Low von Willebrand factor: sometimes a risk factor and sometimes a disease.

A sufficiently low level of von Willebrand factor (VWF) predisposes to bleeding that can be quite serious, and low VWF is a diagnostic feature of von Willebrand disease (VWD) type 1, which is characterized by partial quantitative deficiency of VWF. Recent groundbreaking studies of patients with VWD type 1 have delineated several pathophysiologic mechanisms that determine the plasma concentration of VWF, but the relationship between VWF level and the likelihood of bleeding remains less well understood. In part, this problem reflects the broad range of VWF levels in the population, so that the distinction between "normal" and "low" is arbitrary. The risk of bleeding certainly increases as the VWF level decreases, but the relationship is not very strong until the VWF level is very low. Furthermore, mild bleeding symptoms are common in apparently healthy populations and have many causes other than defects in VWF, which can make it impossible to attribute bleeding to any single factor, such as low VWF. These difficulties might be resolved by an epidemiologic approach to VWF and other risk factors for bleeding, analogous to how physicians manage multiple risk factors for cardiovascular disease or venous thromboembolism.