Vitamin B12 deficiency schilling test

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Step 1: One µg of radioactive crystalline B12 is taken orally. Step 2: Gastric acid and pepsin free vitamin B12 from food proteins. This step is not required when crystalline B12 is utilized as the test dose. B12 attaches to "R" binders (R) produced by the stomach, which have a higher affinity for B12 than intrinsic factor (IF), also produced by the stomach. Step 3: Pancreatic proteases degrade the "R" binders, allowing formation of the B12-IF complex, the specific form absorbed by the terminal ileum. Step 4: One hour after the test dose, a 1000 µg "flushing" dose of non-radioactive B12 is given to saturate B12 binders (transcobalamin). Step 5:If present, bacteria in "blind loops" in the duodenum or jejunum preferentially utilize vitamin B12, allowing none to be available at the site of absorption. Step 6: With blood and tissue B12 binding sites blocked by the "flushing" dose, the B12/IF complex is absorbed by the terminal ileum, and the B12 is excreted in the urine (step 7), which is assayed for its content of radioactivity. The test can be repeated with the addition of missing factors (eg, intrinsic factor, pancreatic extract), or following the use of nonabsorbable antibiotics (blind loops and/or bacterial overgrowth present), or gluten-free diet (celiac disease). 

Note: While the Schilling test is rarely used today, steps 2, 3, and 6, as described above, delineate the normal pathway for B12 absorption.

Test	Gastrectomy, pernicious anemia	Celiac disease*	Bacterial overgrowth	Ileal resection or disease•	Pancreatic insufficiency
Vitamin B12	Low	Low	Low	Low/normal	Low
Vitamin B12 + intrinsic factor	Normal	Low	Low	Low/normal	Low
Vitamin B12 + antibiotics	n/a	Low	Normal	Low/normal	Low
Vitamin B12 + gluten-free diet	n/a	Normal	n/a	Low/normal	Low
Vitamin B12 + pancreatic enzymes	n/a	n/a	n/a	n/a	Normal

* The Schilling test may be normal in patients with celiac disease because the terminal ileum is frequently spared. n/a, these stages of the Schilling test are not needed for the disorder. 

• Results depend upon the length of resection or the extent of disease. Values will not normalize with >100 cm of resection. Values may normalize after treatment of active Crohn's disease.