VWF and von Willebrand disease 2: Samples from patients with severe type 1 VWD, defined as VWF antigen (VWF:Ag) below 10U/dL, and those with type 2 VWD (absence of large VWF multimers) failed to occlude either AR- or PL-chips but the occlusion times were normal in patients with milder type 1 VWD.