von Willebrand factor: measuring its antigen or function? Correlation between the level of antigen, activity, and multimer size using various detection systems

The Role of the von Willebrand Factor Collagen-Binding Assay (VWF:CB) in the Diagnosis and Treatment of von Willebrand Disease (VWD) and Way Beyond: A Comprehensive 36-Year History

The von Willebrand factor (VWF) collagen binding (VWF:CB) assay was first reported for use in von Willebrand diagnostics in 1986, by Brown and Bosak. Since then, the VWF:CB has continued to be used to help diagnose von Willebrand disease (VWD) (correctly) and also to help assign the correct subtype, as well as to assist in the monitoring of VWD therapy, especially desmopressin (DDAVP). However, it is important to recognize that the specific value of any VWF:CB is predicated on the use of an optimized VWF:CB, and that not all VWF:CB assays are so optimized. There are some good commercial assays available, but there are also some "not-so-good" commercial assays available, and these may continue to give the VWF:CB "a bad reputation." In addition to VWD diagnosis and management, the VWF:CB found purpose in a variety of other applications, from assessing ADAMTS13 activity, to investigation into acquired von Willebrand syndrome (especially as associated with use of mechanical circulatory support or cardiac assist devices), to assessment of VWF activity in disease states in where an excess of high-molecular-weight VWF may accumulate, and lead to increased (micro)thrombosis risk (e.g., coronavirus disease 2019, thrombotic thrombocytopenic purpura). The VWF:CB turns 37 in 2023. This review is a celebration of the utility of the VWF:CB over this nearly 40-year history.

Validation of an automated latex particle-enhanced immunoturbidimetric von Willebrand factor activity assay

BACKGROUND

Laboratory diagnosis of von Willebrand disease (VWD) requires accurate measurement of plasma von Willebrand factor (VWF) activity.

OBJECTIVES

To evaluate laboratory characteristics, diagnostic accuracy and testing utilities of an automated latex particle-enhanced immunoturbidimetric VWF assay (VWF:Lx) based on a monoclonal antibody recognizing the VWF-platelet glycoprotein (GP) Ib binding domain.

METHODS

Laboratory characteristics including lower detection limit, linearity, precision, sample stability, and method comparison between VWF:Lx and VWF ristocetin cofactor activity by platelet aggregometry (VWF:RCo) were examined. To assess VWF:Lx diagnostic accuracy, 492 patient plasma samples, including 40 previously characterized VWD patient samples, were tested for VWF antigen (VWF:Ag) and VWF:RCo by either aggregometry or flow cytometry, and VWF:Lx with supplemental VWF multimer analysis when indicated. Based on results of VWF:Ag, VWF:RCo and VWF multimer analysis, and available clinical information, samples were categorized as: normal; VWD types 1, 2A/B, 2M, or severe 1 vs. 2M; or acquired VWF abnormalities (AVWA) due to subtle loss of highest molecular weight multimers.

RESULTS

VWF:Lx had excellent laboratory characteristics and linear correlation with VWF:RCo (R(2) = 0.93). VWF:Lx accurately classified virtually all normal and VWD patient samples. Compared with VWF:RCo, VWF:Lx had superior sensitivity and specificity for distinguishing severe type 1 vs. 2M VWD and identifying AVWA. A proposed screening panel comprising VWF:Ag and VWF:Lx had 100% and 83% sensitivity for detecting VWD and AVWA, respectively.

CONCLUSIONS

VWF:Lx has excellent laboratory characteristics and diagnostic accuracy compared with VWF:RCo, and can be used as part of an initial VWD screening panel and as a supplementary test.

Von Willebrand factor--two sides and the edge of a coin

Von Willebrand factor (VWF) has multiple functions in coagulation. It is a clotting protein and its deficiency causes a primary haemostatic bleeding disorder. Excess VWF, particularly high molecular weight multimers can cause thrombosis. There is also a debatable function of protecting factor VIII (FVIII) in circulation with the prevention of development of FVIII inhibitors. This commentary addresses these functions.

Comparative analysis of von Willebrand factor profiles in pulsatile and continuous left ventricular assist device recipients

A higher rate of nonsurgical bleeding has been observed in nonpulsatile left ventricular assist device (LVAD) recipients. von Willebrand factor (vWF) profiles were compared for nonpulsatile and pulsatile LVAD recipients to explore mechanisms that may contribute to the development of postimplant nonsurgical bleeding. The nonpulsatile mechanism may impair vWF function by creating a deficiency in vWF high molecular weight multimers (HMWMs), essential for hemostasis. High molecular weight multimer deficiency should result in low ristocetin cofactor (RCo) to vWF antigen ratios (vWF:RCo/vWF:Ag) because of impaired platelet (plt)-binding ability. von Willebrand factor profiles and HMWM were measured pre- and post-LVAD placement in 11 nonpulsatile (HeartMate II [HM II[) and 3 pulsatile (HeartMate XVE [HM XVE]) recipients. All the nonpulsatile LVAD recipients exhibited loss of HMWM 30 days postimplant. The vWF:RCo/vWF:Ag ratio was significantly lower after LVAD placement in the nonpulsatile group when compared with the pulsatile group. In addition, the vWF:RCo/vWF:Ag ratio decreased significantly from baseline 30 days postimplant within the nonpulsatile recipients. All nonpulsatile LVAD recipients had low vWF:RCo/vWF:Ag ratios 30 days post-LVAD even if the values were normal at baseline. These data suggest that nonpulsatile HM II recipients develop HMWM loss and impaired vWF platelet (plt)-binding ability after LVAD placement. Similar results were not observed in our small series of pulsatile HM XVE recipients. This finding could suggest a contributing factor to the increase in nonsurgical bleeding observed in nonpulsatile LVAD patients. Further investigation is ongoing to identify specific causes of vWF impairment.

The prevalence of the cysteine1584 variant of von Willebrand factor is increased in type 1 von Willebrand disease: co-segregation with increased susceptibility to ADAMTS13 proteolysis but not clinical phenotype

The molecular pathogenesis of type 1 von Willebrand disease (VWD) is uncertain in most patients. We examined 30 type 1 VWD families in the UK Haemophilia Centre Doctors' Organization study. Heterozygosity for Y/C1584 was present in eight of 30 (27%) families and 19 of 76 (25%) individuals with type 1 VWD recruited into the study. Eighteen (95%) of these 19 individuals were blood group O. C1584 did not co-segregate with VWD in four families, and co-segregated in one family; the results were equivocal in three families. In all families increased susceptibility of von Willebrand factor (VWF) to a disintegrin and metalloprotease with thrombospondin motifs (ADAMTS) 13 proteolysis co-segregated with C1584 in affected and unaffected individuals. These data show that C1584, associated with blood group O, is prevalent among patients with type 1 VWD but not necessarily causative of disease and should not be used in isolation to diagnose VWD. Increased susceptibility of C1584 VWF to ADAMTS13 proteolysis may be physiologically significant and increase an individual's risk of bleeding and presenting with VWD.

Measurement of von Willebrand factor activity: relative effects of ABO blood type and race

Tests based on three different principles are reported to measure the activity of von Willebrand factor (VWF): ristocetin cofactor (VWF:RCo), collagen binding (VWF:CB), and the so-called "activity ELISA" (VWF:MoAb). We measured these and other diagnostic parameters in a population of 123 randomly selected female study controls, age 18-45 years. Type O subjects had significantly lower levels than non-O subjects in each test. Race differences were seen in all tests except VWF:RCo, with Caucasians having significantly lower levels than African-Americans. ABO differences accounted for 19% of the total variance in VWF:Ag (P < 0.0001) and race for 7% (P < 0.0001), for a total of 26%. Both effects were mediated through VWF:Ag and were independent. VWF:Ag level was the primary determinant of VWF function, accounting for approximately 60% of the variance in VWF:RCo and VWF:CB and 54% of the variance in factor VIII. The ratio VWF:RCo/VWF:Ag differed significantly by race within blood group. The median ratios were 0.97 for type O Caucasians vs. 0.79 for type O African-Americans and 0.94 for non-O Caucasians vs. 0.76 for non-O African-Americans. The ratio VWF:CB/VWF:Ag did not vary. This suggests racial differences in the interaction of VWF with GP1b but not with subendothelium. Alternatively, VWF:RCo may be regulated to maintain a relatively constant plasma level in the presence of excessive VWF:Ag. This heterogeneity within the normal population is partially responsible for the difficulty in defining diagnostic limits for von Willebrand disease.

An influence of ABO blood group on the rate of proteolysis of von Willebrand factor by ADAMTS13

The susceptibility of von Willebrand factor (VWF) of blood group O, A, B and AB to proteolysis by the ADAMTS13 metalloprotease was investigated. Multimeric analysis indicated that the rate of VWF proteolysis differed between blood groups and was greater for group O VWF than for non-O VWF in the rank order O >/= B > A >/= AB. Measurement of the collagen binding activity of VWF of each blood group following proteolysis for a fixed time interval corroborated the results obtained on multimer analysis: the loss of collagen binding activity was greater for VWF of group O compared with non-O VWF, in the rank order O >/= B > A >/= AB. Ristocetin was found to increase the rate of VWF proteolysis approximately two-fold; the differential between blood groups was retained in the presence of ristocetin.

Development and applications of surface plasmon resonance-based von Willebrand factor-collagen binding assay

Von Willebrand factor (vWf) functions both as a carrier of factor VIII (fVIII) in plasma and as an adhesive protein providing the primary link between collagen of the extracellular matrix and platelets sequestered from blood flow. The functional activity of vWf correlates with the level of its binding to collagen, which is commonly measured in the enzyme-linked immunosorbent assay (ELISA). We developed an automated collagen-binding assay employing the surface plasmon resonance (SPR) phenomenon, which allows one to quantitatively measure the binding of purified vWf and vWf-containing therapeutic fVIII concentrates to collagen type III immobilized on a biosensor chip. The results of the SPR-based assay highly correlated (r = 0.987) with collagen-binding ELISA. The advantages of the SPR-based assay are its higher accuracy and reproducibility in comparison with ELISA. We applied the developed assay for monitoring structural changes in the vWf component of plasma-derived fVIII/vWf concentrates during a virus inactivation procedure performed by heat treatment. We determined the critical residual moisture content of 2% that can be present in lyophilized concentrates during heat-treatment procedures without causing deteriorative changes in vWf properties. Our data suggest that the SPR-based assay is a useful tool in the development of industrial virus-inactivation procedures, allowing one to preserve vWf activity and achieve the maximal therapeutic efficacy of fVIII/vWf concentrates.

Appropriate laboratory assessment as a critical facet in the proper diagnosis and classification of von Willebrand disorder

The correct diagnosis and classification of von Willebrand disease or disorder (VWD) is crucial because the presenting biological activity of von Willebrand factor (VWF) determines both the haemorrhagic risk and the subsequent clinical management. A variety of laboratory assays may be employed, not necessarily restricted to assessments of VWF. Because of assay limitations and von Willebrand disease heterogeneity, no single test procedure is sufficiently 'robust' to permit the detection of all VWD variants. Classically, the test panel might include any combination of: (a) skin bleeding time, (b) von Willebrand factor antigen assay, (c) factor VIII C level, (d) assessment of 'functional' von Willebrand factor (collagen-binding activity or ristocetin co-factor assay), (e) ristocetin-induced platelet aggregation, and (f) multimer analysis. There have also been many new diagnostic developments that have begun to influence the diagnostic process. These include the automation of existing assay procedures, new automated platelet function analyzers such as the PFA-100, and specific von Willebrand factor-factor VIII-binding assays. This chapter focuses on the recommended laboratory process for the investigation of VWD. The selection of an appropriate combination test panel and testing sequence is crucial for the proper diagnosis and classification of congenital von Willebrand disease.

Abnormal von Willebrand factor in bleeding angiodysplasias of the digestive tract

BACKGROUND & AIMS

Involvement of an abnormal von Willebrand factor in the bleeding expression of gastrointestinal angiodysplasias has been suggested but not assessed by prospective studies.

METHODS

To address this issue, 27 patients with either nonbleeding (group A, n = 9) or bleeding (group B, n = 9) digestive angiodysplasias or telangiectasias or diverticular hemorrhage (group C, n = 9) were enrolled. In all patients, an analysis of von Willebrand factor and a screening for the most common disorders associated with an acquired von Willebrand disease were performed.

RESULTS

In all patients from groups A and C, von Willebrand factor was normal, and no underlying disease could be found. In contrast, all but 1 patient from group B had a variable selective loss of the largest multimeric forms of von Willebrand factor, associated in 7 cases with a stenosis of the aortic valve.

CONCLUSIONS

This study indicates that most patients with bleeding angiodysplasia or telangiectasia have a deficiency of the largest multimers of von Willebrand factor induced by a latent acquired von Willebrand disease. Because these multimers are the most effective in promoting primary hemostasis at the very high shear conditions related to these vascular malformations, we suggest that their deficiency is likely to contribute to the bleeding diathesis.

Assessment of current diagnostic practice and efficacy in testing for von Willebrand's disorder: results from the second Australasian multi-laboratory survey

This study reports an evaluation of current laboratory practice for the diagnosis of von Willebrand's disorder (VWD) by means of a multi-laboratory (n = 19) survey (the 'Second Australasian VWD Survey'). Results are compared with a previously conducted but similarly comprehensive survey ('Survey-1'). Samples comprised a new set of seven plasmas: (i) Type 3 VWD; (ii) Type 2B VWD; (iii) Moderate Type 1 VWD/Haemophilia A combined defect; (iv) Normal individual; (v) Mild Type 1 VWD; (vi) Type 2M/2A VWD; (vii) Type 2N VWD. Overall, many current findings confirmed those reported in Survey-1 [including between-method analysis, within-method analysis, inter-laboratory assay variation, sensitivity to low levels of von Willebrand Factor (vWF), detection of functional vWF 'discordance', and appropriateness of diagnostic predictions]. Novel findings include: (i) although vWF:collagen binding activity (vWF:CBA) performed better than vWF:ristocetin cofactor (vWF:RCof) assay in identification of functional discordance in Type 2B VWD, both assays performed equally in identification of discordance in the Type 2M/2A VWD; (ii) most laboratories failed to identify the Type 2N VWD as a potential 2N utilizing vWF antigen (vWF:Ag) and factor VIII coagulant (FVIII:C) testing as a screening process; (iii) this particular survey was followed up by a dry workshop attended by over 45 scientists from Australia and New Zealand, and comprising representatives from most survey participants. Discussion covered many topics including the effect of blood group, the role (if any) of the bleeding time, the role of the PFA-100, confirmatory and additional tests, and the possibility of restricting testing to specialized centres. Consensus was reached on the following points: (i) diagnosis of VWD requires both clinical and laboratory assessment; (ii) testing should comprise FVIII:C, vWF:Ag and either/or both vWF:RCof and vWF:CBA; (iii) laboratory results should be reviewed in the light of clinical findings; and (iv) confirmatory repeat testing should be performed on a sample taken 6 weeks later.

Detection of von Willebrand disorder and identification of qualitative von Willebrand factor defects. Direct comparison of commercial ELISA-based von Willebrand factor activity options

Two von Willebrand factor (vWF):collagen binding (activity) assay (CBA) kit methods are commercially available. A monoclonal antibody (MAB)-based enzyme-linked immunosorbent assay (ELISA) system reported to correlate with a standard vWF:ristocetin cofactor (RCof) assay is also commercially available. It is marketed as a vWF:Activity assay and is available in 2 assay version formats. In the present study, these 4 vWF-activity options were compared directly with in-house vWF:CBA ELISAs for their ability to detect von Willebrand disease (vWD) and identify qualitative vWF defects. The 2 MAB-based systems detected vWD but could not specifically identify qualitative vWF defects, although the recently modified Mark II kit was more effective for the latter compared with the original Mark I kit. All vWF:CBA methods, including in-house and commercial, also effectively detected vWD but differed in their ability to identify qualitative vWF defects. Effectiveness was highest using the in-house reference vWF:CBA (using a type I/III collagen mix product from equine tendon), the Gradipore vWF:CBA (also uses equine tendon-derived collagen), or the in-house vWF:CBA methods using type III human collagen at a relatively low concentration (1 or 3 micrograms/mL, without covalent linkage). The IMMUNO vWF:CBA seemed to be the least effective among the vWF:CBA methods for detection of qualitative vWF defects.

Sulfatide-binding assay for von Willebrand factor. Detection of von Willebrand's disease without discrimination of vWD subtypes

The appropriate diagnosis of von Willebrand's disease requires a panel of laboratory assays. Subclassification of a patient's von Willebrand's disease is also important as it will influence management. The minimal test panel should comprise an assay for factor VIII coagulant, assessment of von Willebrand factor antigen and at least one functional von Willebrand factor assay (collagen-binding assay and/or Ristocetin cofactor assay). This panel will provisionally determine whether von Willebrand's disease is present and whether the von Willebrand's disease provisionally detected is a qualitative (Type 2) or quantitative (Types 1 or 3) defect, and results will help guide further investigation and the need for further testing. In view of reports indicating that sulfatides bind to von Willebrand factor by a specific high-affinity mechanism distinct from collagen, and with affinity reportedly dependent on von Willebrand factor size, the author has devised and tested a novel enzyme linked immunosorbant assay (ELISA)-based von Willebrand factor sulfatide-binding assay for its potential use in von Willebrand's disease diagnosis and von Willebrand's disease subtype discrimination. Results obtained using this assay were compared with those using a standard von Willebrand factor antigen assay (by ELISA), a Ristocetin cofactor assay for von Willebrand factor (by platelet agglutination procedure), and a collagen-binding assay for von Willebrand factor (by ELISA). All four von Willebrand factor assays were capable of detecting von Willebrand factor in a dose-dependent manner (i.e., were von Willebrand factor quantitative), and all were capable of detecting deficiencies in von Willebrand factor associated with von Willebrand's disease (i.e., all could detect the presence of von Willebrand's disease; Type 1 and Type 2 von Willebrand's disease). Linear regression analysis showed good correlation of results of sulfatide-binding assay for von Willebrand factor with results of the von Willebrand factor antigen, Ristocetin cofactor assay for von Willebrand factor, and collagen-binding assay for von Willebrand factor. However, only the collagen-binding assay for von Willebrand factor and Ristocetin cofactor assay for von Willebrand factor could detect the presence of von Willebrand factor functional discordance associated with Type 2 von Willebrand's disease. That is, in the current study, the sulfatide-binding assay for von Willebrand factor yielded laboratory data for von Willebrand's disease patients that was essentially similar to that obtained using the von Willebrand factor antigen assay, and which therefore could not be used to discriminate between von Willebrand's disease subtypes.

Laboratory assessment as a critical component of the appropriate diagnosis and sub-classification of von Willebrand's disease

von Willebrand's disease (VWD) is now recognized to be most common inherited bleeding disorder. It arises from defects or deficiencies in a protein called von Willebrand factor (VWF). VWD is a heterogeneous disorder, and patients are typed according to pathophysiology. The correct diagnosis and sub-classification of a patient's VWD is crucial because the presenting biological activity of VWF determines the haemorrhagic risk, and since subsequent clinical management will differ accordingly. Although clinical assessment of the propositus will provide the initial clue to, or an index of clinical suspicion for, a diagnosis of VWD, it is the laboratory process that will confirm or discount the diagnosis. A variety of assays may be employed by the laboratory undertaking the investigation, and these will not necessarily be restricted to an assessment of VWF. Due to the limitations of each potential laboratory assay, and because of VWD heterogeneity, no single test procedure is sufficiently 'robust' to permit detection of all VWD variants. This situation often leads to some clinical confusion in the process of laboratory interpretation regarding the likelihood of VWD, and the subtype of VWD. Classically, the test panel might include any combination of the following: (i) determination of (skin) bleeding times, (ii) VWF antigen (VWF:Ag) levels, (iii) 'functional' activity of Factor VIII (i.e. FVIII:coagulant or FVIII:C), (iv) 'functional' activity of VWF (e.g. Ristocetin Cofactor [VWF:RCof] assay), and/or Ristocetin induced platelet aggregation [RIPA] analysis), and (v) assessment of the VWF molecular weight or structural profile (i.e. VWF multimeric analysis or VWF:Multimers). There have also been a number of new diagnostic developments, and these are beginning to significantly influence the overall clinical VWD-diagnostic process. These include automation of existing assay procedures, a relatively new functional VWF assay called the Collagen Binding Assay (VWF:CBA), new automated platelet function analysers such as the PFA-100 and the Xylum Clot Signature Analyser, and specific VWF:FVIII binding assays. The current report focuses on the recommended laboratory process for investigation of VWD. An analysis of this process shows that selection of an appropriate test panel is a critical component for the proper diagnosis and classification. This review also outlines those new and emerging technologies that will help streamline the diagnostic process. Because VWD is just one manifestation of a 'bleeding' disorder (albeit the most common), the review also briefly mentions other related diagnostic processes and general approaches to the investigation of 'bleeding disorders'. The review also provides two algorithms to assist clinicians in making appropriate diagnostic choices in response to the clinical findings. A number of summary tables describing each laboratory assay in detail, and summarising the likely diagnostic findings for each Type of VWD, are also provided. This review should be of value to both haemostasis scientists and clinical specialists involved in VWD diagnosis.