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Subject: von Willebrand Disease (VWD) Assays
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VWD is a bleeding disorder manifested by mucocutaneous bleeding.
No single laboratory test can detect all forms of VWD, hence the following four-assay panel is recommended: von Willebrand factor (vWF) activity by a ristocetin (RCoF) or an immunoassay, von Willebrand factor (VWF) antigen, factor VIII activity which is decreased in parallel to VWF levels, and ristocetin-induced platelet aggregation (RIPA); once a diagnosis of vWD is established with this panel, VWF multimer analysis is recommended to distinguish various subtypes.
Determination of the blood group is also helpful in interpreting low values since patients with group 0 run 20–30% lower values that those established for a random normal population.
RCoF. A quantitative test for VWF is used whenever a history of mucocutaneous bleeding suggests VWD. In the presence of ristocetin, the VWF causes agglutination of platelets, measured in an aggregometer by the change in optical density. It is unaffected by anticoagulants.
Normal range for RCoF: 48–172%.
Various types of VWD
Acquired inhibitors to VWF
Acute inflammatory conditions (VWF is an acute-phase reactant).
High levels are seen in some patients with thromboembolic events. There is some evidence that patients with very elevated VWF levels may be predisposed to thromboembolism.
Great variability in results (see above the broad range for normal values).
Therapy with factor VIII concentrates that contain VWF, or with DDAVP, raise the RCoF levels.
Interfering substances: Lipemia, clotted or hemolyzed blood, blood collected in wrong anticoagulant, or improperly filled test tube.
VWF antigen: normal values: 60–150. The antigen values may be higher than the vWF activity in certain subtypes with qualitative defects, resulting in a ratio <0.7 of activity/antigen.
Factor VIII coagulant: normal range: 70–150%
RIPA is a semiquantitative assay for VWF, used when there is a strong suspicion for VWD. It uses ristocetin as the platelet agglutinating agent in the presence of VWF. The changes in optical density (OD) are recorded in an aggregometer. Abnormal response to ristocetin is the result of von Willebrand disease or of the platelet receptors responsible for binding von Willebrand factor
To estimate VWF activity and rule in or out type 2B VWD (see below.)