Most instruments used for platelet function testing require the use of citrated blood. Blood is collected by venipuncture into a plastic tube containing buffered 3.2% sodium citrate, which is then mixed gently immediately upon filling.
Collection tubes must be filled completely to ensure the proper 9:1 ratio of blood to anticoagulant. Alterations in the blood-to-anticoagulant ratio can have an adverse effect on platelet response to agonists (platelet activators) used in the test procedure. It is also important to the outcome of the test that the venipuncture is not traumatic, as this can result in platelet activation. A needle gauge size between 19 and 21 is recommended to prevent vein trauma or reduced blood flow, leading to activated platelets. If the vein collapses or there is a stoppage of blood flow during collection, the specimen should be discarded, and a venipuncture should be repeated from another site.
Some test manufacturers may recommend that the whole blood specimen rest on the laboratory benchtop for 30 minutes prior to platelet function testing. Platelets, which may be activated as a result of venipuncture, will return to an inactive (resting) state during the 30 minutes. Specimens should be held at room temperature (18-24°C) only and not refrigerated or frozen. Testing should be completed within the time limit specified by the manufacturer to ensure platelet viability (usually within 3-4 hours of collection); platelet functionality becomes abnormal after a period of approximately 3-4 hours.
Specimens for platelet function testing should be rejected if they are clotted, grossly hemolyzed, or if the collection time limit has been exceeded.
