Which additive is used in coagulation testing tubes?

• A. Citrate
• B. Heparin
• C. EDTA
• D. Fluoride

Answer: (A)

Citrate, specifically sodium citrate, is the additive used in coagulation testing tubes. It works by binding calcium ions, which are essential for the coagulation cascade, so blood doesn’t clot in the collection tube. This keeps the sample stable as plasma is prepared. During the actual coagulation test, calcium is added back in a controlled way to start clotting, allowing accurate measurement of clotting times (like PT/INR and aPTT). Citrate is preferred because it preserves the activity of clotting factors and yields reliable results, whereas the other additives either interfere with testing or are used for different purposes. Heparin can alter clotting times, EDTA disrupts calcium-dependent processes and is used for complete blood counts, and fluoride (often with oxalate) is for glucose testing, not coagulation.