Schilling test
Indications
Procedure
Stage I.
Tests the ability to absorb cobalamin from the small intestine. The patient is given a 0.5-2.0 ug oral dose of radiolabeled cobalamin in a fasting state, followed in 2 hours by a large parenteral dose of unlabeled cobalamin in order to saturate the tissue-binding sites. Saturation allows the absorbed radiolabeled cobalamin to be excreted in the urine. Radioactivity is measured in the urine collected over the next 24 hours. Normal levels of radioactivity in the urine (> 7% of a 1 ug dose) indicate normal absorption of cobalamin. An abnormally low level of radioactivity in the urine indicates poor absorption & the need for Stage II testing.
Stage II.
Tests whether lack of intrinsic factor is the cause of cobalamin deficiency. The test is repeated, but this time the oral dose is given together with intrinsic factor (hog or recombinant). Normal radioactivity in the 24 hour urine specimen indicates a lack of intrinsic factor is the cause of the patient's cobalamin deficiency. Low urine radioactivity in Stage II testing indicates intestinal malabsorption.
Stage III
Test is conducted after a course of treatment with antibiotics or anti-inflammatory agents.
Stage IV
In patients with inflammatory diseases affecting the terminal ileum, the ileal receptor site for intrinsic factor may be damaged. The Schilling test may normalize after treatment with prednisone or sulfasalazine.
Stage V
Some patients may absorb vitamin B12 in water as in the Schilling test, but not in food bound to protein. A modification of the Schilling test using vitamin B12 bound to egg albumin in Stage I testing may be useful.
Interpretation
Distinguishing features of malabsorption syndromes
| Disorder | Co-58 | IF | Pancreatic Enzymes | Antibiotics |
|---|---|---|---|---|
| pernicious anemia | low | normal | low | low |
| chronic anemia | low | low | normal | low |
| bacterial overgrowth | low | low | low | normal |
| ileal disease | low | low | low | low |
Co-58: Stage 1 (excretion of Co-58 in urine)
IF: Stage 2 (addition of intrinsic factor)
Pancreatic Enzymes: as in stage 2, but inclusion of pancreatic enzymes rather than intrinsic factor
Antibiotics: Stage 3 (after 5 day treatment with antibiotics)