In the first part of the test, the patient is given radiolabeled vitamin B to drink or eat. The most commonly used radiolabels are Co and Co. An intramuscular injection of unlabeled vitamin B is given an hour later. This is not enough to replete or saturate body stores of B. The purpose of the single injection is to temporarily saturate B receptors in the liver with enough normal vitamin B to prevent radioactive vitamin B 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 B will be absorbed into the body. Since the body already has liver receptors for transcobalamin/vitamin B saturated by the injection, much of the ingested vitamin B will be excreted in the urine.
* A normal result shows at least 10% of the radiolabeled vitamin B 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 B 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.