Schilling Test: Diagnosing Vitamin B12 Malabsorption & Pernicious Anemia
Abstract
The Schilling Test evaluates vitamin B12 absorption and is used to identify causes of deficiency such as pernicious anemia or terminal ileum disease.
Synonyms
Radioactive Vitamin B12 Absorption Test With or Without Intrinsic Factor, Vitamin B12 Absorption Test
Test Commonly Includes
Measurement of vitamin B12 absorption before and after administration of intrinsic factor to determine the cause of B12 deficiency.
Patient Care/Preparation
- Patient must fast from midnight prior to the test.
- Discontinue all B-vitamin supplements at least 3 days before testing.
Specimen
24-hour urine collection after radiolabeled B12 ingestion.
Reason to Reject Sample
- Incomplete 24-hour urine collection
- Prior radioisotope scan
- Contaminated urine (e.g., with stool)
- Parenteral B12 not administered
- Patient not fasting
Reference Range
- >10% radioactive B12 excretion: Normal absorption and intrinsic factor function
- <6–7% excretion without and with IF: Suggests intestinal malabsorption
- Low excretion improved by IF: Indicates pernicious anemia
Use
To diagnose vitamin B12 malabsorption due to intrinsic factor deficiency (e.g., pernicious anemia), assess terminal ileum function (e.g., Crohn’s disease), or rule out other small intestine disorders.
Limitations
- Must be the first isotopic test if others are planned.
- Renal insufficiency, pancreatic disease, liver dysfunction, bacterial overgrowth, or antibodies to intrinsic factor may yield false results.
- Incomplete urine collection skews data.
Methodology
The patient is orally administered radiolabeled B12 (usually tagged with 57Co). One hour later, a large parenteral dose of unlabeled B12 is injected to saturate serum binding proteins. Over 24 hours, urine is collected and measured for radioactivity. Part II includes intrinsic factor to distinguish gastric from intestinal causes of malabsorption.
Additional Information
This test is contraindicated in pregnant or lactating women unless necessary. A therapeutic B12 dose or a prior Schilling test must be spaced 2–3 days apart. Bone marrow evaluation and serum B12/folate levels should be performed before testing.
If absorption improves upon addition of intrinsic factor in Part II, pernicious anemia is likely. If absorption remains poor, the cause may lie in the small intestine (e.g., Crohn’s disease, celiac sprue, or surgical resection).
References
- Carethers M. “Diagnosing Vitamin B12 Deficiency,” Geriatrics, 1988, 43:89–112.
- Lindenbaum J. “Laboratory Testing in Megaloblastic Anemia,” Blood, 1983, 61:624–7.
- Williams WJ et al. Hematology, 4th ed, McGraw-Hill, 1990, pp. 462–4.
- Jacobs et al. Laboratory Test Handbook, Lexi-Comp Inc, 1994.
