Shilling

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.