Ristocetin cofactor

Last updated: Thursday, 25, March, 2004

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Item Process

4.5 mL blood added to 0.5 mL citrate.


Ristocetin-induced agglutination of fixed, washed platelets is dependent on added VWF.

Test and pooled normal plasma assayed in parallel dilutions.

Activity of test plasma expressed as % of activity in pooled plasma.

The reference assay is performed on a platelet aggregometer. Abbreviated VWF:RCo.

Reference Interval

Generally 50-200% of the activity in pooled normal plasma.


Additional functional VWF assay complimentary to the collagen binding assay in the investigation of possible bleeding disorder.

Diagnosis of von Willebrand’s disease and its variant forms, see von Willebrand factor multimers.


Levels are reduced in classical (type I) von Willebrand’s disease, usually to a similar extent as the reduction in factor VIII and VWF:Ag.

Levels of VWF (ristocetin cofactor and collagen binding) are reduced disproportionately to VWF:Ag in type 2 (A and B) VWD.

In Type 2M VWD, levels of VWF:RCo and/or VWF:CB (that is, either or both) may be reduced disproportionately to VWF:Ag, depending on the specific VWF defect represented.

See also von Willebrand's disease, von Willebrand factor antigen (VWF:Ag), and Collagen binding assay.


Favaloro EJ. Clin Haematol 2001; 14: 299-319.

Favaloro EJ. Haemophilia 2002; 8: 828-831.

McFarlane DE et al. Thromb Diath Haemorrh 1975; 34: 306.

Marder VJ et al. Thromb Haemost 1985; 54: 871-872.