Novel insights into the clinical phenotype and pathophysiology underlying low VWF levels.

Critical clinical questions remain unanswered regarding diagnosis and management of patients with low von Willebrand factor (VWF) levels (30-50 IU/dL). To address these questions, the Low VWF Ireland Cohort (LoVIC) study investigated 126 patients registered with low VWF levels. Despite marginally reduced plasma VWF levels, International Society of Thrombosis and Haemostasis Bleeding Assessment Tool (ISTH BAT) confirmed significant bleeding phenotypes in the majority of LoVIC patients. Importantly, bleeding tendency did not correlate with plasma VWF levels within the 30 to 50 IU/dL range. Furthermore, bleeding phenotypes could not be explained by concurrent hemostatic defects. Plasma factor VIII to VWF antigen (VWF:Ag) ratios were significantly increased in LoVIC patients compared with controls (P < .0001). In contrast, VWF propeptide to VWF:Ag ratios >3 were observed in only 6% of the LoVIC cohort. Furthermore, platelet-VWF collagen binding activity levels were both significantly reduced compared with controls (P < .05). In response to 1-desamino-8-D-arginine vasopressin (DDAVP), peak VWF:Ag levels exceeded 100 IU/dL in 88% of patients and was sustained >100 IU/dL after 4 hours in 72% of subjects. In conclusion, our novel data suggest that low VWF levels can be associated with significant bleeding and are predominantly due to reductions in VWF synthesis and/or constitutive secretion. Although enhanced VWF clearance may contribute to the pathophysiology in some individuals, the absolute reduction in VWF plasma half-life is usually mild and not sufficient to significantly impact upon the duration of DDAVP-induced VWF response. This trial was registered at www.clinicaltrials.gov as #NCT03167320.