A 3.2% sodium citrate (blue top) tube collected for PT and APTT is filled to 80% of the stated line. What will happen to the test results?

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Multiple Choice

A 3.2% sodium citrate (blue top) tube collected for PT and APTT is filled to 80% of the stated line. What will happen to the test results?

The test times are affected by how much citrate is present relative to the plasma. Sodium citrate in blue-top tubes chelates calcium to prevent clotting, and the assay adds calcium back to start coagulation. If the tube is only 80% full, there is more citrate relative to plasma, so more calcium remains bound and less free calcium is available to initiate the clotting cascade. That makes PT and APTT longer than they should be, giving falsely prolonged results. This is why the correct choice attributes the prolongation to excess citrate binding calcium and reducing the calcium needed to start clotting.

The test times are affected by how much citrate is present relative to the plasma. Sodium citrate in blue-top tubes chelates calcium to prevent clotting, and the assay adds calcium back to start coagulation. If the tube is only 80% full, there is more citrate relative to plasma, so more calcium remains bound and less free calcium is available to initiate the clotting cascade. That makes PT and APTT longer than they should be, giving falsely prolonged results. This is why the correct choice attributes the prolongation to excess citrate binding calcium and reducing the calcium needed to start clotting.

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