Limitations of the ristocetin cofactor
assay in measurement of von Willebrand factor function.
(70) Additional laboratory studies show normal VWF antigen and ristocetin cofactor
activity to distinguish Bernard-Soulier syndrome from von Willebrand disease.
Determination of a decrease in ristocetin cofactor
activity (vWF:RCo), clinical symptoms related to bleeding, and family history are diagnostic for vWD.
Comparison of a new automated von Willebrand factor activity assay with an aggregation von Willebrand ristocetin cofactor
activity assay for the diagnosis of von Willebrand disease.
(2,3) The latter typically entails initial plasma testing (laboratory 'screening') of factor VIII coagulant (FVIII:C), and VWF protein ('antigen'; VWF:Ag) and VWF function, with this classically assessed using the ristocetin cofactor
These variants are differentiated on the basis of a panel of lab tests that include the bleeding time, platelet count, partial thromboplastin time, yon Willebrand factor (vWF) activity and antigen, factor VIII levels, ristocetin cofactor
, and vWF multimeric composition.
In this case, the initial VWD testing panel identified significantly decreased plasma von Willebrand factor ristocetin cofactor
activity (VWF:RCo), with a discordantly low ratio of VWF:RCo to von Willebrand factor antigen level (VWF:Ag) and normal coagulation factor VIII activity, suggesting types 2A, 2B, or 2M VWD (or acquired von Willebrand syndrome).
Because of its varying penetration, the diagnosis of von Willebrand disease is elusive and requires repeated ristocetin-induced platelet aggregometry assays supplemented with additional assays such as ristocetin cofactor
and VWF immunoassay.
VWF:Ag, ristocetin cofactor
activity (VWF:RCo), collagen binding with type III VWF:CB, and multimer distribution were performed on all samples as previously described (11).
Because factor VIII reductions can be associated with impaired protection by its carrier protein, von Willebrand factor, it is advisable to quantitatively measure the von Willebrand factor antigen and test for the functional von Willebrand factor ristocetin cofactor
. von Willebrand disease can occur in congenital or acquired variants.
Current functional tests for vWF include the ristocetin cofactor
activity assay and the collagen-binding assay.