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Definition / General
- Also called ristocetin cofactor activity
- Ristocetin is an antibiotic that causes vWF to bind to and activate platelets
- Test measures function of von Willebrand factor
- Platelets from healthy individuals are mixed with standard concentrations of ristocetin, and patient plasma is added to cause platelet agglutination (measured in aggregometer), which is proportional to the vWF concentration
- Used to confirm type 2B von Willebrands disease (increased agglutination due to increased affinity of vWF for GPIb); similar results for platelet type von Willebrands disease (although the defect is in GPIb)
- Note: aggregation of platelets implies linkage via fibrinogen and GP IIb / IIIa; ristocetin links platelets through vWF and GP Ib; appropriate term is actually agglutination
- Other functional tests: collagen-binding ELISA assay, functional vWF binds to collagen and is detected (Br J Haematol 2002;116:187)
Laboratory
- Low dose ristocetin platelet aggregation assay
- To diagnose type 2B von Willebrand disease
- Similar to von Willebrand factor activity test, but uses patients platelets and lower dose of ristocetin
- Platelets from patient are mixed with standard concentrations of ristocetin and patient plasma is added to cause platelet agglutination (measured in aggregometer)
- Increased aggregation in type 2B vWD in this assay due to GP Ib mutation, which increases affinity for vWF
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