vWF threshold required for confirming von Willebrand disease

What is the threshold of von Willebrand factor required for the diagnosis of von Willebrand disease?

Alfonso Iorio, MD, PhD: It is controversial. The threshold of 0.5 U/mL or 50% is more accepted. There is a tendency towards considering 0.4 U/mL or 40% a more useful threshold, and there are guidelines likely coming up proposing 40%. Some groups suggest to use a lower threshold for blood group type O where normal values can be as low as 35%, but not everyone agrees on that.

For sure, clinically relevant von Willebrand starts at 20%. You make the diagnosis at 40% but you consider mild cases at 20% to 40%.