Collection ProceduresQuestion 8 of 100
The light blue stopper tube used for PT/INR and aPTT contains which additive, and why must it be filled completely?
a.Heparin, because it must be balanced against the plasma volume
b.Sodium citrate, because a fixed 9:1 blood-to-anticoagulant ratio is required
c.EDTA, because underfilling shrinks the red cells
d.Thrombin, because clotting must be accelerated
Explanation
Sodium citrate reversibly binds calcium and requires a 9 parts blood to 1 part anticoagulant ratio. An underfilled tube leaves excess citrate that binds reagent calcium during testing and falsely prolongs the PT and aPTT. Heparin and EDTA are not used for routine coagulation, and thrombin is found in rapid-serum tubes, not coagulation tubes.
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