This test measures the amount of factor XI in your blood. Factor XI is a protein that plays an important role in blood clotting. 6 mL Whole blood in 2 Blue Top (Sodium Citrate) tube. Mix thoroughly by inversion. Transport to Lab within 4 hours. If this is not possible, make PPP within 1 hour of collection as follows: Centrifuge sample at 3600 rpm for 15 min. & transfer supernatant to a clean plastic tube. Centrifuge this supernatant again at 3600 rpm for 15 mins. & finally transfer the supernatant (PPP) to 1 labelled, clean plastic screw capped vial. FREEZE IMMEDIATELY. Ship frozen. DO NOT THAW.