Final answer:
The anticoagulant used in a blood specimen for a complete blood cell count is EDTA, due to its ability to chelate calcium ions and thereby prevent coagulation. The correct option is a.
Step-by-step explanation:
The additive used as an anticoagulant in a blood specimen collected to determine a complete blood cell count is EDTA (Ethylene Diamine Tetraacetic Acid). EDTA functions as an anticoagulant by chelating (binding strongly to) the calcium ions in the blood.
Calcium is a critical factor in the blood clotting cascade, and its removal prevents coagulation. Other substances like sodium chloride do not have this chelating property, while citrate and heparin are also known anticoagulants but are not the correct answer in this context.
For example, when blood was collected into EDTA tubes, it could then be centrifuged to obtain plasma without the concern of clotting. Similarly, EDTA's ability to bind metal ions is what makes it effective at inhibiting coagulation, as calcium ions are necessary for the coagulation process.
Blood samples can also be collected into tubes containing other anticoagulants like lithium heparin, but for a complete blood count, EDTA is the preferred additive.
Therefore, the correct option is a.