I believe you are referring to a nuclear scan test, where a quantity of blood is withdrawn in order to tag it with a radioactive isotope or to mix it with medicine, whereupon it is put back into the patient. This allows for its detection during certain medical tests, such as MUGA scan to examine how well one's heart is functioning as a pump, and also useful for some GI bleeds. At our facility, a tube of blood is drawn, taken to the lab, tagged, and then reinjected into the patient before having the scan completed. Hope this helps!