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The test is then started, and platelet aggregation is recorded at approximately 0.5 second intervals for six minutes. Three parameters are calculated: aggregation, velocity and area under the curve (AUC). Aggregation (in AU) is the maximum height of the aggregation curve, and velocity (in AU/min) is the maximum slope of the curve.
It measures platelet aggregation with the help of von Willebrand factor The ristocetin-induced platelet aggregation ( RIPA ) is an ex vivo assay for live platelet function. It measures platelet aggregation with the help of von Willebrand factor (vWF) and exogenous antibiotic ristocetin added in a graded fashion. [ 1 ]
Blood compatibility testing is routinely performed before a blood transfusion.The full compatibility testing process involves ABO and RhD (Rh factor) typing; screening for antibodies against other blood group systems; and crossmatching, which involves testing the recipient's blood plasma against the donor's red blood cells as a final check for incompatibility.
In all forms of the ristocetin assay, the platelets are fixed in formalin prior to the assay to prevent von Willebrand's factor stored in platelet granules from being released and participating in platelet aggregation. Thus, the ristocetin cofactor activity depends only upon high-molecular multimers of the factor present in circulating plasma.
Platelet aggregation plays a critical role in the genesis of a resulting thrombus. Adhesion should remain local, but platelet aggregation must grow exponentially to form a platelet thrombus and prevent blood loss. Platelet aggregation factors are the regulators that enhance the adhesion and stimulate the platelets to secrete its granules.
Light transmission aggregometry is widely accepted as the gold standard diagnostic tool for assessing platelet function, and a result of absent aggregation with any agonist except ristocetin is highly specific for Glanzmann's thrombasthenia. [9] Following is a table comparing its result with other platelet aggregation disorders:
The test is started by adding appropriate reagents. The instrument measures and graphically displays the changes in elasticity at all stages of the developing and resolving clot. The typical test temperature is 37 °C, but different temperatures can be selected, e.g. for patients with hypothermia. [4]
Thromboxane A 2 (TXA 2) is a type of thromboxane that is produced by activated platelets during hemostasis and has prothrombotic properties: it stimulates activation of new platelets as well as increases platelet aggregation.