Aggregation of platelets with ristocetin (von Willebrand Factor)

The reference values (norm) of von Willebrand factor activity are 58-166%.

The von Willebrand factor synthesizes endothelial cells and megakaryocytes. It is necessary for normal adhesion of platelets and has the ability to prolong the half-life of factor VIII. The plasma clotting factor VIII - anti-hemophilic globulin A - circulates in the blood in the form of a complex of three subunits, designated VIII-k (coagulating unit), VIII-Ar (the main antigen marker) and VIII-PV (von Willebrand factor bound to VIII-Ar ). It is believed that von Willebrand factor regulates the synthesis of the coagulation part of anti-hemophilic globulin A (VIII-k) and participates in vascular platelet hemostasis.

Von Willebrand disease is a hereditary disease characterized by an increase in the duration of bleeding, a decrease in ristocetin cofactor activity and a decrease in the coagulant activity of factor VIII (to varying degrees). Clinical manifestations of the disease are similar to those in thrombocytopathy. However, in patients with a significant decrease in the activity of factor VIII, hematomas and hemarthroses can be observed.

Based on laboratory studies that allow determining the structure and activity of von Willebrand factor, the following forms of the disease are distinguished.

• Type I (70% of all cases) is characterized by a slight decrease in ristocetin-cofactor (von Willebrand factor) and coagulant activity (VIII-k) under normal macromolecular structure of von Willebrand factor.
• Type II: the basis is the selective deficit of high molecular weight Willebrand factor polymers due to disturbances in the structure of this protein.
  • Type IIB is due to increased interaction between von Willebrand factor and platelets, and increased clearance of platelet aggregates leads to thrombocytopenia.
• Type III is characterized by a severe quantitative deficit of von Willebrand factor, which leads to a clinically significant decrease in the activity of factor VIII (VIII-k).

The content of von Willebrand factor is determined in those cases when bleeding time is increased, the number of platelets is within the reference values and there are no obvious causes of platelet dysfunction. For the evaluation of von Willebrand factor, the quantitative content of von Willebrand factor (study of ristocetin-cofactor activity) is determined, rustocetine-induced platelet agglutination and the antigenic structure of factor VIII vWF (vWF) are investigated.

The determination of platelet aggregation with ristocetin in plasma is used to quantify the von Willebrand factor. A linear relationship was established between the degree of ristocetinic aggregation and the amount of von Willebrand factor. The method is based on the ability of this antibiotic (ristocetin) to stimulate in vitro interaction of von Willebrand factor with the platelet glycoprotein Ib. In most cases of von Willebrand's disease there is a violation of ristocetin-aggregation with a normal response to the effects of ADP, collagen and adrenaline. Disturbance of ristocetin-aggregation is also found in Bernard-Soulier's macrocytic thrombodynastrophy (absence of ristocetinic aggregation receptors on the platelet membrane). For differentiation, a test with the addition of normal plasma is used: with von Willebrand's disease after the addition of normal plasma, ristocetin-aggregation is normalized, while in Bernard-Soulier's syndrome this does not happen.

The study can also be used in the differential diagnosis of hemophilia A (a deficiency of factor VIII) and von Willebrand's disease. With hemophilia, the content of VIII-k is sharply reduced, and the content of VIII-PV is within the normal range. Clinically, this difference is manifested in the fact that hemophilia causes a hematoma type of increased bleeding, and in the case of von Willebrand's disease - a petechial-hematoma type.

Ristocetin-induced agglutination of platelets is reduced in most cases of von Willebrand disease, except type IIB.

The antigenic structure of vWF related to factor VIII (VIII-PV) is detected by various immunological methods, and the distribution of vWF by size of the molecules is determined by electrophoresis in an agarose gel. These tests are used to establish the type of von Willebrand disease.

The study of platelet aggregation with various inducers is carried out not only to detect abnormalities of aggregation functions of platelets. This study allows to evaluate the effectiveness of antiplatelet therapy, to select individual doses of drugs and to conduct drug monitoring.

[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]