How does sodium citrate prevent clotting in blood samples?

Sodium citrate prevents clotting in blood samples primarily by binding calcium ions. Calcium plays a crucial role in the coagulation cascade; it is essential for several steps in the clotting process. When sodium citrate is added to a blood sample, it chelates (or binds) free calcium in the sample, thereby removing it from the circulation involved in coagulation. Without available calcium, various clotting factors cannot function appropriately, effectively inhibiting the clotting process and allowing for accurate laboratory testing on the sample.

The other options do not accurately describe the mechanism of sodium citrate. While it is true that some anticoagulants inactivate clotting factors, sodium citrate specifically sequesters calcium, rather than directly inactivating the clotting factors themselves. Similarly, sodium citrate does not promote platelet aggregation or neutralize acids as part of its anticoagulant function. Thus, binding calcium is the defining characteristic of how sodium citrate prevents clotting, making it the correct answer.