American Society for Clinical Pathology (ASCP) Histotechnician Certification Practice Exam

Heparin is an anticoagulant that works through a different mechanism than the other options listed. It inhibits thrombin and factor Xa in the coagulation cascade, thereby preventing the formation of fibrin. It does not function by chelating or binding calcium ions, which is the action seen in the other options.

EDTA, citrate, and oxalate all bind calcium ions, which is crucial for their anticoagulant effect. EDTA chelates calcium in the blood, effectively preventing coagulation by removing calcium needed for various clotting factors to function. Similarly, citrate binds calcium in a reversible manner, while oxalate forms insoluble complexes with calcium, leading to a reduction of free calcium in the plasma. Because heparin does not rely on calcium binding for its anticoagulant properties, it stands out from the other options provided.