This test may be performed in four different stages to find the cause of low vitamin B12 levels.
Stage I: You will get two doses of vitamin B12 (cobalamin). You will take a small, first dose (a radioactive form of B12) by mouth. You will a second, larger dose by a shot 1 hour later. You will need to collect your urine over the next 24 hours, and deliver it to a lab or your doctor's office. The urine will be checked to see if you are absorbing vitamin B12 normally. For information on collecting the urine sample, see: 24-hour urine collection
If Stage I is abnormal, Stage II may be done 3 - 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 efficiently.
Stage II of the test can tell whether low vitamin B12 levels are caused by problems in the stomach that prevent it from producing intrinsic factor.
If a Stage II test is abnormal, a Stage III test is performed.
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 will take pancreatic enzymes for three days, followed by a radioactive dose of vitamin B12.