Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1302 page)

   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%.

Decreased In

   Various types of VWD

   Platelet-type VWD

   Hypothyroidism

   Acquired inhibitors to VWF

Increased In

   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.

   Limitations of Assay

   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.