VON WILLEBRAND FACTOR ANTIGEN
The diagnosis of von Willebrand Disease (VWD), probably the most common congenital bleeding disorder, requires a number of special tests at the laboratory level. The measurement and comparison of von Willebrand Factor Antigen (VWF:Ag), VWF Activity and Factor VIII (FVIII) levels in plasma aid in the differentiation of quantitative defects (type 1 or type 3) or qualitative defect (type 2) of VWF and therefore to diagnose the different types of VWD.
When an extremely low or undetectable level of VWF:Ag is obtained, a type 3 VWD could be expected. If a moderate or even normal result is obtained, VWF Activity and FVIII assays must be performed and compared with the VWF:Ag result. If all three values are within the normal range, VWD and Hemophilia A may be excluded. If at least one parameter is abnormally low, it is necessary to calculate the ratios VWF Activity/VWF:Ag and FVIII/VWF:Ag. If both ratios are close to 1 (some authors suggest 0.7 as cut-off), a VWD type 1 may be diagnosed.
When the VWF/Activity/VWF:Ag ratio is low (0.7 is also the suggested cut-off), types 2A, 2B or 2M may be diagnosed. These subtypes are characterized by its abnormal multimeric pattern and/or its altered platelet affinity. Additional laboratory tests as RIPA (Ristocetin Induced Platelet Aggregation), multimeric analysis and binding assays are required in order to be able to distinguish the different subtypes. When the FVIII/VWF:Ag ratio is low (0.7 is also the suggested cut-off), a type 2N or Hemophilia A may be diagnosed and a FVIII binding assay is necessary to discriminate among them.