This test may be done in four different stages to find the cause of a low vitamin B12 level.
Stage I: You get two doses of vitamin B12 (cobalamin). You take a small, first dose (a radioactive form of B12) by mouth. You get a second, larger dose by a shot 1 hour later. You then collect your urine over the next 24 hours and deliver it to a lab or your doctor's office. The urine is checked to see if you are absorbing vitamin B12 normally. If Stage I is abnormal, Stage II may be done 3 to 7 days later.
Stage II: You are given radioactive B12 along with intrinsic factor. Intrinsic factor is a protein produced by cells in the stomach lining. The body needs it so the intestines can absorb vitamin B12.
Stage II of the test can tell whether a low vitamin B12 level is caused by problems in the stomach, preventing it from producing intrinsic factor.
If Stage II is abnormal, a Stage III test is done.
Stage III: This test is done after you have taken antibiotics for 2 weeks. It can tell whether abnormal bacterial growth has caused the low vitamin B12 levels.
Stage IV: This test determines whether low vitamin B12 levels are caused by problems with the pancreas. With this test, you take pancreatic enzymes for 3 days. You then take a radioactive dose of vitamin B12.