Additive that chelates calcium in banked blood is which?

• A. Oxalate
• B. Heparin
• C. ACD (citrate)
• D. EDTA

Answer: (C)

Calcium chelation to prevent coagulation is the key idea. Citrate binds ionized calcium to form calcium citrate, removing calcium from the plasma and stopping the coagulation cascade. In banked blood, citrate-based anticoagulants are used to keep the blood from clotting during storage, with acid-citrate-dextrose (ACD) being a classic example. This is different from heparin, which works by enhancing antithrombin III and does not chelate calcium, and from EDTA or oxalate, which are calcium chelators used for other types of specimens or tests rather than routine banked-blood storage. So, the additive that chelates calcium in banked blood is citrate, such as ACD.