Type of test Blood
Normal findings
Dependent on the platelet agonist used
Test explanation and related physiology
Platelet aggregation is an important part of hemostasis. Surrounding an area of acute blood vessel endothelial injury is a clump of platelets. Normal platelets adhere to this area of injury, and through a series of chemical reactions, they attract other platelets to the area. This is platelet aggregation, the first step of hemostasis. After this step, the normal coagulation factor water fall occurs (see Figure 1).
Fig1. Secondary hemostasis (fibrin clot formation) and fibrinolysis (fibrin clot dissolution). Primary hemostasis involves platelet plugging of the injured blood vessel. Secondary hemostasis, as described here, takes place most rapidly on the platelet surface after attachment to the fractured endothelium. Four different reactions result in the formation of fibrin. As seen beneath the dark line in the figure, the fibrin clot supports the platelet clump so that the clot does not get swept away by the tremendous shear forces of the fast moving blood cells. Fibrinolysis follows formation of the fibrin clot in order to prevent complete occlusion of the injured blood vessel.
Certain diseases that affect either platelet number or function can inhibit platelet aggregation and prolong bleeding times. If blood is passed through a heart–lung or dialysis pump, platelet injury can occur and reduce aggregation.
Ristocetin is most commonly used to induce platelet agglutination. The agglutination of the patient’s blood with the addition of this type of product can help differentiate the disease that may be affecting platelet aggregation.
Interfering factors
• Factors that may cause increased platelet aggregation include blood storage temperature, hyperbilirubinemia, hemoglobinemia, hyperlipidemia, and platelet count.
* Drugs that may cause decreased platelet aggregation include antibiotics, aspirin, nonsteroidal antiinflammatory agents, and thienopyridine antiplatelet drugs.
Procedure and patient care
• See inside front cover for Routine Blood Testing.
• Fasting: no • Blood tube commonly used: blue
• Abnormalities in platelet aggregation can prolong bleeding time, and hematoma at the venipuncture site may occur.
Abnormal findings
- Hypoactive platelet aggregation
- Various congenital disorders (e.g., Wiskott–Aldrich syndrome, Bernard–Soulier syndrome, glycogen storage, some types of von Willebrand disease)
