Abstract
The radiopharmaceutical used in nuclear medicine to detect gastrointestinal bleeding is Technetium 99m-labeled autologous red blood cells (Tc-99m RBC). Labeling of red blood cells is possible because the radionuclide technetium 99m pertechnetate (TcO
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) freely crosses the red blood cell membrane via anion exchange. However, once TcO
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is reduced, it can no longer pass through the red blood cell membrane. Therefore, it follows that if TcO
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is allowed to cross the RBC membrane and subsequently reduced within the cell, Tc-99m will be trapped within the RBC. This is precisely how RBCs are labeled for a nuclear medicine GI bleeding study. The agent used to reduce TcO
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is tin (Sn+2). The protocol involves pretreating red blood cells with tin followed by exposure to TcO
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. The intracellular tin reduces the TcO
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trapping it within the cell. The reduced technetium 99m binds to the beta chain of hemoglobin (see Fig. 36.1