Von Willebrand factor assay proficiency testing. The North American Specialized Coagulation Laboratory Association experience

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.

Flow affects the structural and mechanical properties of the fibrin network in plasma clots

The fibrin network is one of the main components of thrombi. Altered fibrin network properties are known to influence the development and progression of thrombotic disorders, at least partly through effects on the mechanical stability of fibrin. Most studies investigating the role of fibrin in thrombus properties prepare clots under static conditions, missing the influence of blood flow which is present in vivo. In this study, plasma clots in the presence and absence of flow were prepared inside a Chandler loop. Recitrated plasma from healthy donors were spun at 0 and 30 RPM. The clot structure was characterized using scanning electron microscopy and confocal microscopy and correlated with the stiffness measured by unconfined compression testing. We quantified fibrin fiber density, pore size, and fiber thickness and bulk stiffness at low and high strain values. Clots formed under flow had thinner fibrin fibers, smaller pores, and a denser fibrin network with higher stiffness values compared to clots formed in absence of flow. Our findings indicate that fluid flow is an essential factor to consider when developing physiologically relevant in vitro thrombus models used in researching thrombectomy outcomes or risk of embolization.

How Do Laboratories Perform von Willebrand Disease Diagnostics and Classification of von Willebrand Disease Patients? Results from External Quality Data and an International Survey

BACKGROUND

 Reduced or dysfunctional von Willebrand factor (VWF) may lead to von Willebrand disease (VWD), which is a common inherited bleeding disorder. VWD is classified into three major types: type 1 is a partial quantitative deficiency of VWF, type 3 is a complete quantitative deficiency of VWF, and type 2 consists of qualitative abnormalities of VWF. To arrive at a correct VWD diagnosis, multiple tests and a correct interpretation of these tests are needed.

AIM

 The aim of the present study was to gain insight into the approach of laboratories toward VWD diagnosis.

METHODS

 Data from four samples of the external quality assessment (EQA) VWF surveys of the ECAT (External Quality Control for Assays and Tests) were evaluated. Furthermore, results were analyzed of a questionnaire that was sent to hemostasis laboratories about VWD diagnostic approaches.

RESULTS

 For most EQA samples, the majority of participants indicated the correct classification. However, 6 to 60% indicated another classification. For all samples, significant differences in VWF results were observed between the correct and incorrect classifications. The questionnaire demonstrated that the testing approach varied between the laboratories, especially for parameters that were essential for discrimination between VWD type 1 and healthy individuals, as well as the cutoff values used to discriminate VWD types 1 and 2.

CONCLUSIONS

 Diagnosis of VWD is heterogeneous in diagnostic approach, guidelines, and cutoff values within large ranges of VWF results between laboratories. Harmonization of approaches and increased accuracy of VWF measurements may help to establish a correct diagnosis.

How Do Laboratories Perform von Willebrand Disease Diagnostics and Classification of von Willebrand Disease Patients? Results from External Quality Data and an International Survey

Background Reduced or dysfunctional von Willebrand factor (VWF) may lead to von Willebrand disease (VWD), which is a common inherited bleeding disorder. VWD is classified into three major types: type 1 is a partial quantitative deficiency of VWF, type 3 is a complete quantitative deficiency of VWF, and type 2 consists of qualitative abnormalities of VWF. To arrive at a correct VWD diagnosis, multiple tests and a correct interpretation of these tests are needed.

Aim The aim of the present study was to gain insight into the approach of laboratories toward VWD diagnosis.

Methods Data from four samples of the external quality assessment (EQA) VWF surveys of the ECAT (External Quality Control for Assays and Tests) were evaluated. Furthermore, results were analyzed of a questionnaire that was sent to hemostasis laboratories about VWD diagnostic approaches.

Results For most EQA samples, the majority of participants indicated the correct classification. However, 6 to 60% indicated another classification. For all samples, significant differences in VWF results were observed between the correct and incorrect classifications. The questionnaire demonstrated that the testing approach varied between the laboratories, especially for parameters that were essential for discrimination between VWD type 1 and healthy individuals, as well as the cutoff values used to discriminate VWD types 1 and 2.

Conclusions Diagnosis of VWD is heterogeneous in diagnostic approach, guidelines, and cutoff values within large ranges of VWF results between laboratories. Harmonization of approaches and increased accuracy of VWF measurements may help to establish a correct diagnosis.

How Do Laboratories Perform von Willebrand Disease Diagnostics and Classification of von Willebrand Disease Patients? Results from External Quality Data and an International Survey

BACKGROUND

 Reduced or dysfunctional von Willebrand factor (VWF) may lead to von Willebrand disease (VWD), which is a common inherited bleeding disorder. VWD is classified into three major types: type 1 is a partial quantitative deficiency of VWF, type 3 is a complete quantitative deficiency of VWF, and type 2 consists of qualitative abnormalities of VWF. To arrive at a correct VWD diagnosis, multiple tests and a correct interpretation of these tests are needed.

AIM

 The aim of the present study was to gain insight into the approach of laboratories toward VWD diagnosis.

METHODS

 Data from four samples of the external quality assessment (EQA) VWF surveys of the ECAT (External Quality Control for Assays and Tests) were evaluated. Furthermore, results were analyzed of a questionnaire that was sent to hemostasis laboratories about VWD diagnostic approaches.

RESULTS

 For most EQA samples, the majority of participants indicated the correct classification. However, 6 to 60% indicated another classification. For all samples, significant differences in VWF results were observed between the correct and incorrect classifications. The questionnaire demonstrated that the testing approach varied between the laboratories, especially for parameters that were essential for discrimination between VWD type 1 and healthy individuals, as well as the cutoff values used to discriminate VWD types 1 and 2.

CONCLUSIONS

 Diagnosis of VWD is heterogeneous in diagnostic approach, guidelines, and cutoff values within large ranges of VWF results between laboratories. Harmonization of approaches and increased accuracy of VWF measurements may help to establish a correct diagnosis.

Establishing reference intervals for von Willebrand factor multimers

Background

von Willebrand factor (VWF) multimers (VWF:MM) methodologies are technically difficult, laborious, time consuming, non-standardized and results vary between laboratories. A new semi automated VWF:MM assay is available for routine use (Sebia). Due to lack of reference values for VWF:MM fractions, results interpretation can be challenging in some cases. The aim of this study was to determine reference intervals for low molecular weight (LMWM), intermediate molecular weight (IMWM) and high molecular weight (HMWM) multimers.

Methods

By the international cooperation initiated between 4 countries (Estonia, Latvia, France, and USA) 131 samples of relatively healthy individuals were analyzed for VWF:MM (in total 51 males and 80 non-pregnant females aged 17-69 years). Reference intervals were calculated according to CLSI C28-A3 standard.

Results

The proposed reference intervals for VWF:MM were calculated for LMWM 10.4-22.5%, IMWM 22.6-37.6%, HMWM 45.6-66.6%. Age related differences were seen in IMWM and HMWM (p<0.001 and 0.038). There was no gender related difference observed. Geographically LMWM results of France were different from the other regions (p<0.05).

Conclusions

Quantification of VWF:MM fractions, in addition to qualitative assessment of VWF:MM patterns, has the potential to aid in differential diagnosis of von Willebrand disease (VWD) subtypes. The reference values calculated in this study can be used in future research to establish clinical decision limits.

Diagnosis of von Willebrand disease: An assessment of the quality of testing in North American laboratories

BACKGROUND

Laboratory diagnosis of von Willebrand Disease (VWD) is complex. Reliance on laboratory testing can be problematic as different VWD screening panels, assays and methodologies can produce analytic variability in test results.

OBJECTIVES

To compare the degree of imprecision among the VWD assays and within the platelet binding activity (PBA) assays, to determine the consensus among the VWD assays for correct classification of sample results, and to determine consensus among laboratories' von Willebrand factor (VWF) multimer interpretations and final interpretations of the VWD panels.

PATIENTS/METHODS

Proficiency testing results from the North American Specialized Coagulation Laboratory Association (NASCOLA) submitted by laboratories from 2010 to 2019 for all normal, type (T) 1 VWD and T2 VWD samples were analysed.

RESULTS AND CONCLUSIONS

Imprecision was lowest for VWF antigen and highest for collagen binding activity (CBA) with median coefficient of variation (CV) of 12% (interquartile range (IQR) 7%) and 23% (IQR 21%) respectively. Within the VWF PBA assays, the gain-of-function mutant GP1b binding (VWF: GP1bM) methods had the least imprecision (CV 9%, IQR 10%). All assays, including the various PBA methods had excellent consensus. The majority of laboratories agreed that normal (median consensus-82%, IQR 16%) and T1 VWD (median consensus-100%, IQR 9%) samples had normal multimer distribution. Consensus among laboratories for final interpretations was excellent for normal samples (median 81%, IQR 8%), good for T1 VWD samples (median 59%, IQR 9%), and fair for T2 VWD samples (median 44%, IQR 21%). Consensus on final interpretation decreased as sample complexity increased.

Why is Misdiagnosis of von Willebrand Disease Still Prevalent and How Can We Overcome It? A Focus on Clinical Considerations and Recommendations

Despite von Willebrand disease (VWD) being the most common inherited bleeding disorder, its accurate diagnosis is frequently shrouded by diagnostic pitfalls. VWD is frequently under-diagnosed, over-diagnosed and misdiagnosed, leading to significant avoidable patient morbidity and health care system burden. At the heart of this dilemma lies the heterogeneity and complexity of von Willebrand factor (VWF) and associated defects, and the necessity of coalescing clinical and laboratory features to obtain an accurate diagnosis. Common pitfalls include poor clinical and scientific understanding and familiarity with VWD, incomplete clinical history and lack of routine use of standardised bleeding assessment tools (BAT), difficulty in accessing a comprehensive repertoire of laboratory tests, significant pre-analytical, analytical and post-analytical issues, and lack of expertise in laboratory testing and interpretation. Errors, resulting in under-diagnosis, over-diagnosis, and misdiagnosis of VWD, are presented and discussed. Strategies to minimise errors include better education of clinicians and laboratory staff on VWD, routine use of validated BAT, utilising a comprehensive gamut of laboratory investigations according to a standardised algorithm, and repeating testing to minimise pre-analytical errors. Recommendations on appropriate patient selection for VWD testing, how VWD should be investigated in the laboratory, and how to ensure test results are accurately interpreted in the correct clinical context are detailed.

How we diagnose 2M von Willebrand disease (VWD): Use of a strategic algorithmic approach to distinguish 2M VWD from other VWD types

INTRODUCTION

von Willebrand disease (VWD) is the most common inherited bleeding disorder and caused by an absence, deficiency or defect in von Willebrand factor (VWF). VWD is currently classified into six different types: 1, 2A, 2B, 2N, 2M, 3. Notably, 2M VWD is more often misdiagnosed as 2A or type 1 VWD than properly identified as 2M VWD.

AIM

To describe an algorithmic approach to better ensure appropriate identification of 2M VWD, and reduce its misdiagnosis, as supported by sequential laboratory testing.

METHODS

Comparative assessment of types 1, 2A, 2B and 2M VWD using various laboratory tests, including VWF antigen and several VWF activity assays, plus DDAVP challenge data, ristocetin-induced platelet agglutination (RIPA) data, multimer analysis and genetic testing.

RESULTS

Types 1, 2A, 2B and 2M VWD give characteristic test patterns that can provisionally classify patients into particular VWD types. Notably, type 1 VWD shows low levels of VWF, but VWF functional concordance (VWF activity/Ag ratios >0.6), with both baseline assessment and post-DDAVP. Types 2A, 2B and 2M VWD show VWF functional discordance (low VWF activity/Ag ratio(s)) dependent on the defect, but type 2M separates from 2A/2B VWD based on specific test patterns, especially with collagen binding vs glycoprotein Ib binding assays. RIPA identifies 2B VWD. Multimers separate 2M from 2A/2B.

CONCLUSION

We provide strategies to improve correct diagnosis of VWD, especially focussed on 2M VWD, and which can be used by most diagnostic haemostasis laboratories, reserving genetic analysis (if required) for confirmation.

Improving diagnosis of von Willebrand disease: Reference ranges for von Willebrand factor multimer distribution

BACKGROUND

Phenotypic von Willebrand disease (VWD) classification requires multiple tests including analysis of multimeric distributions von Willebrand factor (VWF) and evaluation of its structure. VWF multimer analysis is labor intensive, nonstandardized, and limited to specialized laboratories. A commercial semiautomatic assay, HYDRAGEL VW multimer assay (H5/11VWM, Sebia), has become available.

OBJECTIVES

Establishment of reference ranges for H5/11VWM to improve VWD classification.

METHODS

Implementation validation, establishment and validation of normal and pathological reference intervals (NRIs/PRIs), comparison with in-house method using 40 healthy volunteers and 231 VWD patients.

RESULTS

Qualitative and quantitative validation of NRI obtained sensitivity of 88% and 79%, respectively, for type 2. Comparison of the two methods showed an overall concordance of 86% with major conflicting results in all atypical 2B (n = 7) and 50% 2M-GPIb (n = 41) showing quantitative and qualitative multimeric loss, that was not detected with in-house method. We were able to use established PRIs, with 73% validity in type 2 cases, to distinguish individual type 2A subtypes (IIA, IIC, IID, IIE) from 2M and 2B.

CONCLUSION

H5/11VWM could be used for all clinical purposes because its reliability and its rapid and accurate diagnostic ability and reduced observer bias. Although H5/11VWM cannot evaluate triplet structures, we were able to define 2A subtypes by stripping back to the percentage of intermediate/high-molecular-weight multimers. H5/11HWM could be an efficient and widely available alternative for the "gold standard" technique.

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.

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.

Comparative assessment of von Willebrand factor multimers vs activity for von Willebrand disease using modern contemporary methodologies

INTRODUCTION

Diagnosis of von Willebrand disease (VWD) is challenging due to heterogeneity of VWD and test limitations. Many von Willebrand factor (VWF) assays are utilized, including antigen (Ag), activity and multimer analysis. Activity assays include ristocetin cofactor using platelets (VWF:RCo) or other particles incorporating recombinant glycoprotein I ('VWF:GPIbR'), or other GPI binding assays using gain-of-function mutations ('VWF:GPIbM'), or collagen binding (VWF:CB).

AIM

To comparatively evaluate modern contemporary VWF activity assays vs VWF multimer analysis using modern contemporary methods.

MATERIALS AND METHODS

Several VWF activity assays (VWF:RCo, VWF:GPIbR, VWF:GPIbM, VWF:CB) assessed (typically as a ratio against VWF:Ag) against a new semi-automated procedure for different types of VWD (1, 3, 2A, 2B, 2M), plus control material (n = 580). The evaluation also focussed on relative loss of high and very high molecular weight multimers (HMWM and VHMWM) by densitometric scanning.

RESULTS

All evaluated VWF activity/Ag ratios showed high correlation to the presence/absence of HMWM and VHMWM, although VWF:CB/Ag and VWF:GPIbR/Ag ratios using an automated chemiluminescence method yielded highest correlation coefficients (r = .909 and .874, respectively, for HMWM). Use of the investigative procedure (VHMWM) identified fewer false positives for 'loss' in type 1 VWD.

CONCLUSIONS

This comparative investigation identified that new automated chemiluminescence VWF activity assays best identified relative loss or presence of HMWM and VHMWM according to activity to Ag ratios and an alternative investigative method for identifying VHMWM in multimer testing for a new commercial multimer method may lead to fewer false identifications of HMW loss in type 1 VWD.

Utility of repeat testing in the evaluation for von Willebrand disease in pediatric patients

BACKGROUND

Von Willebrand disease (VWD) is the most common inherited bleeding disorder and is caused by quantitative and qualitative defects in von Willebrand factor (VWF). The laboratory diagnosis of VWD in pediatric patients is complicated by VWF interassay and intra-assay variability, stress-induced elevations in VWF levels, and a lack of significant bleeding history with which to correlate test results.

OBJECTIVE

Guidelines recommend repeat testing in patients with a high suspicion of VWD and unclear laboratory assay results; however, no studies have evaluated the utility of repeat VWF testing in pediatric patients.

METHODS

This retrospective single-center cohort study aimed to determine clinical variables associated with requiring more than one test to diagnose VWD and to establish a cutoff VWF value above which further testing is not informative.

RESULTS

Of 811 patients evaluated for a suspected bleeding disorder, 22.2% were diagnosed with VWD, with ~70% diagnosed on the first test. Patients with VWD were younger (5.8 vs. 8.5 years, P = .002) and more likely to have a family history of VWD (38% vs. 22%, P < .001) than those without VWD. Univariate analysis failed to identify any clinical variables that correlated with needing multiple tests for a VWD diagnosis. A cutoff of 100 IU/dL for VWF antigen or activity on the first test yielded negative predictive values >95%.

CONCLUSIONS

We demonstrate that the majority of pediatric patients had diagnostic VWF values on the first set of testing. Pediatric patients without a family history of VWD and VWF levels >100 IU/dL may not need further testing to rule out the diagnosis of VWD.

Semi-automated von Willebrand factor multimer assay for von Willebrand disease: Further validation, benefits and limitations

INTRODUCTION

Accurate diagnosis of von Willebrand disease (VWD) enables effective patient management. von Willebrand factor (VWF) multimer analysis provides useful information regarding VWF multimer structure, thereby aiding VWD subtyping and management; however, historically technically challenging assays have had limited utility. This study evaluates the Sebia Hydrasys Hydragel-11 semi-automated VWF multimer assay and further validates the Hydragel-5 gel system, as primarily pertaining to VWD diagnostics and monitoring of therapy.

METHODS

Provisionally diagnosed (via a reference assay test panel) archived patient samples and prospective test patient samples, including those undergoing desmopressin trial or therapy monitoring, along with commercial and in-house control material and various external quality assessment (EQA) samples, were analysed. VWF multimers were evaluated for presence, loss or partial loss of high molecular weight (HMWM) and intermediate molecular weight (IMWM) multimers by both visual inspection and densitometric scanning, and comparison with reference assay results.

RESULTS

All anticipated multimer patterns were reproduced, with patients generally showing multimer profiles matching expected patterns according to VWD type based on reference test panel 'diagnosis'. Occasional discrepancies were resolved by retesting. The increase in plasma VWF following desmopressin therapy was also clearly demonstrated. Multimer profiles of EQA samples complemented reference test panel results and matched EQA targets. There were some 'technical' limitations noted.

CONCLUSION

This easy to use, standardised, semi-automated multimer analysis system can demonstrate the multimer profile of VWD patients, thus representing an additional laboratory tool for improved diagnosis, thereby facilitating appropriate patient management.

Automated assays for von Willebrand factor activity

von Willebrand factor (VWF) ristocetin cofactor activity (VWF:RCo) by platelet aggregometry has been considered the gold standard for evaluating the ability of VWF to bind platelets for over 40 years. Many automated systems no longer require platelets and rather rely on agglutination of latex particles. Automated methods of measuring VWF activity have improved performance characteristics and are performed on the same coagulation instruments used for routine testing via immunoturbidimetric methodology. Alternatively, a newer chemiluminescence assay system for measuring VWF activity demonstrates excellent performance characteristics. As these methods are becoming widely used, it is important to assess their performance in diagnosing and monitoring different types of von Willebrand disease. We review the automated methodologies and the published performance of these VWF assays. Advantages and limitations of these automated methods are discussed.

von Willebrand factor/ADAMTS-13 interactions at birth: implications for thrombosis in the neonatal period

von Willebrand factor (VWF) and its cleaving protease ADAMTS-13 (A Disintegrin and Metalloproteinase with Thrombospondin type 1 motif, member 13) are essential components to hemostasis. These plasma proteins have also been implicated in a number of disease states, including those affecting children. The best described abnormality is the congenital form of thrombotic thrombocytopenic purpura (TTP) resulting from germline mutations in the ADAMTS-13 gene. The VWF/ADAMTS-13 interaction has more recently emerged as a causative risk factor in the pathogenesis of pediatric stroke and secondary microangiopathies. There is now increasing interest and need to measure these coagulation factors during the neonatal period and throughout childhood. Methods adopted from a multitude of technically diverging studies have been used to understand their role during this period. To date, studies of VWF/ADAMTS-13 in this group of patients have reported conflicting results, which makes interpreting values in the clinical setting especially challenging. In this review we describe the historical evolution of the methodology used to measure VWF/ADAMTS-13 and how it may influence the results obtained during the first days of life. We review the individual assays used to analyze VWF/ADAMTS-13 as well as published reference values. Finally, we bring attention to the potential pathophysiologic role of VWF/ADAMTS-13 in neonatal thrombosis. This has significant implications because the pathologic processes that explain thrombosis in neonates remain poorly characterized and thromboembolism remains a significant source of morbidity and mortality, particularly in sick children.

Von Willebrand factor and the aortic valve: Concepts that are important in the transcatheter aortic valve replacement era

Since the approval of the first transcatheter aortic valve replacement (TAVR) device in 2011, this technology has undergone substantial enhancements and exponential growth. However, valve thrombosis and residual paravalvular leaks (PVL) are among the challenges that require further investigation. Recently, monitoring von Willebrand factor (vWF) multimers has emerged as a tool to help evaluate the severity of PVL after TAVR. Following TAVR, vWF large multimers recovery have been documented. The role of large vWF multimers recovery and their interactions with platelets, and the endothelium have not been entirely elucidated. In this review, we discuss vWF synthesis and its role in aortic stenosis. We further provide an overview of the studies that investigated changes affecting vWF multimers following TAVR and the role of HMW vWF multimers monitoring in the determination of PVL severity. We also offer potential future directions for what will be fertile ground for research in this field.

Towards harmonization of external quality assessment/proficiency testing in hemostasis

Quality in diagnostic testing represents a key target of laboratory medicine, for which an assurance around the quality of testing is expected from all involved in the process. Laboratories attempt to assure the quality of their testing by various processes, but especially by performance of internal quality control and external quality assessment (EQA). This is especially true for tests of hemostasis and coagulation. EQA in general provides information on test accuracy and on evaluation of long-term laboratory performance. EQA providers support laboratory performance by various means, including distribution of material for testing of analytes (“proficiency testing”), educational support through expert advice, distribution of publications or case series. Participation in EQA is often a laboratory accreditation requirement. This review aims to identify some of the strengths and weaknesses of EQA, and targets attempts towards harmonization of EQA practice, in order to achieve the best outcome for participant laboratories and, thus, for patients and their clinical care providers.

Preclinical evaluation of a semi-automated and rapid commercial electrophoresis assay for von Willebrand factor multimers

BACKGROUND

The von Willebrand factor (VWF) multimer test is required to correctly subtype qualitative type 2 von Willebrand disease (VWD). The current VWF multimer assays are difficult, nonstandardized, and time-consuming. The purpose of this study was to evaluate the clinical utility of the commercial VWF multimer kit by Sebia (Lisses, France), an electrophoresis technique yielding same-day results.

METHODS

Ten healthy volunteer plasma samples, in-house reference plasma (IRP) and commercial normal plasma (CNP) samples, 10 plasma samples from patients with a known VWD type, 1 hemophilia A plasma sample, and 7 external quality assurance (EQA) samples were analyzed using the commercial VWF multimer kit. Additional coagulation testing included measurements of VWF antigen (VWF:Ag), VWF activity (VWF:Ac), and FVIII activity (FVIII:C).

RESULTS

The CNP results revealed a relative loss of the highest molecular weight multimers; therefore, IRP was preferred as the reference sample. The interpretations of 10 patients with a known VWD type could be successfully reproduced and agreed with previous VWF multimer results. In all EQA surveys, the multimer results and final VWD diagnosis agreed with expert opinion.

CONCLUSIONS

The VWF multimer assay by Sebia is easy to perform and can be successfully implemented in any clinical laboratory for second-stage evaluation of VWD. The resolution power of multimer distribution is adequate to correctly classify VWD types 1, 2A, 2B, and 3.

A novel flow cytometry single tube bead assay for quantitation of von Willebrand factor antigen and collagen-binding

Deficiency of or defects in the plasma protein von Willebrand factor (VWF) lead to bleeding and von Willebrand disease (VWD), which may be congenital or acquired. VWD is considered the most common inherited bleeding disorder and laboratory testing for VWF level and activity is critical for appropriate diagnosis and management. We have designed and established a novel Flow Cytometry (FC) based method for measuring VWF antigen (VWF:Ag) and collagen binding (VWF:CB), together in the same tube and at the same time. The results of the novel FC method have been compared against existing reference methods using a range of normal and pathological material. Methods correlated well (VWF:Ag, r=0.866; VWF:CB, r=0.888) and generally permitted similar discrimination of quantitative versus qualitative VWD types (e.g. type 1 vs type 2A or 2B VWD). The novel procedure is expected to permit future streamlined performance of VWD screening, either using stand-alone FC systems or potentially incorporated into FC-capable automated blood cell and particle counters to allow for improved, automated and faster identification or exclusion of VWD.

Association between Carotid Intima-media Thickness and Aldehyde Dehydrogenase 2 Glu504Lys Polymorphism in Chinese Han with Essential Hypertension

BACKGROUND

Aldehyde dehydrogenase 2 (ALDH2) is involved in the pathophysiological processes of cardiovascular diseases. Recent studies showed that mutant ALDH2 could increase oxidative stress and is a susceptible factor for hypertension. In addition, wild-type ALDH2 could improve the endothelial functions, therefore reducing the risk of developing atherosclerosis. The aim of the present study was to explore the frequency of the Glu504Lys polymorphism of the ALDH2 gene and its relation to carotid intima-media thickness (CIMT) in a group of patients with essential hypertension (EH) and to investigate the association between the Glu504Lys polymorphism and CIMT in Chinese Han patients with EH.

METHODS

In this study, 410 Chinese Han patients with EH who received physical examinations at the People's Hospital of Sichuan Province (China) were selected. DNA microarray chip was used for the genotyping of the Glu504Lys polymorphism of the ALDH2 gene. The differences in CIMT among patients with different Glu504Lys ALDH2 genotypes were analyzed.

RESULTS

The mean CIMT of the patients carrying AA/AG and GG genotypes was 1.02 ± 0.31 mm and 0.78 ± 0.28 mm, respectively. One-way ANOVA showed that the CIMT of the patients carrying the AA/AG genotype was significantly higher than in the ones carrying the GG genotype (P < 0.001). Multivariate logistic regression showed that the Glu504Lys AA/AG genotype of the ALDH2 gene was one of the major factors influencing the CIMT in patients with EH (odds ratio = 3.731, 95% confidence interval = 1.589-8.124, P = 0.001).

CONCLUSIONS

The Glu504Lys polymorphism of the ALDH2 gene is associated with the CIMT of Chinese Han patients with EH in Sichuan, China.

Utility of the von Willebrand factor collagen binding assay in the diagnosis of von Willebrand disease

von Willebrand Disease (VWD) is the most common inherited bleeding disorder and also arises as an acquired defect (AVWS). VWD and AVWS are due to quantitative deficiencies and/or qualitative defects in von Willebrand factor (VWF), an adhesive plasma protein with multiple activities. Diagnosis of VWD is problematic, being subject to overdiagnosis, underdiagnosis, and misdiagnosis. This is largely due to limitations in current test procedures and an over-reliance on these imperfect test systems for clinical diagnosis. VWF essentially acts to assist in the formation of a platelet thrombus to stop blood loss from sites of injury, achieving this by binding to platelets (primarily through the glycoprotein Ib receptor), binding to subendothelial matrix components (primarily collagen), and binding to factor VIII (FVIII), thus protecting FVIII from degradation and enabling its delivery to sites of vascular injury. VWD is classified into six separate types, which may each be differentially managed therapeutically, and this underscores the importance of a correct diagnosis. The current report concisely reviews the utility of a relatively underutilised assay, the VWF collagen binding assay (VWF:CB), in facilitating the correct diagnosis and typing of VWD. In particular, if laboratories do not utilise the VWF:CB, then (i) type 2M VWD will continue to be missed, and/or misdiagnosed as types 2A or 1 VWD, and (ii) types 2A, 2B and PT-VWD will continue to be missed, or else be misdiagnosed as type 1 VWD or ITP. Am. J. Hematol. 92:114-118, 2017. © 2016 Wiley Periodicals, Inc.

Rapid discrimination of the phenotypic variants of von Willebrand disease

Approximately 20% to 25% of patients with von Willebrand disease (VWD) have a qualitative defect of the von Willebrand factor (VWF) protein activities. Variant VWD typically is classified as type 1C, 2A, 2B, 2M, or 2N depending on the VWF activity defect. Traditionally, diagnosis has relied on multiple clinical laboratory assays to assign VWD phenotype. We developed an enzyme-linked immunosorbent assay (ELISA) to measure the various activities of VWF on a single plate and evaluated 160 patient samples enrolled in the Zimmerman Program for the Molecular and Clinical Biology of von Willebrand Disease with type 2 VWD. Using linear discriminate analysis (LDA), this assay was able to identify type 1C, 2A, 2B, 2M, or 2N VWD with an overall accuracy of 92.5% in the patient study cohort. LDA jackknife analysis, a statistical resampling technique, identified variant VWD with an overall accuracy of 88.1%, which predicts the assay's performance in the general population. In addition, this assay demonstrated correlation with traditional clinical laboratory VWF assays. The VWF multiplex activity assay may be useful as a same-day screening assay when considering the diagnosis of variant VWD in an individual patient.

Misfolding of vWF to pathologically disordered conformations impacts the severity of von Willebrand disease

The primary hemostatic von Willebrand factor (vWF) functions to sequester platelets from rheological blood flow and mediates their adhesion to damaged subendothelium at sites of vascular injury. We have surveyed the effect of 16 disease-causing mutations identified in patients diagnosed with the bleeding diathesis disorder, von Willebrand disease (vWD), on the structure and rheology of vWF A1 domain adhesiveness to the platelet GPIbα receptor. These mutations have a dynamic phenotypical range of bleeding from lack of platelet adhesion to severe thrombocytopenia. Using new rheological tools in combination with classical thermodynamic, biophysical, and spectroscopic metrics, we establish a high propensity of the A1 domain to misfold to pathological molten globule conformations that differentially alter the strength of platelet adhesion under shear flow. Rheodynamic analysis establishes a quantitative rank order between shear-rate-dependent platelet-translocation pause times that linearly correlate with clinically reported measures of patient platelet counts and the severity of thrombocytopenia. These results suggest that specific secondary structure elements remaining in these pathological conformations of the A1 domain regulate GPIbα binding and the strength of vWF-platelet interactions, which affects the vWD functional phenotype and the severity of thrombocytopenia.

Comparison of several von Willebrand factor (VWF) activity assays for monitoring patients undergoing treatment with VWF/FVIII concentrates: improved performance with a new modified automated method

BACKGROUND

The ability of von Willebrand factor (VWF) to bind platelet GP Ib and promote platelet plug formation is measured in vitro using the ristocetin cofactor (VWF:RCo) assay. Automated assay systems make testing more accessible for diagnosis, but do not necessarily improve sensitivity and accuracy.

OBJECTIVE

We assessed the performance of a modified automated VWF:RCo assay protocol for the Behring Coagulation System (BCS(®) ) compared to other available assay methods.

METHODS

Results from different VWF:RCo assays in a number of specialized commercial and research testing laboratories were compared using plasma samples with varying VWF:RCo activities (0-1.2 IU mL(-1) ). Samples were prepared by mixing VWF concentrate or plasma standard into VWF-depleted plasma. Commercially available lyophilized standard human plasma was also studied. Emphasis was put on the low measuring range. VWF:RCo accuracy was calculated based on the expected values, whereas precision was obtained from repeated measurements.

RESULTS

In the physiological concentration range, most of the automated tests resulted in acceptable accuracy, with varying reproducibility dependent on the method. However, several assays were inaccurate in the low measuring range. Only the modified BCS protocol showed acceptable accuracy over the entire measuring range with improved reproducibility.

CONCLUSIONS

A modified BCS(®) VWF:RCo method can improve sensitivity and thus enhances the measuring range. Furthermore, the modified BCS(®) assay displayed good precision. This study indicates that the specific modifications - namely the combination of increased ristocetin concentration, reduced platelet content, VWF-depleted plasma as on-board diluent and a two-curve calculation mode - reduces the issues seen with current VWF:RCo activity assays.

Laboratory aspects of von Willebrand disease: test repertoire and options for activity assays and genetic analysis

The deficiency or abnormal function of von Willebrand factor (VWF) causes von Willebrand disease (VWD), the most frequent inherited bleeding disorder. The laboratory diagnosis of VWD can be difficult as the disease is heterogeneous and an array of assays is required to describe the phenotype. Basic classification of quantitative (type 1 and 3) and qualitative (type 2) VWD variants requires determination of VWF antigenic (VWF:Ag) levels and assaying of VWF ristocetin cofactor (VWF:RCo) activity, determining the capacity of VWF to interact with the platelet GPIb-receptor. Knowing the VWF:RCo activity is essential for identifying, subtyping and monitoring VWD, but the assay is poorly standardized and many protocols do not fulfil the clinical need in all situations. This has led to the development of novel activity assays, independent of ristocetin, with enhanced assay characteristics. Results from the first independent clinical evaluations are promising, showing that they are reliable and suitable for VWD diagnosis. The qualitative type 2 VWF deficiency can be further divided into four different subtypes (A, B, M and N) using specific assays that explore other activities or the size distribution of VWF multimers. These methods are discussed herein. However, in a number of patients it may be difficult to correctly classify the VWD phenotype and genetic analysis may provide the best option to clarify the disorder, through mutation identification.

Translational medicine advances in von Willebrand disease

Following the recognition of von Willebrand disease (VWD) in 1926 and the cloning of the gene for von Willebrand factor (VWF) in 1985, significant advances have been made in our fundamental knowledge of both the disease and the protein. Some of this new knowledge has also begun to impact the clinical management of VWD. First, the progressive increase in our understanding of the molecular genetic basis of VWD has resulted in rational applications of molecular testing to complement the current range of phenotypic tests for VWD. These molecular genetic strategies are most effectively directed at the prenatal diagnosis of type 3 VWD and confirmatory testing for types 2B and 2N disease. In contrast, the use of molecular testing to clarify the diagnosis of type 1 VWD is of marginal benefit, at best. In terms of VWD therapies, a new recombinant VWF concentrate has recently completed successful clinical trials and is now awaiting more widespread application. There have even been some preclinical successes with VWF gene transfer although the clinical rationale for this therapeutic strategy needs careful consideration. Much more remains to be learnt about the biology of VWF and further translational advances for the enhancement of VWD care will inevitably be realized.

The molecular characterization of von Willebrand disease: good in parts

Since the cloning of the gene that encodes von Willebrand factor (VWF), 27 years ago, significant progress has been made in our understanding of the molecular basis of the most common inherited bleeding disorder, von Willebrand disease (VWD). The molecular pathology of this condition represents a range of genetic mechanisms, some of which are now very well characterized, and others that are still under investigation. In general, our knowledge of the molecular basis of type 2 and 3 VWD is now well advanced, and in some instances this information is being used to enhance clinical management. In contrast, our understanding of the molecular pathogenesis of the most common form of VWD, type 1 disease, is still at an early stage, with preliminary evidence that this phenotype involves a complex interplay between environmental factors and the influence of genetic variability both within and outside of the VWF locus.

Indexes of von Willebrand factor as biomarkers of aortic stenosis severity (from the Biomarkers of Aortic Stenosis Severity [BASS] study)

We correlated von Willebrand factor (VWF) activity indexes and brain natriuretic peptide (BNP) with measures of aortic stenosis (AS) severity, bleeding, symptoms, and freedom from death or aortic valve replacement. Patients with AS (n = 66 [16 mild, 20 moderate, and 30 severe]) and aortic valve replacement (n = 21) were assessed with VWF antigen, VWF latex agglutination immunoturbidic activity, platelet function analyzer collagen plus adenosine diphosphate (PFA-CADP), VWF multimer ratio, and BNP level after echocardiography. In patients with AS, the mean gradient correlated with BNP (Spearman r = 0.29, p = 0.02), VWF latex agglutination immunoturbidic activity/VWF antigen ratio (r = -0.41, p <0.001), PFA-CADP (r = 0.49, p <0.001), and VWF multimer ratio (r = -0.76, p <0.001). The area under the curve for detection of severe AS was 0.62 (95% confidence interval [CI] 0.48 to 0.77) by elevated BNP, 0.81 (95% CI 0.69 to 0.92) by PFA-CADP closure time, 0.69 (95% CI 0.55 to 0.82) by VWF latex agglutination immunoturbidic activity/VWF antigen ratio, and 0.86 (95% CI 0.76 to 0.95) by VWF multimer ratio. For the VWF multimer ratio, a threshold of 0.15 yielded a sensitivity and specificity for severe AS of 77% and positive predictive value of 74%. Bleeding (in 14%) was associated with a prolonged PFA-CADP time and reduced VWF latex agglutination immunoturbidic activity/VWF antigen ratio. Symptoms were associated with elevated BNP and low Duke Activity Status Index score. In 66 patients with AS, freedom from death (n = 4) or aortic valve replacement (n = 22) was associated with PFA-CADP (p = 0.003), VWF high-molecular-weight multimers (p = 0.009), and VWF latex agglutination immunoturbidic activity/VWF antigen ratio (p <0.001) but not BNP (p = 0.32). In severe AS versus aortic valve replacement, the PFA-CADP and VWF multimer ratio differed (p <0.001), but BNP and the VWF latex agglutination immunoturbidic activity/VWF antigen ratio did not. In conclusion, the VWF activity indexes were associated with AS severity and bleeding and were predictive of cardiovascular outcomes.

Establishment and characterization of a new and stable collagen-binding assay for the assessment of von Willebrand factor activity

Introduction

Laboratory diagnosis of von Willebrand disease (VWD) requires determination of both von Willebrand factor (VWF) protein levels and activity. Current VWF activity tests include the ristocetin cofactor assay and the collagen-binding assay (VWF:CB). The goal of this investigation is to characterize a new collagen-binding assay and to determine its effectiveness in identifying VWD.

Methods

Analytical studies were carried out to characterize the performance of a new VWF:CB ELISA. Additionally, samples from a normal population were tested as were well-characterized type 1 and type 2 VWD samples.

Results

Repeatability and within-laboratory precision studies resulted in coefficients of variation (CVs) of ≤11%. A linear range of 1–354% (0.01–3.54 IU/mL) was determined, along with a limit of detection and a lower limit of quantitation of 1.6% and 4.0% (0.016 and 0.04 IU/mL), respectively. Samples tested from apparently healthy individuals resulted in a normal range of 54–217% (0.54–2.17 IU/mL). Known VWD type 1 and type 2 samples were also analyzed by the ELISA, with 99% of samples having VWF:CB below the normal reference range and an estimated 96% sensitivity and 87% specificity using a VWF collagen-binding/antigen cutoff ratio of 0.50.

Conclusion

This new VWF:CB ELISA provides an accurate measure of collagen-binding activity that aids in the diagnosis and differentiation of type 1 from type 2 VWD.

von Willebrand factor as new noninvasive predictor of portal hypertension, decompensation and mortality in patients with liver cirrhosis

von Willebrand factor antigen (vWF-Ag) is elevated in patients with liver cirrhosis, but the clinical significance is unclear. We hypothesized that vWF-Ag levels may correlate with portal pressure, measured by hepatic venous pressure gradient (HVPG), and predict clinically significant portal hypertension (CSPH; HVPG ≥ 10 mmHg), decompensation and mortality. Portal hemodynamics were assessed by HVPG measurement, whereas vWF-Ag levels were measured by enzyme-linked immunosorbent assay. During follow-up, complications of liver cirrhosis, death or transplantation were recorded. Two hundred and eighty-six patients (205 male and 81 female; mean age, 56 years) with liver cirrhosis were included. vWF-Ag correlated with HVPG (r = 0.69; P < 0.0001) and predicted CSPH independently of Child Pugh score. Higher vWF-Ag levels were associated with varices (odds ratio [OR] = 3.27; P < 0.001), ascites (OR = 3.93; P < 0.001) and mortality (hazard ratio: 4.41; P < 0.001). Using a vWF-Ag cut-off value of ≥ 241%, the AUC for detection of CSPH in compensated patients was 0.85, with a positive predictive value and negative predictive value of 87% and 80%, respectively. Compensated patients had 25% mortality after 53 months if the vWF-Ag was <315% compared to 15 months in patients with vWF-Ag >315% (P < 0.001). Decompensated patients had a mortality of 25% after 37 and 7 months if their vWF-Ag was <315% and >315%, respectively (P = 0.002). In compensated patients with a vWF-Ag >315% median time to decompensation or death was 32 months compared with 59 months in patients with vWF-Ag <315%. vWF-Ag equals Model for End-Stage Liver Disease (MELD) in mortality prediction (area under the curve [AUC] = 0.71 for vWF-Ag versus AUC = 0.65 for MELD; P = 0.2).

CONCLUSION

vWF-Ag is a new, simple and noninvasive predictor of CSPH. A vWF-Ag cut-off value at 315% can clearly stratify patients with compensated and decompensated liver cirrhosis in two groups with completely different survival. vWF-Ag may become a valuable marker for the prediction of mortality in patients with liver cirrhosis in clinical practice.

Clinical measurement of von Willebrand factor by fluorescence correlation spectroscopy

BACKGROUND

Identification of von Willebrand factor (vWF) abnormalities in a variety of conditions is hampered by the limitations of currently available diagnostic tests. Although direct multimer visualization by immunoelectrophoresis is a commonly used method, it is impractical as a routine clinical test. In this study, we used a biophysical analysis tool, fluorescence correlation spectroscopy (FCS), to measure vWF distributions. The goals were to develop a method that is quicker and simpler than vWF gel electrophoresis and to evaluate the potential of FCS as a clinical diagnostic technique.

METHODS

We analyzed plasma from 12 patients with type 1 von Willebrand disease (vWD), 14 patients with type 2 vWD, and 10 healthy controls using a fluctuation-based immunoassay approach.

RESULTS

FCS enabled identification and proper classification of type 1 and type 2 vWD, producing quantitative results that correspond to qualitative gel multimer patterns. FCS required minimal sample preparation and only a 5-min analysis time.

CONCLUSIONS

This study represents the first implementation of FCS for clinical diagnostics directly on human plasma. The technique shows potential for further vWF studies and as a generally applicable laboratory test method.