Why should the tube with sodium citrate be filled completely?

The tube with sodium citrate must be filled completely to maintain the correct ratio of blood to anticoagulant. Sodium citrate is an anticoagulant used in laboratory tests, especially in coagulation studies such as PT (Prothrombin Time) and aPTT (Activated Partial Thromboplastin Time). The effectiveness of sodium citrate in preventing clot formation is dependent on the precise balance between the volume of blood and the volume of anticoagulant. If the tube is underfilled, there will be an inadequate volume of anticoagulant in proportion to the blood, which can lead to inaccurate test results by potentially allowing some clotting to occur. Therefore, ensuring the tube is completely filled guarantees that the intended ratio is maintained, leading to reliable and valid test outcomes.