When collecting a specimen for a prothrombin time (PT) or aPTT, what is the critical fill requirement for the sodium citrate (blue-top) tube?
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A
Fill to at least 50% of tube capacity
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B
Fill exactly to the line — the 9:1 blood-to-citrate ratio must be maintained
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C
Fill no more than 75% to prevent clotting
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D
Fill level does not affect coagulation results