NHA Phlebotomy Certification Practice Exam

Most anticoagulants in blood collection tubes prevent clotting by:

1. Binding calcium

2. Acting as an antithrombin agent

3. Binding fibrinogen

4. Releasing heparin

The correct answer is: Binding calcium

Most anticoagulants in blood collection tubes function primarily by binding calcium. Calcium ions play a crucial role in the coagulation cascade; they are necessary for various steps in the blood clotting process. By chelating or binding free calcium in the blood, these anticoagulants effectively prevent the formation of fibrin, which is key in developing a stable blood clot. This mechanism is essential for ensuring that blood samples remain in a liquid state, allowing for accurate laboratory analysis. While other options mention agents like antithrombin and heparin, their actions differ from the direct chelation of calcium. Antithrombin is a natural inhibitor that targets specific clotting factors, while heparin works by enhancing the activity of antithrombin rather than actively binding calcium. Fibrinogen, though a critical protein in blood coagulation, is not the target for most anticoagulants used in blood collection; instead, preventing its conversion into fibrin is the primary goal, which is achieved through calcium chelation.