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.
How does the Schilling test work?
Source: Pathology Student