Q. I don’t think I have a good understanding of the Schilling test. If a patient has B12 in their urine does that mean they are absorbing it? If it isn’t in the urine does that indicate that they aren’t absorbing it and may lack intrinsic factor?

A. That’s a good question! For those of you reading this post who haven’t heard of the Schilling test, it is an in-vivo test designed to see if patients are able to absorb B12. It is usually performed to figure out if the patient has a deficiency of intrinsic factor, which you need to make in order to bind and absorb B12. The test has nothing to do with the British (or Scottish) Shilling. It was actually named after Robert Schilling, MD, Professor Emeritus at the University of Wisconsin.

Here’s the physiologic basis behind the Schilling test.
If you are able to absorb B12 normally, you will absorb it through your gut cells and it will get into your bloodstream where it will circulate throughout your body and do its job. If you can’t absorb it correctly, then it won’t get in through your gut mucosa, and it will just stay in your gut and be excreted in your feces.

So, the Schilling test is kind of ingenious because it uses radioactive B12 (so you can measure where it comes out). Also, the patient gets injections of regular (non-radioactive) B12 at the same time to saturate any open B12 binding sites throughout the body.

In a patient who CAN absorb B12 properly, here’s what happens:
1. Give IM B12 to sop up any empty B12 binding sites.
2. Give patient a drink of radioactive B12.
3. Wait a while and see if the B12 is absorbed through the gut mucosal cells and circulates around the body in the blood. It won’t have many places to drop off B12 (because you’ve thought ahead to block all those sites) – so most of the B12 will just stay in the blood and get peed out in the urine. You measure the radioactivity of the urine, and that tells you how much B12 made it through the gut (was absorbed through the gut) and got into the blood.

But in a patient who CAN’T absorb B12 properly, here’s what happens:
1. Give IM B12 to sop up any empty B12 binding sites.
2. Give patient a drink of radioactive B12.
3. Wait a while and see if the B12 is absorbed through the gut mucosal cells and gets into the blood. If the patient can’t absorb B12, then the B12 won’t get through the gut mucosal cells, and it will just stay in the gut (never making it into the blood!) and get pooped out in the feces (instead of in the urine).

If the patient doesn’t have radioactive urine, that means the radioactive B12 was not absorbed. Then, you can perform the test again, giving the patient some intrinsic factor this time to see if the B12 will be absorbed. If there is now radioactive B12 in the urine, you know that the problem was lack of intrinsic factor. If the urine still doesn’t have radioactive B12, then you need to figure out some other cause.

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9 Responses to How does the Schilling test work?

  1. David says:

    I don’t think the Schilling Test is performed anymore in the UK due to lack of the radioactive vitamin and lack of supplies of IF.

    It was quite a useful test to diagnose pernicious anaemia but it did require the patient to collect a 24 hour urine. Incomplete collection made the test inaccurate.

    Is it still used in the rest of the world?

  2. Ramesh Wadhwani says:

    It is better to Measure IF rather than doing all this cumbersome procedure.

  3. Kristine says:

    You’re right – the Schilling test was considered to be the gold standard test for diagnosing pernicious anemia – but it is used less commonly now. There are other tests that are less sensitive – like serum anti-IF antibody testing. Here’s an article that lists the various tests and describes the pros and cons of each.

  4. sun says:

    schilling’s test is not practical but it is highly tested (medical boards), mainly because it requires a conceptual understanding of the different causes of B12 deficiency. For example, if the patient peed radioactive b12 right away, that means that they were simply deficient due to poor intake. if they peed it only after you give exogenous IF WITH b12, that means that they had pernicious anemia. if you are required to give antibiotics beforehand and then they pee the B12, that indicates bacterial overgrowth (most likely ileal). If they pee it after you give pancreatic enzymes that means they had pancreatic insufficiency (recall that it is a pancreatic protease that cleaves rbinder from B12 in the duodenum). Lastly if none of the above work I think you are supposed to assume that they have had an ilectomy or that their ileum is otherwise not working.

    whew. i should probably go study one of the many things I haven’t mastered, but I love me some Schilling’s test!

  5. Kristine says:

    Yup! I bet it will remain a highly-testable topic years from now because it makes you think about how B12 is absorbed and what can cause a deficiency. Nicely put!

  6. Stan says:

    Wow! What an excellent concise explanation. Outstanding job of putting this in a student friendly manner. As a former teacher and current medical student I have to say that you are an outstanding instructor. I will pass this website on to my classmates.

  7. Kristine says:

    Thanks so much, Stan! That means a lot coming from a teacher AND a medical student!!

  8. Hero Omar Hamad says:

    Thank you so much,
    now I’m understanding the schilling test correctly.

  9. Madiha says:

    Very nice explanation.

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