New developments in von Willebrand disease

Von Willebrand disease (VWD) constitutes the most common inherited human bleeding disorder. It is associated with a mucocutaneous bleeding phenotype that can significantly impact upon quality of life. Despite its prevalence and associated morbidity, the diagnosis and subclassification of VWD continue to pose significant clinical challenges. This is in part attributable to the fact that plasma von Willebrand factor (VWF) levels vary over a wide range in the normal population, together with the multiple different physiological functions played by VWF in vivo. Over recent years, substantial progress has been achieved in elucidating the biological roles of VWF. Significant advances have also been made into defining the pathophysiological mechanisms underpinning both quantitative and qualitative VWD. In particular, several new laboratory assays have been developed that enable more precise assessment of specific aspects of VWF activity. In the present review, we discuss these recent developments in the field of VWD diagnosis, and consider how these advances can impact upon clinical diagnostic algorithms for use in routine clinical practice. In addition, we review some important recent advances pertaining to the various treatment options available for managing patients with VWD.     

Presentation and diagnosis of patients with type 3 von Willebrand disease in resources-limited laboratory

Von Willebrand disease (VWD) is a bleeding disorder that results from decreased von Willebrand factor (VWF) activity <0.30 iu/mL. Therefore, the diagnosis of type 3 VWD in patients with bleeding requires finding a VWF:Ag and/or VWF:platelet ristocetin cofactor (RiCof) <0.03 iu/mL, no further testing is usually necessary. This is a cohort study that included 64 patients with type 3 VWD who were presented and diagnosed at the National Center of Hematology (NCH) from October 2014 to October 2016. In this study the sensitivity of VWF:Ag is only 78%, the sensitivity of VWF:RiCof is 92% of diagnosed cases. From our results it can be concluded that patients with type 3 VWD are usually presented with moderate/severe mucocutaneous bleeding that is associated with prolonged bleeding time test of >10 min and a family history of similar type of bleeding. This fact was frequently utilized to provisionally diagnose several members of the same family, forming a cohort of patients that is larger than the number of objectively-diagnosed patients included in this study, when they cannot afford to be all tested with VWF:Ag/VWF:RiCof.     

Acquired von Willebrand syndrome: diagnostic problems and therapeutic options

We present a case of acquired von Willebrand syndrome (AVWS) due to a monoclonal gammopathy of undetermined significance. Initially this case was diagnosed as congenital von Willebrand disease (VWD); however, re-examination of the medical history rendered a congenital bleeding disorder unlikely. A normal plasma von Willebrand factor (VWF) propeptide level and a very short half-life of VWF after a test infusion with factor VIII/VWF concentrate confirmed the diagnosis AVWS. Two major surgical procedures were successfully managed using high-dose intravenous immunoglobulin. The differential diagnosis with congenital VWD and the diagnostic and therapeutic approaches of AVWS are discussed. We conclude that the diagnosis of AVWS relies primarily on clinical suspicion and a careful bleeding history. A correct diagnosis is essential for optimal perioperative management and treatment of bleeding episodes.     

The genetic basis of von Willebrand disease

The common autosomally inherited mucocutaneous bleeding disorder, von Willebrand disease (VWD) results from quantitative or qualitative defects in plasma von Willebrand factor (VWF). Mutation can affect VWF quantity or its functions mediating platelet adhesion and aggregation at sites of vascular damage and carrying pro-coagulant factor VIII (FVIII). Phenotype and genotype analysis in patients with the three VWD types has aided understanding of VWF structure and function. Investigation of patients with specific disease types has identified mutations in up to 70% of type 1 and 100% of type 3 VWD cases. Missense mutations predominate in type 1 VWD and act through mechanisms including rapid clearance and intracellular retention. Many mutations are incompletely penetrant and attributing pathogenicity is challenging. Other factors including blood group O contribute to low VWF level. Missense mutations affecting platelet- or FVIII-binding through a number of mechanisms are responsible for the four type 2 subtypes; 2A, 2B, 2M and 2N. In contrast, mutations resulting in a lack of VWF expression predominate in recessive type 3 VWD. This review explores the genetic basis of each VWD type, relating mutations identified to disease mechanism. Additionally, utility of genetic analysis within the different disease types is explored.     

Von Willebrand factor testing ratios in the diagnosis and subtyping of von Willebrand disease

Von Willebrand disease (VWD) is a common bleeding disorder of platelet adhesion with six currently recognized subtypes. Laboratory diagnosis consists of an initial test panel including antigen, activity and factor VIII measurements, sometimes followed by further specialized testing. VWF activity/antigen testing ratios help to differentiate type 1 and type 2 disease, which is important for selection of proper therapy. Recommended ratio cutoffs differ by guideline, ranging from 0.5 to 0.7, with 0.7 commonly recommended. The ratio cutoff used affects the sensitivity and specificity for type 2 diagnosis. Variability in VWD due to underlying mutations and patient factors, as well as variability in VWF tests, impact the accuracy of ratios for VWD subtyping. This review discusses the use of activity/antigen ratios in the diagnosis and subtyping of VWD with a focus on technical aspects of the tests.     

Type 1 von Willebrand disease - a clinical retrospective study of the diagnosis, the influence of the ABO blood group and the role of the bleeding history

This clinical retrospective study investigated the difficulties in diagnosing type 1 von Willebrand disease (VWD). A total of 246 patients previously diagnosed with type 1 VWD were reclassified into 'possible' type 1 VWD (patients with low levels of VWF adjusted for the blood group and either a significant bleeding history or family history) and 'definite' type 1 VWD, requiring low levels of von Willebrand factor (VWF), a bleeding history and inheritance. On reclassification, only 144/246 (59%) patients had low VWF levels adjusted for blood group, 88/246 (36%) patients met all the criteria for 'definite' type 1 VWD and 51/246 (21%) patients were 'possible' type 1 VWD. A significant proportion of patients, 102/246 (42%), remained an indeterminate group with blood type O, VWF levels between 35 and 50 U/dl and personal and/or family bleeding history. This subgroup might require reclassification as 'not VWD'. However, a similar bleeding tendency was found in two matched groups of patients of blood groups O and non-O and VWF levels between 35 and 50 U/dl. These results suggest that the use of ABO adjusted ranges for VWF levels might not be essential for diagnosis, because bleeding symptoms may depend on the VWF level regardless of the ABO type. Of the diagnostic criteria, the bleeding history was of prime importance in the clinical decision to diagnose and treat type 1 VWD. These observations could help in the reconsideration of how the criteria for diagnosing type 1 VWD could be adjusted in order to maximize their clinical relevance.     

Inverse relationship between plasma von Willebrand factor and soluble P selectin in patients with type 1 but not type 2 von Willebrand disease

P selectin is a component of endothelial Wiebel-Palade body membrane and platelet alpha granule membrane. Patients with von Willebrand disease (VWD) have low plasma von Willebrand factor (VWF). To examine the relationship between plasma P selectin and VWF, both were measured by ELISA in 38 patients with VWD and 40 controls. The patient had lower VWF (P < 0.001) but similar P selectin levels (P = 0.458). In 24 type 1 VWD patients, VWF and P selectin correlated inversely (P = 0.005) but in 14 type 2 VWD patients there was no correlation (P = 0.997). These data imply major differences in VWF/P-selectin regulation between types 1 and 2 VWD.     

Laboratory monitoring of replacement therapy for major surgery in von Willebrand disease

Von Willebrand disease (VWD) is an inherited haemorrhagic disorder caused by a quantitative or qualitative defect of von Willebrand factor (VWF), a multimeric plasma glycoprotein that plays a key role in platelet adhesion to the subendothelium and acts as a carrier of factor VIII (FVIII) in blood. Patients with VWD experience bleeding symptoms that are mainly localized in mucous membranes and soft tissues, and their severity depends on the degree of the primary reduction in VWF and the secondary deficiency of FVIII in plasma. Because VWD patients are also at increased risk of perioperative bleeding, a prophylactic treatment aimed to correct the dual haemostatic defect (i.e. VWF and FVIII) is warranted. This review summarizes knowledge on the current management of patients undergoing major surgery, focusing on the peri‐surgical laboratory monitoring of replacement therapy with VWF/FVIII concentrates. We suggest to monitor plasma levels of FVIII coagulant activity in the postoperative period rather than a surrogate maker of platelet‐binding VWF activity as the ristocetin cofactor assay and its recent modifications.     

Acquired von Willebrand disease in a patient with angiodysplasia resulting from immune-mediated clearance of von Willebrand factor

A patient with a severe bleeding tendency due to acquired von Willebrand disease (VWD) is presented. Although no underlying disorder has emerged during 6 years of follow-up, an immune-mediated mechanism was responsible for acquired VWD in this patient as demonstrated by detection of von Willebrand factor (VWF)/anti-VWF complexes in the patient's plasma and their removal by protein A–sepharose beads and resumption of normal haemostasis with correction of VWF antigen, VWF activity and VWF multimeric pattern after treatment of the patient with high-dose gammaglobulin. Detection of anti-VWF antibodies in the patient's plasma had a significant impact on the choice of therapeutic intervention to control bleeding.     

Association of acquired von Willebrand syndrome with AL amyloidosis

Acquired loss of functional von Willebrand factor (VWF) has been termed the acquired von Willebrand syndrome (AVWS). AVWS is a rare adult-onset bleeding diathesis that is clinically similar to congenital von Willebrand disease (VWD), and occurs with a variety of autoimmune, lymphoproliferative, or myeloproliferative disorders. We have identified four patients with AVWS in association with immunoglobulin light chain (AL) amyloidosis. These patients, lacking any pre-existing or family history of abnormal bleeding, developed cutaneous, mucosal, or gastrointestinal bleeding in the course of their disease without deficiency of clotting factor X or other factors; the activated partial thromboplastin time (aPTT) was prolonged in three out of the four cases. Despite normal VWF antigen levels, VWF ristocetin cofactor activity (VWF:RCo) was low. Electrophoresis patterns of high molecular weight (HMW) VWF multimers were abnormal in two of the four cases. Two of the patients were treated with high-dose intravenous melphalan followed by autologous stem cell transplantation (HDM/SCT) and achieved hematologic remission. In these two patients, the bleeding diathesis improved and the coagulation parameters normalized, confirming a causal relationship between the plasma cell dyscrasia and the AVWS. AVWS should be considered in AL amyloidosis patients with hemorrhagic diatheses and normal clotting factor levels.     

The impact of bleeding history,von Willebrand factor and PFA–100® on the diagnosis of type 1 von Willebrand disease: results from the European study MCMDM‐1VWD

The relationships between the Platelet Function Analyzer (PFA)‐100 and von Willebrand factor (VWF) levels and bleeding score (BS) were evaluated within a multicentre project on Molecular and Clinical Markers for the Diagnosis and Management of type 1 von Willebrand disease (MCMDM‐1VWD). PFA‐100 closure time, either with epinephrine (EPI) or adenosine diphosphate (ADP)‐cartridges, was measured in 107 index cases, 105 affected and 71 unaffected family members, and 79 healthy controls. By regression analysis VWF levels were strongly related to both closure times, with a non‐linear progression. In a multiple stepwise regression model, age‐ and sex‐adjusted PFA‐100 ADP and VWF ristocetin cofactor activity (VWF:RCo) were independently associated with BS. Most of the variation of BS was predicted by PFA‐100 ADP and VWF:RCo alone. In the subgroup of patients with subtle abnormalities of the multimeric pattern, VWF was invariably reduced and closure time prolonged in almost all of them. Neither PFA‐100 ADP nor EPI closure times appeared to significantly improve the diagnostic capability of VWF antigen (VWF:Ag) measurement. Thus, in an unselected population a normal PFA‐100 would be useful to exclude VWD, but whether it could replace the more specific VWF assay in patients with significant mucocutaneous bleeding symptoms remains to be investigated prospectively.     

Screening for von Willebrand disease: contribution of an automated assay for von Willebrand factor activity

Measuring von Willebrand factor (VWF) activity is essential to the diagnosis of von Willebrand disease (VWD). The VWF activity is usually assessed based on measurement of the ristocetin cofactor (VWF:RCo). However, that test is technically challenging and has high intra- and inter-assay variabilities. The HemosIL VWF activity (VWF:AC) is a fully automated assay, recently proposed as a good alternative to VWF:RCo for VWD diagnosis. This study was undertaken to assess this new method. First, the analytical performance of VWF:AC on an automated coagulo-meter (ACLTop) was determined, and then this new method was compared with VWF:RCo and the platelet function analyzer (PFA100) for 160 patients referred for VWD screening. The VWF:AC achieved acceptable precision with within-run and between-run coefficients of variation ranging from 2.3% to 14.1%, and linearity from 10% to 100%. Despite some marked differences between VWF:AC and VWF:RCo for 10 plasmas tested, their agreement for VWD diagnosis was good. The VWF:AC had sensitivity similar to that of PFA100 (close to 100%), but better specificity (97.7% vs. 66% or 60%, depending on the cartridge used). The good analytical performance, and the sensitivity and specificity of VWF:AC to detect VWF deficiency renders it a suitable method for VWD screening. Our findings support VWF:AC use for the diagnostic work-up of VWD, paying close attention to concomitant clinical signs and bleeding score, as recommended for VWD.     

Studies of multimerin in patients with von Willebrand disease and platelet von Willebrand factor deficiency

In normal platelet α-granules von Willebrand factor (VWF) is stored with multimerin and factor V in an eccentric electron-lucent zone. Because the platelet stores of VWF are deficient in 'platelet low' type 1 and type 3 von Willebrand disease (VWD), we investigated their electron-lucent zone proteins. The patients with VWD had partial to complete deficiencies of plasma and platelet VWF but normal α-granular multimerin and factor V, and normal α-granular fibrinogen, thrombospondin-1, fibronectin, osteonectin and P-selectin. In type 3 VWD platelets, α-granular electron-lucent zones lacking VWF-associated tubules were identified and multimerin was found in its normal α-granular location. These findings indicate that the formation of the electron-lucent zone and the sorting of multimerin to this region occur independent of VWF. The isolated abnormalities in VWF suggests a VWF gene mutation is the cause of 'platelet low' type 1 VWD.     

von Willebrand disease (VWD): evidence‐based diagnosis and management guidelines,the National Heart,Lung, and Blood Institute (NHLBI) Expert Panel report (USA)1

Summary. von Willebrand disease (VWD) is a commonly encountered inherited bleeding disorder affecting both males and females, causing mucous membrane and skin bleeding symptoms, and bleeding with surgical or other haemostatic challenges. VWD may be disproportionately symptomatic in women of child‐bearing age. It may also occur less frequently as an acquired disorder (acquired von Willebrand syndrome). VWD is caused by deficiency or dysfunction of von Willebrand factor (VWF), a plasma protein that mediates platelet haemostatic function and stabilizes blood coagulation factor VIII. The pathophysiology, classification, diagnosis and management of VWD are relatively complex, but understanding them is important for proper diagnosis and management of patients with VWD. These evidence‐based guidelines for diagnosis and management of VWD from the National Heart, Lung, and Blood Institute (NHLBI) Expert Panel (USA) review relevant publications, summarize current understanding of VWD pathophysiology and classification, and present consensus diagnostic and management recommendations based on analysis of the literature and expert opinion. They also suggest an approach for clinical and laboratory evaluation of individuals with bleeding symptoms, history of bleeding or conditions associated with increased bleeding risk. This document summarizes needs for further research in VWF, VWD and bleeding disorders, including clinical research to obtain more objective information about bleeding symptoms, advancements in diagnostic and therapeutic tools, and enhancement in the education and training of clinicians and scientists in bleeding and thrombotic disorders. The NHLBI Web site ( http://www.nhlbi.nih.gov/guidelines/vwd ) has a more detailed document, a synopsis of these recommendations, and patient education information.     

An approach to outreach patients with von Willebrand disease in Egypt by targeting women with heavy menstrual bleeding and/or bleeding symptoms

von Willebrand disease (VWD) is frequently ignored as a cause of menorrhagia. We investigated Egyptian women complaining of heavy menstrual bleeding (HMB) and/or other bleeding symptoms to detect potential VWD cases. Seventy‐five female patients complaining of HMB and/or bleeding symptoms and 38 age‐matched healthy female controls went through a family history questionnaire, a physical examination and were evaluated for bleeding score, pictorial blood assessment chart (PBAC), complete blood count, serum ferritin, blood group, prothrombin time, activated partial thromboplastin time, factor VIII (FVIII) activity, von Willebrand factor (VWF) ristocetin cofactor (RCo) activity, antigen (Ag), and RCo/Ag ratio. Sixty‐eight of 75 patients presented with HMB, out of which 46 had no organic pathology and 7 presented other bleeding symptoms. Six patients were diagnosed with VWD, three with HMB, two with other bleeding symptoms and one with family history of VWD. Two related VWD patients were diagnosed in the control group. There were significant differences in bleeding and PBAC scores, ferritin level, FVIII activity, VWF:RCo and VWF:Ag between VWD patients and controls. This study indicated a high prevalence of VWD among patients with HMB without organic pathology (6.5%) and demonstrated the sensitivity of diagnostic parameters of VWD patients in an outreach campaign. The inexpensive bleeding and PBAC scoring systems are valuable to exclude cases without objective bleeding symptoms. Raising gynaecologists awareness about hereditary bleeding disorders is important to ensure a proper diagnosis and possible referral of these patients. Management of these patients with comprehensive medical care services under a multidisciplinary team would be ideal.     

Variability in platelet‐ and collagen‐binding defects in type 2M von Willebrand disease

Type 2M von Willebrand disease (VWD) includes qualitative defects in von Willebrand factor (VWF) function, with normal multimer distribution but a defect in VWF activity with respect to platelet or collagen binding. We characterized novel VWF gene mutations found in type 2M VWD subjects enrolled in the Zimmerman Program for the Molecular and Clinical Biology of VWD. Subjects were enrolled based on a pre‐existing diagnosis of type 2M VWD. Testing included full‐length gene sequencing, VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), VWF collagen binding and multimer distribution. Recombinant VWF variants were synthesized using site‐directed mutagenesis and expressed in HEK293T cells. Platelet binding was measured by flow cytometry with fixed platelets and ELISA with recombinant glycoprotein Ibα (GPIbα). Four novel VWF A1 domain mutations were found in individuals with type 2M VWD: S1358N, S1387I, S1394F and Q1402P. All subjects had a history of bleeding, VWF:RCo < 40 IU dL^?1, VWF:RCo/VWF:Ag ratios <0.6 and normal multimer distribution. No defect in expression, secretion, or multimerization was found for any of the mutations. All showed decreased binding to intact platelets, and decreased or absent binding to a mutant GPIbα construct with spontaneous VWF binding. 1387I had decreased binding to all collagen types tested. 1402P had reduced binding exclusively to type VI collagen. Type 2M VWD is a heterogeneous category comprised of both collagen‐ and platelet‐binding defects. Understanding the precise defect for each mutation may ultimately lead to better diagnosis and treatment.     

Lack of multimer organization of von Willebrand factor in an acquired von Willebrand syndrome

We report a case of acquired von Willebrand syndrome (AVWS) in a 20-year-old-woman with systemic lupus erythematosus, in whom severe bleeding complications followed kidney biopsy. Coagulation studies demonstrated undetectable levels of ristocetin-induced platelet aggregation (RIPA), von Willebrand factor antigen (VWF:Ag) and VWF ristocetin cofactor activity (VWF:RCo), associated with significantly prolonged bleeding time; unlike type 3 von Willebrand disease (VWD), platelet VWF was reduced but not undetectable. The plasma VWF multimer pattern was characterized by the presence of only two bands, one of low molecular weight (MW) running as the protomer of plasma VWF in normals, the other of abnormally high MW without detectable intermediate multimers; this pattern resembles that of VWF present in endothelial cells. A search for an anti-VWF antibody demonstrated the presence of an inhibitor at high titre. This anti-VWF antibody did not interfere in the interaction of VWF with platelet glycoprotein (GP) Ib through the A1 domain, and did not react with the A2 domain of VWF; instead, it seemed to modify the relative representation of high and low MW VWF multimers released by normal human umbilical vein endothelial cells (HUVEC). After Azathioprine and corticosteroid treatment, the anti-VWF antibody disappeared and the patient's haemostatic profile normalized, except for the platelet VWF content which still remained decreased. We suggest that the anti-VWF antibody present in the AVWS described compromised both circulating VWF levels and their multimeric organization, inducing the maintenance of the multimer structure that VWF normally has before or in the early phase after secretion from endothelial cells.     

Bleeding symptoms and laboratory correlation in patients with severe von Willebrand disease

Summary.  Type 3 von Willebrand disease (VWD) is a rare bleeding disorder with markedly decreased or absent von Willebrand factor (VWF) protein, accompanied by a parallel decrease in VWF function and factor VIII (FVIII) activity. The goal of this study was to describe the population of patients enrolled in the USA Centers for Disease Control Universal Data Collection (UDC) study with type 3 VWD, defined as a VWF:Ag of <10%, and to correlate bleeding symptoms with VWF and FVIII levels. Data on 150 patients were analysed. Almost all patients experienced bleeding episodes (98%) and required blood and/or factor product treatment (92%). While oral mucosal bleeding (the site of first bleed in 54%) was most common, subsequent muscle and joint bleeds were also seen (28%, 45%, respectively), and intracranial haemorrhage occurred in 8% of individuals. Mean age of first bleed was lower in those with either a FVIII ≤5% or a VWF:Ag <1%. Univariate marginal model analysis showed lower levels of FVIII and VWF:Ag both predicted a higher risk of joint bleeding. Longitudinal multivariate analysis found a lower FVIII level ( P  = 0.03), increasing age ( P  < 0.0001), history of joint bleeding ( P  = 0.001), higher body mass index (BMI) ( P  < 0.0001), and use of home infusion ( P  = 0.02) were all negatively associated with joint mobility. Low levels of VWF:Ag ( P  = 0.003) and male sex ( P  = 0.007) were also negatively associated with joint function. This study documents the strong bleeding phenotype in severe VWD and provides data to help target therapy, including prophylaxis, for patients most at risk of bleeding complications.     

Autosomal dominant C1149R von Willebrand disease: phenotypic findings and their implications

Background

Mutation C1149R in the von Willebrand factor (VWF) gene has been thought to cause autosomal dominant severe type 1 von Willebrand disease (VWD).

Design and Methods

Eight patients from three unrelated families with this mutation were included in the present study who had distinct VWF abnormalities, not described in earlier studies.

Results

The patients showed notably low levels of VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), VWF collagen binding (VWF:CB), and a reduced ristocetin-induced platelet aggregation (RIPA). VWF:RCo/VWF:Ag and VWF:CB/VWF:Ag ratios were lower than 0.7. At basal conditions, all the VWF multimers were decreased in plasma, with a clearly lower relative proportion of the high molecular weight VWF multimers (HMWM). In high-resolution agarose gels, a large decrease in the relative proportions of the satellite bands was seen. The patients had a brief good response to desmopressin (DDAVP) administration, but the released VWF half-life was shorter than normal, indicating an accelerated clearance of their VWF. Platelet VWF was abnormal.

Conclusions

We conclude from the results obtained in these patients for plasma phenotypic data that this mutation should be classified as a VWD type 2A (IIE). DDAVP therapy may be somewhat helpful for this mutation, at least for mild to moderate bleeding. These data provide evidence that for VWD classification factors other than basal VWF, such as DDAVP response and platelet VWF, should be considered.     

Measurement of von Willebrand factor binding to a recombinant fragment of glycoprotein Ibalpha in an enzyme-linked immunosorbent assay-based method: performances in patients with type 2B von Willebrand disease

Type 2B von Willebrand disease (VWD) is characterised by an increased affinity of von Willebrand factor (VWF) for its platelet receptor glycoprotein Ib (GPIb). This feature is usually studied in vitro by a ristocetin-dependent VWF platelet-binding assay, which has some limitations as it requires [e.g. (radio)-labelled anti-VWF antibodies and normal formaldehyde-fixed platelets]. We, here, extended the applicability of an enzyme-linked immunosorbent assay-based method previously described for the measurement of ristocetin co-factor activity that used a recombinant fragment of GPIb (rfGPIb alpha) and horseradish peroxidase-labelled rabbit anti-human VWF antibodies for measuring the captured ristocetin-VWF complexes on the rfGPIb alpha. Thirty-one type 2B VWD patients from 15 families with eight different known mutations were studied. VWF in plasma from 28 of these patients bound better than normal VWF at 0.2 mg/ml ristocetin, with the ratio, optical density (OD) patient/OD normal pool plasma, higher than 1.8. For two of the three other patients with no enhanced response of plasma VWF, the platelet lysate VWF showed an enhanced binding capacity; for the last patient, the results in other members of the family are unequivocal. We conclude that, this new method for measurement of plasma or platelet VWF-binding capacity offers great advantages for correct type 2B VWD diagnosis.