The von Willebrand factor (VWF) is one of the proteins that act in cascade to promote blood clotting. For individuals with a hereditary or acquired VWF deficiency, blood takes longer to clot and, as a result, bleeding is prolonged. Von Willebrand Disease (VWD) is the most common hemorrhagic disorder. There are three types of VWD: In Type 1 (75% of cases), the protein functions normally but is present in insufficient quantities. In type 2, there is enough protein, but it does not function normally. Bleeding can be more severe than in type 1. Type 3 is very rare and is caused by a significant or complete VWF deficiency. The VWF AG test measures the amount of protein, while the VWF activity test measures both quantity and quality.
Individuals with a mild form of VWD may have normal VWF activity and levels. Individuals without clear signs of the disease may have moderately lower results on both tests.