vonWillebrand Factor Assessment

• To diagnose vonWillebrand Disease
• This can be done via:

1. Antigen assay
2. Ristocetin cofactor activity
3. Multimeric analysis

• Normal VWF is a series of glycoproteins with a high molecular weight, ranging from 500 to 20,000 kD.
• Ristocetin is an antibiotic that was withdrawn due to a  side effect of thrombocytopenia.  It induces aggregation of platelets in a reaction that is dependent on large VWF.  The rate of aggregation can be used to estimate the functional level of VWF.

• Antigen assay

- Measures the total plasma concentration of VWF
- Estimated via ELISA or electroimmunodiffusion
 
• Ristocetin cofactor activity

- Measures the extent of agglutination (via transmitted light) of fixed platelets in response to the patient's plasma and ristocetin.
- The same test is done with plasma pooled from normal individuals.
- VWF activity in patient's plasma is expressed as % of activity in the normal plasma pool.
- Can also test the patient's platelet-rich plasma with ristocetin and compare this with platelet rich plasma from healthy donors.  This is useful in the diagnosis of Type IIB and platelet type VWD, where there is enhanced platelet aggregation to low concentrations of ristocetin.  These two can then be distinguished by measuring platelet aggregation in response to cryoprecipitate.
 
• Multimeric analysis

- Two dimensional electrophoresis:  One direction to separate VWF multimers by size.  The second direction through antibodies to VWF.
- The shape of the immunoprecipitate indicates the size distribution and concentrations of multimers in the sample.
 
• Measure aPTT/aPT
• Measure Bleeding Time
• Measure Factor VIII activity