In the first part of the test, the patient is given radiolabeled vitamin B to drink or eat. The most commonly used radiolabels are 57Co and 58Co. 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.