VWF and von Willebrand disease (hereditary or acquired): In fact, even assuming in the general population a VWD prevalence as high as 1%,11 plasma VWF levels below 20 IU/dL would be required to diagnose VWD based on laboratory data alone.12,13 Therefore, laboratory measurement of VWF levels in unselected patients is not effective, further confirming the notion that preoperative screening tests are of minimal value in the prediction of post-surgical bleeding.14