Which compound is commonly used as an anticoagulant in blood collection tubes?

• A. Sodium tartrate
• B. Sodium citrate
• C. Sodium phosphate
• D. Sodium thiosulfate

Answer: (B)

Sodium citrate works as the anticoagulant in blood collection tubes by binding calcium ions in plasma. Calcium is essential for the clotting cascade, so chelating it stops clot formation and preserves whole blood for accurate testing. In coagulation assays, the citrate amount is carefully controlled so that, after plasma is separated, calcium can be added back to measure true clotting times. The other compounds listed do not provide this reliable, reversible anticoagulation in routine tubes: one mainly serves as a buffer, another isn’t a standard anticoagulant for blood collection, and the third is a reducing agent not suitable for preventing clotting.