von Willebrand factor antigen (VWF:Ag)

Last updated: Wednesday, 31, March, 2004
Item Process
Specimen

4.5 mL blood added to 0.5 mL citrate.

Method

ELISA, electroimmunoassay, immunoradiometric assay or EIA: level of VWF:Ag in test plasma expressed as a percentage of that in pooled, normal plasma.

Reference Interval

Generally 50-200% of the level in pooled normal plasma, sometimes expressed as 0.5-2.0 U/mL.

Application

Investigation of a possible bleeding disorder: diagnosis of von Willebrand’s disease (VWD) and its variant forms.

Detection, in association with Factor VIII assay, of carriers of haemophilia A.

Interpretation

Levels of VWF:Ag are reduced in classical von Willebrand’s disease, usually to a similar extent as the reduction in factor VIII and functional (collagen binding and ristocetin cofactor) VWF assays.

Normal levels may be found in some variant forms of VWD (eg Types 2B, 2N, 2M).
See von Willebrand factor multimers.

Levels of VWF:Ag are normal in haemophilia A and this serves to distinguish mild haemophilia A and VWD, if doubt exists. Carriers of haemophilia A have normal VWFAg levels and may have reduced levels of factor VIII; a low factor VIII/VWF:Ag ratio (<0.7) is suggestive of the carrier state, although a normal ratio does not exclude this possibility.

A low factor VIII/ VWF:Ag ratio (<0.7) may also be suggestive of Type 2N VWD (consult laboratory).

See also von Willebrand disease; Ristocetin cofactor; Collagen binding assay.

Reference

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

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