Mild functional defects of VWF are common in patients with low VWF. (A) VWF:Act and VWF:Ag levels in the total low VWF cohort (n = 214) at the time of diagnosis. The blue area indicates patients with plasma VWF:Ag in the 30 to 50 IU/dL range (ie, low VWF–QT). The red area indicates patients with isolated VWF:Act in the 30 to 50 IU/dL range and plasma VWF:Ag levels of >50 IU/dL (ie, low VWF–QL). (B) In 103 of the 214 (48.1%) in our total cohort, a low VWF diagnosis was based on an isolated VWF:Act in the 30 to 50 IU/dL range, whereas VWF:Ag was above 50 IU/dL (low VWF–QL). The remaining 111 patients (51.9%) had VWF:Ag levels in the 30 to 50 IU/dL range (low VWF–QT). (C) Diagram illustrating the number and types of VWF assays that showed reduced levels in patients with low VWF–QL. (D) The plasma VWF:Act/VWF:Ag ratio at diagnosis and at enrolment into the study (with mean time interval of 8.2 years) for patients with low VWF–QL (red) and those with low VWF–QT (blue).