In this patient, which anticoagulant was used to obtain PT and aPTT results from the blood sample?

• A. EDTA
• B. Sodium citrate
• C. Heparin
• D. Fluoride

Answer: (B)

Coagulation tests such as PT and aPTT are performed on citrated plasma because citrate binds calcium to prevent clotting in the collection tube, while still allowing the test to proceed once calcium is reintroduced during the assay. The calcium reversal is what lets the clotting cascade run in a controlled, measured way, giving accurate times.

Sodium citrate is the anticoagulant of choice here because it preserves the coagulation factors in plasma without permanently disabling them. It’s a reversible chelator, so when the lab adds calcium during the test, clotting can occur normally and the results reflect the factors’ true activity.

Other options don’t fit. EDTA also chelates calcium, but it’s used mainly for preserving blood cells in CBCs and would distort coagulation times. Fluoride is used with oxalate to preserve glucose and is not appropriate for coagulation testing. Heparin prevents clotting by enhancing antithrombin and would alter the coagulation cascade in a way that makes PT/aPTT results unreliable for assessing the intrinsic and extrinsic pathways.