Which reagent is used to prevent coagulation in red blood cells labeled with Cr51 sodium chromate?

The correct answer is ACD solution, which stands for Acid-Citrate-Dextrose solution. ACD solution is commonly used in laboratory settings to prevent coagulation in blood samples because it chelates calcium ions, which are essential for the clotting process. By binding to calcium, ACD effectively inhibits the activation of various coagulation factors, allowing for the preservation of red blood cells without the formation of clots.

When red blood cells are labeled with Cr51 sodium chromate, it is important to maintain the integrity of the cells and prevent coagulation during the preparation and analysis stages. ACD solution is particularly useful in this context because it provides an optimal environment for maintaining red blood cell viability and the tracer's effectiveness.

Heparin and EDTA can also act as anticoagulants, but they work through different mechanisms, which may not be as effective for radio-labeled red blood cells in certain applications compared to ACD solution. Sodium fluoride, while it can inhibit glycolysis, is not primarily used as an anticoagulant in blood storage.

Thus, ACD solution's ability to prevent clotting while preserving cell integrity makes it the most suitable choice among the options provided.