Low Von Willebrand Factor: Cut-Off Value for Diagnosis and Risk Score to Predict Bleeding Incidence
Authors: Atiq Ferdows , Wuijster Esmee , de Maat Moniek P.M. , Kruip Marieke J.H.A. , Cnossen Marjon H. , Leebeek Frank W.G. 
Published: February 2021

Introduction
Although large studies have recently provided valuable insights on the diagnosis, bleeding phenotype, and treatment outcomes of VWD patients, these aspects remain poorly understood in individuals with low VWF. Firstly, there is no clear evidence which cut-off value should be used to diagnose low VWF. Although 0.50 IU/mL is the most recommended cut-off value, some centers use the lower limit of normal (0.60 IU/mL). Secondly, the incidence of post-surgical bleeding, postpartum hemorrhage (PPH) and traumatic- or spontaneous bleeding after diagnosis of low VWF are still unknown. Lastly, it is hard to predict which individuals with low VWF have an increased bleeding risk. Therefore, we investigated the bleeding phenotype of individuals with historically lowest VWF levels of 0.31-0.50 IU/mL and 0.51-0.60 IU/mL, and the incidence of post-surgical bleeding, PPH and traumatic- and spontaneous bleeding after their initial diagnosis of "low VWF".
Methods
We performed a retrospective cohort study from January 2007 to November 2019 at the Erasmus MC, University Medical Center Rotterdam. All patients evaluated for the presence of a bleeding disorder with VWF antigen (VWF:Ag) and/or VWF activity (VWF:Act) and/or VWF collagen binding (VWF:CB) levels between 0.31-0.60 IU/mL, were included. Patients with VWF:Ag and/or VWF:Act and/or VWF:CB ≤0.30 IU/mL, acquired VWD and those with a concomitant bleeding disorder were excluded.
For each individual we collected data from electronic patient files on baseline characteristics, reason for referral, family history of bleeding disorders, ISTH-BAT and laboratory measurements at diagnosis. Retrospective follow-up started from initial date of low VWF diagnosis through November 2019, during which we collected data on surgical procedures, pregnancies, and incidence of spontaneous- and traumatic bleeding.
Results
We included 439 patients; 269 patients with historically lowest VWF levels 0.31-0.50 IU/mL and 170 patients 0.51-0.60 IU/mL. Mean age at diagnosis was 28.8 ±17.7 years. Most patients were female (74.3%) and had blood group O (76.4%, Table 1).
The bleeding score (BS) was similar in patients with historically lowest VWF levels of 0.31-0.50 IU/mL (3.7 ±3.0) and 0.51-0.60 IU/mL (4.0 ±2.9, p=0.209, Table 1). During the mean follow-up period of 6.3 ±3.7 years, 259 surgical procedures were performed in 146 patients, 81 deliveries in 56 women, and 109 spontaneous- or traumatic bleedings in 71 patients. The incidence of post-surgical bleeding was 7 (2.7%) during follow-up, whereas 8 deliveries (10%) were complicated by PPH.
Overall, 65 out of 439 patients (14.8%) had a bleeding episode requiring treatment during follow-up, resulting in an incidence of bleeding requiring treatment of 0.5 ±1.9 per patient per decade. No difference was found in the incidence of bleeding requiring treatment between patients with historically lowest VWF levels of 0.31-0.50 IU/mL and 0.51-0.60 IU/mL (Figure 2A, p=0.154). We found that referral for a personal bleeding diathesis, a younger age at diagnosis and an abnormal BS at diagnosis were strong and independent risk factors for bleeding requiring treatment during follow-up, respectively HR=2.32 (95%CI: 1.16-4.63), HR=1.18 (95%CI: 1.01-1.38) and HR=1.77 (95%CI: 1.04-3.01). These risk factors were combined to develop a risk score to identify low VWF patients with an increased risk for bleeding requiring treatment (Figure 2B). The risk score performed excellent to differentiate in bleeding requiring treatment between low risk, intermediate risk and high risk patients (p&lt;0.001, Figure 2C). The number of patients with bleeding requiring treatment was 8/126 (6.3%) in patients with low risk, 18/143 (12.6%) in intermediate risk and 39/170 (22.9%) in high risk patients (p&lt;0.001). Likewise, the incidence of bleeding requiring treatment per patient per decade was 0.22 ±1.08 in low risk, 0.28 ±1.25 in intermediate risk and 0.87 ±2.61 in high risk patients (p=0.004, Figure 2D).
Conclusion
To conclude, there is no difference in the bleeding phenotype of individuals with historically lowest VWF levels of 0.31-0.50 IU/mL and 0.51-0.60 IU/mL. Therefore, the cut-off value to diagnose low VWF should be set at 0.60 IU/mL. Furthermore, the risk score developed in the current study may assist to identify low VWF patients with low, intermediate and high risk for future bleeding.

Disclosures
Atiq: SOBI: Other: travel grant; CSL Behring: Research Funding. Kruip:Boehringer Ingelheim: Research Funding; Pfizer: Research Funding; Bayer: Research Funding; Daiichi Sankyo: Research Funding; SOBI: Research Funding; Bayer: Speakers Bureau. Cnossen:Takeda: Research Funding; Shire: Research Funding; Baxter: Research Funding; Bayer: Research Funding; Sobi: Research Funding; CSL behring: Research Funding; Nordic Pharma: Research Funding; Novo Nordisk: Research Funding; Pfizer: Research Funding. Leebeek:CSL Behring: Research Funding; Shire/Takeda: Research Funding; Uniqure: Consultancy; Shire/Takeda: Consultancy; Novo Nordisk: Consultancy; SOBI: Other: Travel grant; Roche: Other: DSMB member for a study.