The Schilling test was a medical investigation used for patients with vitamin B12 (cobalamin) deficiency. The purpose of the test was to determine how well a patient is able to absorb B12 from their intestinal tract. The test is now considered obsolete and is rarely performed, and is no longer available at many medical centers. It is named for Robert F. Schilling. Process
The Schilling test has multiple stages. As noted below, it can be done at any time after vitamin B12 supplementation and body store replacement, and some clinicians recommend that in severe deficiency cases, at least several weeks of vitamin repletion be done before the test (more than one B12 shot, and also oral folic acid), in order to ensure that impaired absorption of B12 (with or without intrinsic factor) is not occurring due to damage to the intestinal mucosa from the B12 and folate deficiency themselves. Stage 1: oral vitamin B12 plus intramuscular vitamin B12 (without IF)
In the first part of the test, the patient is given radiolabeled vitamin B12 to drink or eat. The most commonly used radiolabels are 57Co and 58Co. An intramuscular injection of unlabeled vitamin B12 is given an hour later. This is not enough to replete or saturate body stores of B12. The purpose of the single injection is to temporarily saturate B12 receptors in the liver with enough normal vitamin B12 to prevent radioactive vitamin B12 binding in body tissues (especially in the liver), so that if absorbed from the G.I.
Complications
Note that the B12 shot which begins the Schilling test is enough to go a considerable way toward treating B12 deficiency, so the test is also a partial treatment for B12 deficiency. Also, the classic Schilling test can be performed at any time, even after full B12 repletion and correction of the anemia, and it will still show if the cause of the B12 deficiency was intrinsic-factor related. In fact, some clinicians have suggested that folate and B12 replacement for several weeks be normally performed before a Schilling test is done, since folate and B12 deficiencies are both known to interfere with intestinal cell function, and thus cause malabsorption of B12 on their own, even if intrinsic factor is being made. This state would then tend to cause a false-positive test for both simple B12 and intrinsic factor-related B12 malabsorption. Several weeks of vitamin replacement are necessary, before epithelial damage to the G.I. tract from B12 deficiency is corrected. Many labs have stopped performing the Schilling test, due to lack of production of the cobalt radioisotopes and labeled-B12 test substances. Also, injection replacement of B12 has become relatively inexpensive, and can be self-administered by patients, as well as megadose oral B12. Since these are the same treatments which would be administered for most causes of B12 malabsorption even if the exact cause were identified, the diagnostic test may be omitted without damage to the patient (so long as follow-up treatment and occasional serum B12 testing is not allowed to lapse).