilling test requires that attention be paid to a number of parameters, including the amount of radioactive vitamin B12 administered and the completeness of the urine collection. These factors, and others required for correct performance of the test, are discussed in this article.