Carolinas HealthCare System
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FAQs for Hematology and Coagulation

Why is it so important to properly fill the blue top collection tubes used for coagulation testing?
The blue top tubes contain a liquid anticoagulant (sodium citrate). All reference and analytical limits for coagulation testing are derived using a blood to anticoagulant ratio of nine parts whole blood to one part anticoagulant. When the blood collection tube is less than 90 percent full, it is likely that the coagulation clotting times will be falsely elevated due to the increase of anticoagulant relative to the amount of whole blood in the tube. This may result in a compromise in patient care. So, all blood sample tubes for coagulation testing are visually inspected to assure that quality results are reported.
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