Radionuclide-mediated laboratory studies are known as "in vitro" diagnostic testing, and they are performed to measure the amount of a specific radionuclide in body fluid (blood or urine) samples by laboratory analysis. Feces can also be tested for radionuclide concentration. The studies include procedures that determine the ability of the body to absorb a radionuclide or the ability of the body to localize the radionuclide.65 Some of the studies include scanning procedures to obtain immediate or delayed images of an organ or area of the body. A list of commonly used radionuclides and their tissue sites can be found in Table 20-1.