Carolinas HealthCare System
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FAQs for Hematology and Coagulation

Why does blood clot in tubes collected for a CBC, PT, and APTT?

Clotting begins when blood comes in contact with a foreign surface or from tissue activation. There is a very short time window to successfully procure a blood sample and deliver it into a tube with added anticoagulant.

As soon as the sample is delivered into the collection tube, gently invert the tube five or six times to thoroughly mix the whole blood with the anticoagulant. The clotting cascade begins with a small strand of fibrin and is not always visible to the naked eye.