What substance should be injected into the tubing of an infusion set to maintain its patency during a radio chromium red cell mass determination?

To maintain patency in the tubing of an infusion set during a radiochromium red cell mass determination, heparin is the most appropriate choice. Heparin is an anticoagulant that prevents blood coagulation by inhibiting the formation of fibrin clots. It is especially useful in maintaining the flow of blood through catheters and infusion lines, ensuring that there are no blockages or clots that could compromise the procedure.

Saline, while often used for flushing or maintaining fluid balance, does not have the anticoagulant properties necessary to prevent clotting effectively over a longer duration. Glucose solutions, on the other hand, can provide nutrition but do not prevent clot formation, making them less suitable for this purpose. An antibiotic solution could potentially introduce unnecessary complications, such as allergic reactions or changes in the blood chemistry, and is not designed to maintain patency in infusion lines.

Thus, heparin stands out as the most effective and appropriate solution for maintaining the functionality of the infusion set in the context of a radiochromium red cell mass determination.