Studies have demonstrated that VWF collagen binding (VWF:CB) can serve as a surrogate measure for the presence of HMWM (7-9).
Nine individuals with type 1 VWD had a multimer distribution outside the reference interval, with loss of the highest molecular weight multimers (6 participants), loss of all HMWM (1 participant), or a shift from high to low molecular weight multimers with relatively increased staining of the low molecular weight bands (2 participants).
Six participants with loss of HMWM had VWF: CB/VWF:Ag ratios >0.7, but none of those individuals had VWF mutations exclusively associated with type 2A VWD (Table 3).
One recent study demonstrated greater sensitivity to loss of HMWM with collagen binding assays compared to monoclonal antibody-based VWF activity assays, suggesting that such VWF activity assays cannot substitute for either VWF:RCo or VWF:CB in assessment of multimer distribution (34).
Patients with these bleeding disorders have similar phenotypic parameters and clinical features, ie functional VWF discordance, increased response to low concentration Ristocetin in RIPA testing and in most cases although not ours thrombocytopenia and loss of HMWM
. Incidence of pseudo vWD is unknown but it is feasible that under diagnosis may be evident (5) as some patients previously classified as type 2B vWD may in fact have pseudo vWD.