Which reagent is essential in the preparation of labeled RBC for studies involving in vivo labeling?

In the preparation of labeled red blood cells (RBCs) for studies involving in vivo labeling, sodium citrate plays a crucial role as an essential reagent. Sodium citrate acts as an anticoagulant, preventing the clotting of blood samples. By binding calcium ions in the blood, it inhibits the clotting cascade, which ensures that the RBCs remain in a liquid state and are available for labeling.

This property is particularly important when preparing RBCs for scintigraphy or other nuclear medicine applications, as any clotting can significantly affect the integrity and function of the labeled cells, leading to inaccurate imaging results.

The other options, while they may serve specific functions in certain contexts, do not have the primary role that sodium citrate does in this particular preparation process. For instance, stabilizing buffers are often utilized for maintaining the pH and stability of reactions, ascorbic acid can serve as a reducing agent, and calcium chloride is commonly used in other biological processes but not specifically for maintaining the liquid state of blood during RBC labeling.