The Schilling test is a medical investigation used in people with vitamin B12 deficiency. There are a few variations on the exact procedure used.

In the first part of the test, the patient is given radiolabeled vitamin B12 to drink. The most commonly used radiolabels are ⁵⁷Co and ⁵⁸Co. An intramuscular injection of unlabeled vitamin B12 is given at the same time. The patient's urine is then collected over the next 24 hours.

Normally, the ingested vitamin B12 will be absorbed into the body. Since the body already has an adequate store of vitamin B12 delivered in the injection, much of the ingested vitamin B12 will be excreted in the urine.

A normal result shows at least 10% of the radiolabelled vitamin B12 in the urine over the first 24 hours. In patients with pernicious anemia or with deficiency due to impaired absorption, less than 5% of the radiolabeled vitamin B12 is detected.

If an abnormality is found, the test is repeated, this time with additional oral intrinsic factor. If this second urine collection is normal, this shows pernicious anemia. A low result on the second test implies abnormal intestinal absorption (malabsorption), which could be caused by celiac disease, biliary disease, Whipple's disease or liver disease.