Schilling test was named after Dr. Robert F. Schilling for his work on detection of radio-cobalt-labeled, vitamin B12 in urine. In spite of this test being no longer used due to recent advances in laboratory techniques, it has been very helpful in the past as a means of detecting the absorption rate of an administered load of vitamin B12 in a patient and aiding in understanding the exact cause of the problem.
The patient is given oral doses of vitamin B12, and their 24-hour urine level of cobalamin is studied.
The Schilling Test
This test is divided into 2 main stages.
The patient is given radiolabeled vitamin B12 orally, following an intramuscular (IM) dose of unlabeled vitamin B12 one hour later. The injection is given to ensure that none of the radioactive B12 binds to any vitamin B12 depleted tissues, for example, the liver. A 24-hour urine collection monitors the absorption and the excretion.
If stage I is abnormal, stage II may be done 3 to 7 days later.
If the previous stage provides an abnormal result, stage 2 can be done to assess whether there is a deficiency of intrinsic factor. Stage 1 is repeated along with an oral dose of intrinsic factor. A 24-hour urine collection is carried out to assess the level of vitamin B12.
Patients presenting with signs and symptoms of cobalamin used to undergo this test. The most common features of vitamin-B12 deficiency include severe macrocytic anemia and variable neurologic abnormalities such as shuffling gait, with no improvement observed upon the administration of folic acid.
During Stage 1, a healthy person will be able to absorb the administered radioactive B12 in their terminal ileum. It will then be excreted in the urine. If there are any defects with the cubam receptor at the terminal ileum, the result will show a low level of labeled cobalamin in urine as it will remain in the intestines and is likely to be excreted in feces. A defect associated with low levels of an intrinsic factor will also produce a similar abnormal result.
Patients with an abnormally low level of cobalamin in the urine in stage 1 usually undergo stage 2. If stage 2 shows a normal level of excreted vitamin B12, it means that the patient has low levels of intrinsic factor and one possible cause is pernicious anemia. If the test still indicates a low level of vitamin B12 in the urine, it means that the patient has a poor intestinal absorption of cobalamin. Further tests should be done to assess the cause of malabsorption that can include Crohn’s disease, small bowel bacterial overgrowth syndrome, and infection by Diphyllobothrium latum.
A normal patient will show a normal level of cobalamin in urine upon administration of the oral dose. If the urine level is abnormal, it can be from either an absorption defect in the terminal ileum or a lack of intrinsic factor.
Excreting 8% to 40% of the radioactive vitamin B12 in the urine within 24 hours is considered a normal finding. The values, however, may fluctuate between laboratories.Stage 2
If the abnormal result of stage 1 improves with administration of Intrinsic factor, it means that the patient has intrinsic factor deficiency. If the patient still has a low urine level of cobalamin, it means that there is an absorption defect in the terminal ileum and further tests should be done to investigate.
If the patient has an acute illness leading to nausea, diarrhea, and vomiting, the level of vitamin B12 available for absorption will be lower, and the results will be suboptimal.
The physician should make sure that the patient has no allergy to the radiolabeled cobalamin administered.
The patient should be advised to inform the physician of any known allergy to any of the materials used. Patients are advised to avoid any food intake for 8 hours before starting the test but can drink water. After the test, the patients can resume their normal eating habits.
Vitamin B12, also known as cobalamin, has a vital role in the normal synthesis of myelin. It is a water-soluble vitamin, and its absorption is essential for many other processes such as DNA synthesis, red blood cell maturation, and fatty acid metabolism. Orally taken vitamin B12 first binds to an R-protein, also called haptocorrin and cobalophilin once it is released from the food. This R-Protein, produced by salivary glands, protects vitamin B12 against the acidic environment of the stomach. In the duodenum, the R-protein is cleaved from the vitamin B12 via pancreatic proteases and an intrinsic-factor (IF) that is produced by the parietal cells of the stomach, binds to the vitamin B12. This intrinsic-factor is important for the proper absorption of vitamin B12 via the cubam receptor located at the terminal ileum. Since there are multiple steps and factors involved in the absorption of cobalamin, a defect in R-binder, intrinsic factor or terminal ileum can lead to vitamin B12 deficiency.
In pernicious anemia, there are autoantibodies produced against the parietal cells. This leads to a lower level of IF, and hence the absorption of cobalamin is affected. In patients with terminal ileum defect such as Crohn’s disease , the cubam receptor is likely to be affected, thus resulting in the non-absorption of the IF-B12 complex into the enterocytes.
Currently, Schilling test is not readily available in most countries as a means of diagnosis for vitamin B12 deficiency. The level of serum vitamin B12 is now a test of choice in many hospitals. Patients with >300 pg/mL are normal, 200 to 300 pg/mL borderline and less than 200 pg/mL classified as deficient in vitamin B12. The levels of homocysteine and methylmalonic acid are also usually investigated and elevated in those patients.
Patients with low levels of cobalamin are tested for autoantibodies to intrinsic factor with an immunoassay. Pernicious anemia is usually caused by the presence of autoantibodies against parietal cells that make intrinsic factors and presence of those autoantibodies are considered as a confirmatory test. The immunoassay has low sensitivity and specificity , and if the physician suspects pernicious anemia and has a negative immunoassay, it is advised to measure gastrin levels. Since parietal cells are also responsible for the production of gastrin level, patients with pernicious anemia will have a low level of gastrin also.