How should a specimen be prepared for coagulation testing?

• A. Use light-blue top citrate tubes; fill to the recommended volume; gently invert 8–10 times to mix
• B. Red-top serum tubes and shake vigorously
• C. Heparin tubes with minimal mixing
• D. Plain glass tubes stored upright

Answer: (A)

For coagulation testing you need plasma preserved with an anticoagulant that reversibly binds calcium, so citrate is used. The light-blue top tube contains 3.2% sodium citrate and must be filled to the marked line to achieve the correct blood-to-citrate ratio (about 9:1). After collecting, gently invert the tube 8–10 times to mix thoroughly without causing hemolysis. Proper mixing ensures the citrate evenly binds calcium in the sample, preventing premature clotting and preserving coagulation factors for accurate PT and aPTT results. If the tube isn’t filled to the correct volume, the ratio shifts and can falsely alter clotting times.

Red-top serum tubes lack anticoagulant and would yield serum after clotting, which isn’t appropriate for these tests. Shaking red-top tubes vigorously can cause mechanical activation or hemolysis. Heparin tubes are a different anticoagulant and can affect coagulation test results, making them unsuitable for standard PT/aPTT Testing when citrate is required. Plain glass tubes with no anticoagulant will clot, giving serum rather than plasma, which won’t meet the test requirements.