Laboratory Testing for von Willebrand Factor: Factor VIII Binding (for 2N VWD)

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.

Laboratory diagnosis of von Willebrand disease in the age of the new guidelines: considerations based on geography and resources

von Willebrand disease (VWD) is considered the most common bleeding disorder and arises from deficiency and/or defect in the adhesive plasma protein von Willebrand factor (VWF). Diagnosis of VWD requires clinical assessment and is facilitated by laboratory testing. Several guidelines for VWD diagnosis exist, with the latest American Society of Hematology, International Society on Thrombosis and Haemostasis, National Hemophilia Foundation, and World Federation of Hemophilia 2021 guidelines presenting 11 recommendations, some of which have drawn controversy. In the current narrative review, we provide additional context around difficulties in laboratory diagnosis/exclusion/typing of VWD, with a focus on developing countries/resource-poor settings. In particular, there are many variations in assay methodology, and some methods express high assay variability and poor low-level VWF sensitivity that compromises their utility. Although we favor an initial 4-test assay panel, comprising factor (F) VIII coagulant activity, VWF antigen, VWF glycoprotein Ib binding (VWF:GPIbR or VWF:GPIbM favored over VWF Ristocetin cofactor) and VWF collagen binding, we also provide strategies for laboratories only able to incorporate an initial 3-test assay panel, as favored by the latest guidelines, to improve diagnostic accuracy.

Laboratory Testing for von Willebrand Disease Using a Composite Rapid 3-Test Chemiluminescence-Based von Willebrand Factor Assay Panel

von Willebrand disease (VWD) is the most commonly reported inherited bleeding disorder and may alternatively occur as an acquired von Willebrand syndrome (AVWS). VWD/AVWS develops from defects and/or deficiency in the adhesive plasma protein von Willebrand factor (VWF). VWD/AVWS diagnosis/exclusion remains challenging because of the heterogeneity of VWF defects and the technical limitations of many VWF tests, as well as the VWF test panels (number and type of tests) chosen by many laboratories. Laboratory testing for these disorders utilizes evaluation of VWF level and activity, with activity assessment needing several tests due to the many functions performed by VWF in order to help counteract bleeding. This report explains procedures for evaluating VWF level (antigen; VWF:Ag) and activity by means of a chemiluminescence-based panel. Activity assays comprise collagen binding (VWF:CB) and a ristocetin-based recombinant glycoprotein Ib-binding (VWF:GPIbR) assay that reflects a contemporary alternative to classical ristocetin cofactor (VWF:RCo). This 3-test VWF panel (Ag, CB, GPIbR [RCo]) reflects the only such composite panel available on a single platform and is performed on an AcuStar instrument (Werfen/Instrumentation Laboratory). Certain regional approvals may also allow this 3-test VWF panel to be performed on the BioFlash instrument (Werfen/Instrumentation Laboratory).

Laboratory Testing for von Willebrand Factor: Factor VIII Binding for the Diagnosis or Exclusion of Type 2N von Willebrand Disease: An Update

von Willebrand factor (VWF) is a large adhesive plasma protein that expresses several functional activities. One of these activities is to bind coagulation factor VIII (FVIII) and to protect it from degradation. Deficiency of, and/or defects in, VWF can give rise to a bleeding disorder called von Willebrand disease (VWD). The defect in VWF that affects its ability to bind to and protect FVIII is captured within type 2N VWD. In these patients, FVIII is produced normally; however, plasma FVIII quickly degrades as it is not bound to and protected by VWF. These patients phenotypically resemble those with hemophilia A, where instead, FVIII is produced in lower amount. Both hemophilia A and 2N VWD patients therefore present with reduced levels of plasma FVIII relative to VWF level. However, therapy differs, since patients with hemophilia A are given FVIII replacement products, or FVIII mimicking products; instead, patients with 2N VWD require VWF replacement therapy, since FVIII replacement will only be effective for a short term, given this replacement product will quickly degrade in the absence of functional VWF. Thus, 2N VWD needs to be differentiated from hemophilia A. This can be achieved by genetic testing or by use of a VWF:FVIII binding assay. The current chapter provides a protocol for the performance of a commercial VWF:FVIII binding assay.

Hemostasis and Thrombosis: An Overview Focusing on Associated Laboratory Testing to Diagnose and Help Manage Related Disorders

Hemostasis is a complex but balanced process that permit normal blood flow, without adverse events. Disruption of the balance may lead to bleeding or thrombotic events, and clinical interventions may be required. Hemostasis laboratories typically offer an array of tests, including routine coagulation and specialized hemostasis assays used to guide clinicians for diagnosing and managing patients. Routine assays may be used to screen patients for hemostasis-related disturbances but may also be used for drug monitoring, measuring efficacy of replacement or adjunctive therapy, and other indications, which may then be used to guide further patient management. Similarly, "specialized" assays are used for diagnostic purposes or may be used to monitor or measure efficacy of a given therapy. This chapter provides an overview of hemostasis and thrombosis, with a focus on laboratory testing that may be used to diagnose and help manage patients suspected of hemostasis- and thrombosis-related disorders.

Laboratory Diagnosis of von Willebrand Disease (VWD): Geographical Perspectives

von Willebrand disease (VWD) is reportedly the most common inherited bleeding disorder, and can also arise as an acquired (von Willebrand) syndrome (AVWS). The hemostasis laboratory plays a key role in the diagnosis or exclusion of VWD/AVWS, which may otherwise be suspected due to the patient's clinical (bleeding) history. VWD/AVWS arise from deficiency and/or defects in the adhesive plasma protein, von Willebrand factor (VWF). VWF undertakes various roles within hemostasis, but principally acts within primary hemostasis to anchor platelets to sites of vascular damage, thereby facilitating thrombus formation to arrest bleeding. The diagnosis or exclusion of VWD/AVWS requires laboratory testing for both VWF level and activity, with the latter potentially comprising several of a potential plethora of different assays. Complete diagnosis of VWD also requires the differentiation of VWD type, with six types comprising the current classification (i.e., qualitative [types 2A, 2B, 2M, 2N VWD] vs. quantitative [types 1 and 3 VWD] deficiency/defects). Although appropriate diagnosis and type classification hold important therapeutic consequences, these remain problematic and sometimes elusive for some laboratories to achieve. This report reviews the laboratory aided diagnosis or exclusion of VWD from a geographic perspective, and focuses on the disparities of approaches and methods in different regions of the world. This is primarily done from the perspective of data available from published reports related to external quality assessment (or proficiency testing) from different geographic localities. Moreover, differences in approaches between laboratories may arise due to differential adherence of particular guidelines, as well as regulatory aspects and predominance of local manufacturers and suppliers.

Evaluating Performance of Contemporary and Historical von Willebrand Factor (VWF) Assays in the Laboratory Identification of von Willebrand Disease (VWD): The Australasian Experience

von Willebrand disease (VWD) is a common bleeding disorder that arises from deficiency and/or defects of von Willebrand factor (VWF). Appropriate diagnosis of VWD, including differential identification of qualitative (types 2A, 2B, 2M, 2N VWD) versus quantitative (types 1 and 3 VWD) defects remains problematic but has important management implications, given differential therapy. Complete assessment for VWD in a patient with a bleeding history requires comprehensive test panels, including VWF activity and antigen. We describe the Australasian experience, using data from the Royal College of Pathologists of Australasia (RCPA) Quality Assurance Program (QAP) related to VWF testing in their VWD test module. The RCPAQAP has been providing samples for VWF testing since 1998, representing 25 years of proficiency testing related to VWD diagnosis. A total of 109 samples have been dispatched to participants over these years, with current assessment involving dispatches of two samples (=4 samples) per year. Samples have represented all types of VWD, as well as normal or other samples, including acquired von Willebrand syndrome and plasma VWF concentrates as used in VWD therapy. Different VWF assays and activity/antigen ratios show different utility in VWD and type identification. In the past 9 years of data capture, a total of 166 errors were identified from a total of 1,839 interpretations, representing a base error rate of 9.0%. Identification errors were highest for type 2 VWD samples (15.3%), intermediate for type 1 VWD samples (7.5%), and lowest for normal samples (2.4%). Errors can be linked to assay limitations, including assay variability and low-level VWF detection limits, as well as laboratory issues (including test result misinterpretation, which accounts for approximately 40% of all errors for type 2 VWD). For test-associated errors, VWF:RCo and VWF:GPIbM were associated with the highest variability and error rate, which was up to 10x higher than that using VWF:CB. As a test group, chemiluminescence-based procedures were associated with lowest inter-laboratory variability, best low-level VWF detection (down to <1 U/dL), and least errors overall. These findings inform on reasons behind high rates of errors associated with VWD diagnosis, with some assays and methodologies performing substantially better than others.

Advances in laboratory assessment of thrombosis and hemostasis

Technologies in laboratory diagnostics are changing fast with progress in understanding and therapy of diseases. Unfortunately, new analyzers are often needed to be installed in a clinical laboratory to implement such techniques. The demand for new hardware is a bottleneck in improving the diagnostic services for many facilities with limited resources. In this regard, hemostasis laboratories take a slightly different position. Because many in vitro diagnostic tests target the functional aspects of hemostasis, further meaningful information can be obtained from the same analyzers as in current use. Automated coagulometers are good candidates for such further utilization. Clot waveform analysis is a leading example. Behind the simple values reported as clotting time, clotting curves exist that represent the process of fibrin clot formation. Clot waveform analysis examines the clotting curves and derives new parameters other than clotting times. The clot waveform parameters are now in active use in assessing the hemostatic potential of hemorrhagic patients. Clinical application of coagulometers can also be widened by modifying the reagent formulation. For example, the chromogenic factor VIII assay with bovine source reagent compositions has recently been introduced for hemophilia A patients on emicizumab prophylaxis. Also, new immunoturbidimetric functional assays for von Willebrand factor have been developed recently. Thus, new clinically relevant information can be mined from the automated coagulometers that are based on old technology.

Evaluating errors in the laboratory identification of von Willebrand disease using contemporary von Willebrand factor assays

von Willebrand disease (VWD) arises from deficiency and/or defects of von Willebrand factor (VWF). Assessment requires test panels, including VWF activity and antigen. Appropriate diagnosis including differential identification of qualitative versus quantitative defects remains problematic but has important management implications. Data using a large set (n=27) of varied plasma samples comprising both quantitative VWF deficiency ('Type 1 and 3') vs qualitative defects ('Type 2') tested in a cross-laboratory setting have been evaluated to assess contemporary VWF assays for utility to differentially identify sample types. Different VWF assays and activity/antigen ratios showed different utility in VWD and type identification. Identification errors were linked to assay limitations, including variability, and laboratory issues (e.g., test result misinterpretation). Quantitative deficient (type 1) samples were misinterpreted as qualitative defects (type 2) on 35/467 occasions (7.5% error rate); 11.4% of these errors were due to laboratories misinterpreting their own data, which was instead consistent with quantitative deficiencies. Conversely, qualitative defects were misinterpreted as quantitative deficiencies at a higher error rate (14.3%), but this was more often due to laboratories misinterpreting their data (40% of errors). For test-associated errors, VWF:RCo and VWF:GPIbM were associated with the highest variability and error rate, which was many-fold higher than that using VWF:CB. Chemiluminescence ('CLIA') procedures were associated with lowest inter-laboratory variability and errors overall. These findings in part explain the high rate of errors associated with VWD diagnosis. VWF:GPIbM showed a surprisingly high rate of test associated errors, whilst CLIA procedures performed best overall.

Navigating the Myriad of von Willebrand Factor Assays

von Willebrand factor (VWF) represents a large and complex adhesive plasma protein whose main function is to provide a bridge between blood platelets and damaged endothelium, and thus facilitate primary hemostasis. VWF also binds to FVIII, preventing early proteolysis, and delivers this cargo to sites of vascular injury, thereby promoting clot formation and secondary hemostasis. An absence, deficiency, or defect in VWF can lead to a bleeding diathesis called von Willebrand disease (VWD), considered the most common inherited bleeding disorder. Contemporary laboratory assays used in VWD diagnosis/exclusion comprise a myriad of assays that identify the quantity (level) of VWF, as well as the multitude of VWF activities. These may use the following test abbreviations: VWF:Ag, VWF:RCo, VWF:CB, VWF:GPIbR, VWF:GPIbM, VWF:FVIIB, VWF:Ab. The current review explains what these assays are, as well as their place in VWD diagnostics.

Rare forms of von Willebrand disease

von Willebrand disease (VWD) arises from deficiency and/or defect(s) of plasma von Willebrand factor (VWF). In turn, plasma VWF is an adhesive protein which primarily functions by anchoring platelets to regions of vascular injury, thereby assisting prevention of bleeding. There is a proportional reduction also in Factor VIII, due to the absence of the stabilizing and anti-proteolytic effect that VWF normally exerts. VWD is reportedly the most common inherited bleeding disorder and can be classified into quantitative and qualitative defects, with type 1 and 3 VWD respectively identifying partial and total quantitative deficiency of VWF, and type 2 VWD identifying qualitative defects of VWF. The relative incidence of each subtype of VWD differs according to the locality and the ability of clinicians and laboratories to correctly diagnose and classify cases. In general, type 1 VWD is considered the most common type of VWD, whereas types 2 and 3 represent rarer forms. However, in developing countries, and partly because of consanguinity, type 3 VWD is over-represented. This review primarily focuses on the rarer forms of VWD, which typically comprise types 2 (A, B, M and N) and 3 VWD. The review also mentions type 1 VWD, largely for completeness and comparability, and since purportedly "severe" type 1 VWD, albeit not a formally recognized subtype of type 1 VWD, would represent a relatively "rare" form of VWD.

Postanalytical considerations that may improve the diagnosis or exclusion of haemophilia and von Willebrand disease

von Willebrand disease (VWD) and haemophilia represent the most common inherited or acquired bleeding disorders. However, many laboratories and clinicians may be challenged by their accurate diagnosis or exclusion. Difficulties in diagnosis/exclusion may include analytical issues, where assays occasionally generate an incorrect result (ie representing an analytical error) or have limitations in their measurement range of and/or low analytical sensitivity. Also increasingly recognized is the influence of preanalytical issues on the diagnosis of VWD or haemophilia. Unfortunately, postanalytical considerations are often not well considered in the diagnostic process. Therefore, this narrative review aims to provide an overview of some important postanalytical considerations that may help improve the diagnosis of VWD and haemophilia. This review primarily discusses aspects around reporting of test results. However, we also discuss other less well-recognized postanalytical considerations, including the use of assay ratios to help identify differential diagnoses and then guide further investigation.