The RBCs are tagged with radioisotope in 1 of 2 ways.
1- removing blood from a vein. The red blood cells are separated from the rest of the blood sample and then mixed with the radioactive material. The cells with the radioactive material are considered "tagged." A short time later the tagged RBCs are injected into one of your veins.
2- injecting medicine that will allow the radioactive material to attach to your red blood cells. The radioactive material is injected into a vein 15 or 20 minutes after you receive this medicine
This test is most often done to locate the site of bleeding in patients who have blood loss from the colon or small bowel.
A variation on this test, called a ventriculogram, may be performed to check heart function

http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0004292/