Platelet aggregation testing is useful in the evaluation of suspected hereditary and acquired disorders of platelet function. Activation of platelets causes them to change shape, secrete their intracellular granules, and aggregate with each other. Platelets normally contain two major types of granules - alpha and dense granules. These granules contain substances which promote hemostasis when secreted from platelets at the site of vascular injury. The alpha granules contain fibrinogen, factor V, and von Willebrand factor while the dense granules contain platelet factor 4, ADP, ATP, calcium and serotonin. Abnormalities of platelet function may be hereditary or acquired. Hereditary platelet function disorders include rare defects of adhesion (Bernard Soulier syndrome), rare defects of aggregation (Glanzmann thrombasthenia), and more common defects of secretion (alpha or dense granule deficiency, aspirin-like defects, or other primary secretion defects).
Acquired platelet function disorders are more common than the hereditary disorders and include drug-induced platelet dysfunction (including aspirin, NSAID's, clopidogrel, antibiotics, various cardiovascular and psychotropic drugs), uremia, and myeloproliferative disorders.
Classically, platelet function has been tested in the laboratory by testing platelet aggregation in platelet rich plasma upon addition of various agonists such as ADP, collagen, arachidonic acid, epinephrine and ristocetin. Using modern platelet aggregometers, it is possible to evaluate platelet secretion simultaneously, by measuring release of ATP by the aggregating platelets. Measurement of platelet secretion allows the laboratory diagnosis of secretion defects with greater sensitivity than platelet aggregation testing alone. Platelet aggregation may be normal in some cases, and measurement of platelet secretion may be critical for diagnosis.
The following concentrations of each agonist are used: ADP (2uM and 5uM), collagen (1ug/mL), arachidonic acid (1mM) and ristocetin (1.25 mg/mL). For von Willebrand disease evaluation, dilute ristocetin (0.625 mg/mL) is also added. Evaluation of thrombocytosis (platelet count >400,000/uL) includes epinephrine (10uM). Platelet secretion is measured together with aggregation for all agonists except ristocetin.
Decreased response to ristocetin and normal aggregation with the other agonists is seen in von Willebrand's disease and Bernard Soulier syndrome. For diagnosis of von Willebrand disease, platelet response to both concentrations of ristocetin must be evaluated together with von Willebrand disease screening tests such as ristocetin cofactor, von Willebrand disease antigen, and factor VIII assay. Glanzmann's thrombasthenia is characterized by absent aggregation with ADP, collagen and arachidonic acid and normal response to ristocetin. Inherited disorders of platelet secretion usually demonstrate a decreased secondary aggregation wave with ADP, decreased response to collagen and variable response to arachidonic acid; however, the pattern is not always typical.
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