Schilling test

The Schilling test was a medical investigation used for patients with vitamin B₁₂ (cobalamin) deficiency.^[1] 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.^[2]

The Schilling test has multiple stages.^[3] 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.

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^{[clarification needed]} 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. tract, it will pass into the urine. The patient's urine is then collected over the next 24 hours to assess the absorption.

Normally, the ingested radiolabeled vitamin B₁₂ will be absorbed into the body. Since the body already has liver receptors for transcobalamin/vitamin B₁₂ saturated by the injection, much of the ingested vitamin B₁₂ will be excreted in the urine.

• A normal result shows at least 10% of the radiolabeled vitamin B₁₂ in the urine over the first 24 hours.
• In patients with pernicious anemia or with deficiency due to impaired absorption, less than 10% of the radiolabeled vitamin B₁₂ is detected.

The normal test will result in a higher amount of the radiolabeled cobalamin in the urine because it would have been absorbed by the intestinal epithelium, but passed into the urine because all hepatic B12 receptors were occupied. An abnormal result is caused by less of the labeled cobalamin to appear in the urine because it will remain in the intestine and be passed into the feces.

If an abnormality is found, i.e. the B12 in the urine is only present in low levels, the test is repeated, this time with additional oral intrinsic factor.

• If this second urine collection is normal, this shows a lack of intrinsic factor production. This is by definition pernicious anemia.
• A low result on the second test implies abnormal intestinal absorption (malabsorption), which could be caused by coeliac disease, biliary disease, Whipple's disease, small bowel bacterial overgrowth syndrome, fish tapeworm infestation (Diphyllobothrium latum), or liver disease. Malabsorption of B₁₂ can be caused by intestinal dysfunction from a low vitamin level in-and-of-itself (see below), causing test result confusion if repletion has not been done for some days previously.

This stage is useful for identifying patients with bacterial overgrowth syndrome. The physician will provide a course of 2 weeks of antibiotics to eliminate any possible bacterial overgrowth and repeat the test to check whether radio-labeled Vitamin B12 would be found in urine or not.

This stage, in which pancreatic enzymes are administered, can be useful in identifying patients with pancreatic insufficiency. The physician will give 3 days of pancreatic enzymes followed by repeating the test to check if radio-labeled Vitamin B12 would be detected in urine.

In some versions of the Schilling test, B₁₂ can be given both with and without intrinsic factor at the same time, using different cobalt radioisotopes ⁵⁷Co and ⁵⁸Co, which have different radiation signatures, in order to differentiate the two forms of B₁₂. This is performed with the 'Dicopac' kitset. This allows for only a single radioactive urine collection.^[4]

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,^[5] 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).

It is possible for use of other radiopharmaceuticals to interfere with interpretation of the test.^[6]

• MedlinePlus Encyclopedia: 003572