Identifying carriers of type 2N von Willebrand disease: procedures and significance.

The defective FVIII carrier function of von Willebrand factor (VWF) identifies type 2N von Willebrand disease (VWD), a variant with a pattern resembling hemophilia A. Type 2N characterization is based on the evaluation of the capacity of VWF to bind exogenous FVIII (VWF:FVIIIB). Here we report on a retrospective evaluation of hemostatic laboratory parameters most useful in detecting type 2N carriers. The diagnostic capacity of aPTT, FVIII, VWF:Ag, FVIII/VWF:Ag ratio, VWF:FVIIIB and VWF:FVIIIB/VWF:Ag ratio was evaluated in 21 type 2N VWD carriers. Twenty subjects were heterozygous for the R854Q mutation, one was heterozygous for the R760C missense mutation, which interferes with cleavage of the VWF propeptide. We found that prolongation of aPTT and decrease in FVIII and FVIII/VWF:Ag ratio were not frequent findings in type 2N carriers. The same was true for VWF:FVIIIB which was not always abnormal. On the contrary, VWF:FVIIIB/VWF:Ag ratio was always defective and its values were not related with FVIII and FVIII/VWF:Ag ratio or influenced by plasma VWF concentration. Given these results, we attribute the greatest significance to VWF:FVIIIB/VWF:Ag ratio in the diagnosis of type 2N defects, and only search for type 2N mutations, to validate the diagnosis, if the ratio proves abnormal.