Which additive is used to prevent coagulation by binding calcium?

The correct answer is Potassium EDTA, as it prevents coagulation by binding to calcium ions in the blood. EDTA, or ethylenediaminetetraacetic acid, chelates calcium, a vital factor in the coagulation cascade, effectively stopping the blood from clotting. This property makes EDTA particularly useful in laboratory settings, especially in hematology, where it's commonly used in tube collections for complete blood counts and other tests that require whole blood.

In contrast, sodium citrate is another additive that also chelates calcium, but it is distinct in its chemical structure and might be a more common choice in certain blood coagulation tests. Heparin works differently by enhancing the activity of antithrombin III, which inhibits thrombin and factor Xa, thus preventing coagulation but does not bind calcium. Calcium carbonate, on the other hand, is a form of calcium supplementation and does not serve as an anticoagulant; in fact, it would promote coagulation rather than prevent it. Therefore, the function of potassium EDTA as a calcium binder gives it the spot as the correct choice in this context.