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Discussion Starter · #1 · (Edited)
Although parietal cells antibodies and antibodies to intrinsic factor tests have largely replaced the classic Schilling test the latter is still sometimes seen in USMLE questions because it requires a good grasp of the pathophysiology of pernicious anemia to understand it.

So how they do the Schilling test?
Step 1: We have to saturate the B12 receptors with an Intramuscular injection of cyanocobalamine (vit. B12) because otherwise the body would be hungry for the oral dose of B12 and it won't appear in urine even if there was not pernicious anemia.
Step 2: Give a radiolabled oral B12 dose-> now if B12 appeared in the urine then the absorptive mechanisms must be working and the case is due to dietary deficiency. -> if it did not appear in the urine then we have now a case of B12 malabsorption but we don't know yet it's pernicious anemia or not therefore we proceed to step 3.
Step 3: We give an intrinsic factor and re-give the oral radiolabled B12. -> if the B12 appeared in the urine then the diagnosis is pernicious anemia. -> if the B12 still did not show up in the urine then it's malabsorption but not due to pernicious anemia and most commonly due to ileal resection or small bowel infection and bacterial over growth or fish tapeworm infestation.
Step 4: Giving tetracycline is sometimes done after this. If it corrects the abnormality then bacterial growth is suspected. Similarly giving pancreatic enzyme extracts with the oral B12 should guide towards pancreatic insufficiency if it corrects the abnormality.
 
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