Testing for von Willebrand disease is best performed on citrate anticoagulated plasma (BTT). If the sample is picked up and delivered by an Antech courier or the same day, submission of a full BTT of whole blood is acceptable. For longer transit times, the BTT should be spun at the clinic and plasma transferred to an empty RTT or plastic tube. To separate the plasma, use a plastic transfer pipet or small syringe and needle to aspirate the plasma down to just above the buffy coat layer. Do not attempt to decant (pour-off) the plasma. This is important because von Willebrand factor (VWF) is the largest plasma protein and tends to become layered at the bottom of the plasma compartment.

If all of the plasma is not harvested, the resultant VWF level could be lower than it is actually is in vivo as the largest multimeric forms of this protein have been left behind.