Conformation-specific RNA aptamers for phenotypic distinction between normal von Willebrand factor and type 2B von Willebrand disease

The A1 domain in Von Willebrand Factor (VWF) initiates coagulation through binding to platelet glycoprotein GPIbα receptors. Von Willebrand Disease (VWD)-Mutations in A1 that either impair (type 2M) or enhance (type 2B) platelet adhesion to VWF can locally destabilize and even misfold the domain. We leveraged misfolding in the gain-of-function type 2B VWD phenotype as a target, distinct from the normal conformation. Two nuclease-resistant 2'-fluoropyrimidine RNA aptamers were selected to discriminate normal A1 domains from a type 2B V1314D A1 variant in a glycosylated A1A2A3 tri-domain VWF-fragment. Two aptamers, W9 and V1, were isolated that selectively recognize, bind, and inhibit the A1-GPIbα interaction with WT A1A2A3 and V1314D A1A2A3, respectively. These aptamers were tested against their respective recombinant targets, plasma VWF, VWF concentrates, and patient plasma with the heterozygous type 2B VWD R1306W variant using clinical assays, surface plasmon resonance and inhibition assays of platelet adhesion to recombinant A1 and A1A2A3 domains under shear stress. The specificity of W9 and V1 aptamers confirms that pathological conformations of VWD Type 2B proteins are different from normal VWF. The availability of aptamers that distinguish normal plasma-derived VWF from VWD suggests potential applicability in clinical diagnosis of severe gain-of-function phenotypes.

Analytical and Clinical Validation of a Non-Ristocetin Based VWF Assay on 2 Automated Analyzers in a Large Reference Laboratory

BACKGROUND

Historically, von Willebrand factor (VWF) activity assays utilized ristocetin despite limitations including poor limits of detection and high imprecision. Newer VWF activity assays such as the INNOVANCE® VWF Ac assay, however, do not rely on ristocetin to measure platelet-dependent VWF function. The purpose of this study was to evaluate the analytical and clinical performance of the Siemens Healthineers INNOVANCE VWF Ac Assay on the Siemens BCS® XP and the Sysmex® CS-2500 systems in a large reference laboratory setting.

METHODS

Performance indicators for the INNOVANCE VWF Ac assay were the limit of quantitation (LoQ), precision, and method comparison. Method comparison studies were performed using remnant plasma patient samples from routine coagulation tests and analyzed using both the INNOVANCE VWF Ac assay and the Siemens Healthineers ristocetin-dependent BC von Willebrand Reagent.

RESULTS

Evaluation of the INNOVANCE VWF Ac assay on the BCS® XP and CS-2500 systems demonstrated good precision and a lower LoQ compared to the BC von Willebrand Reagent. Method comparisons support the use of the INNOVANCE VWF Ac assay on the BCS® XP and CS-2500 systems to measure platelet-dependent VWF function. The INNOVANCE VWF Ac assay was able to further assist in von Willebrand disease classification in 6/7 (86%) samples when the result was below the LoQ for the BC von Willebrand Reagent (ristocetin cofactor activity).

CONCLUSIONS

These data are consistent with the 2021 American Society of Hematology/International Society on Thrombosis and Haemostasis/National Hemophilia Foundation/World Federation of Hemophilia von Willebrand disease guidelines that suggest using newer assays such as the INNOVANCE VWF Ac assay in place of ristocetin cofactor activity assays.

Evaluation of a diagnostic platelet aggregation test strategy for platelet rich plasma samples with low platelet counts

INTRODUCTION

Light transmission aggregometry (LTA) is important for diagnosing platelet function disorders (PFD) and von Willebrand disease (VWD) affecting ristocetin-induced platelet aggregation (RIPA). Nonetheless, data is lacking on the utility of LTA for investigating thrombocytopenic patients and platelet rich plasma samples with low platelet counts (L-PRP). Previously, we developed a strategy for diagnostic LTA assessment of L-PRP that included: (1) acceptance of referrals/samples, regardless of thrombocytopenia severity, (2) tailored agonist selection, based on which are informative for L-PRP with mildly or severely low platelet counts, and (3) interpretation of maximal aggregation (MA) using regression-derived 95% confidence intervals, determined for diluted control L-PRP (C-L-PRP).

METHODS

To further evaluate the L-PRP LTA strategy, we evaluated findings for a subsequent patient cohort.

RESULTS

Between 2008 and 2021, the L-PRP strategy was applied to 211 samples (11.7% of all LTA samples) from 192 unique patients, whose platelet counts (median [range] × 109 /L) for blood and L-PRP were: 105 [13-282; 89% with thrombocytopenia] and 164 [17-249], respectively. Patient-L-PRP had more abnormal MA findings than simultaneously tested C-L-PRP (p-values <0.001). Among patients with accessible electronic medical records (n = 181), L-PRP LTA uncovered significant aggregation abnormalities in 45 (24.9%), including 18/30 (60%) with <80 × 109  platelets/L L-PRP, and ruled out PFD, and VWD affecting RIPA, in others. The L-PRP LTA strategy helped diagnose VWD affecting RIPA, Bernard Soulier syndrome, familial platelet disorder with myeloid malignancy, suspected ITGA2B/ITGB3-related thrombocytopenia, and acquired PFD.

CONCLUSION

Diagnostic LTA with L-PRP, using a strategy that considers thrombocytopenia severity, is feasible and informative.

In vitro field study and worldwide survey assessing how clinical haemostasis laboratories analyse recombinant and plasma-derived von Willebrand factor products

INTRODUCTION

Several well-established clinical laboratory methods are available to measure von Willebrand factor (VWF) in plasma samples, but few data are available on their use for analysing recombinant VWF (rVWF).

AIM

To evaluate how clinical diagnostic laboratories analyse rVWF and plasma-derived VWF (pdVWF) spiked in vitro into VWF-deficient plasma using quantitative protein and functional assays of VWF.

METHODS

Human VWF-deficient plasma samples were spiked with rVWF (vonicog alfa; Takeda) or pdVWF/factor VIII (pdVWF/FVIII; antihemophilic factor/VWF complex [human], CSL Behring), each at final concentrations of 1.0, 0.6, 0.2, 0.1 IU/mL VWF:ristocetin cofactor activity (VWF:RCo) according to labelled VWF activity. The ISTH SSC secondary coagulation standard was used as a control. Participating laboratories received three sets of these blinded aliquots. Mean results per assay were compared with the expected potency based on the labelled VWF:RCo activity.

RESULTS

Among 39 laboratories, the most commonly established assay was VWF:RCo; 22 laboratories reported data from 2214 tests. Despite a trend to lower values, VWF:RCo activities for rVWF were in agreement with target concentrations (71%-109%), whereas VWF:platelet glycoprotein Ib (VWF:GpIb) and VWF collagen-binding activity (VWF:CB) assays gave high recoveries (up to 132% and 127%, respectively). In contrast, pdVWF/FVIII was substantially underestimated by VWF:GpIb and VWF:CB assays (56%-86% recoveries), whereas the VWF:RCo assay gave recoveries of 47%-112% for pdVWF/FVIII.

CONCLUSION

The results of VWF assays used in clinical laboratories differ between rVWF and pdVWF, particularly for VWF:GpIb and VWF:CB assays. These differences may arise from the higher multimeric structure of rVWF compared to pdVWF.

Beyond the guidelines: how we approach challenging scenarios in the diagnosis and management of von Willebrand disease

Although von Willebrand disease (VWD) is the most common inherited bleeding disorder, its diagnosis and management are often challenging. Clinical practice guidelines, developed through systematic review of the medical literature and considering the best available evidence, provide guidance for common clinical scenarios. However, in the clinical setting, patients often present with characteristics and nuances that may fall outside the realm of available evidence and guidelines, and hence, shared decision-making will be essential in the evaluation and management of these patients. The challenges in the diagnosis of VWD are mainly attributable to the heterogeneity of the disorder, limitations of laboratory assays, and the significant impact of various physiologic processes on von Willebrand factor. The impact of physiologic normalization of von Willebrand factor, which may occur in various settings such as pregnancy, inflammation, or aging, remains uncertain, as is the optimal management in these scenarios. Multidisciplinary and individualized care, based on evolving evidence supported by clinicians, patients, caregivers, and stakeholders, will be needed to ensure the highest quality care for those who live with VWD.

Hemostatic and thrombotic disorders in the pediatric patient

This review focuses on significant advances in the field of pediatric hemostasis and thrombosis, with a focus on published studies within the past decade. The evaluation and management of patients with excessive bleeding remain cornerstones of consultative hematology. We will describe the development of validated bleeding assessment tools relevant to pediatric practice, laboratory advances in the evaluation of von Willebrand disease, and a shift in clinical practice regarding the interpretation of normal coagulation studies in patients with significant bleeding phenotypes. There have also been critical advances in the management of hemostatic disorders. This review highlights new treatment paradigms in hemophilia and the rise of multidisciplinary medical homes for women living with bleeding disorders. Given the continued increase in the incidence of thrombosis, particularly in the hospital setting, a full call to arms against pediatric venous thromboembolism is now essential. We will describe recently completed clinical trials of direct oral anticoagulants in children and adolescents and ongoing work to elucidate the appropriate duration of therapy for children with provoked thrombosis. Recent work regarding the prevention of pediatric venous thromboembolism is highlighted, including studies of thromboprophylaxis and the development of risk prediction models for hospital-acquired thrombosis. Finally, we review advances in our understanding of thrombotic sequelae and the need for continued refinement of our evaluation tools. Despite the significant advances in pediatric hemostasis and thrombosis over the past decade, many unanswered questions remain for the next generation of investigators.

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.

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.

Von Willebrand Disease: Current Status of Diagnosis and Management

Von Willebrand disease (VWD) is a common bleeding disorder, affecting male and female individuals equally, that often manifests in mucosal bleeding. VWD can be secondary to a quantitative (Type 1 and Type 3) or qualitative (Type 2) defects in Von Willebrand factor (VWF). Initial testing includes VWF antigen, as well as a platelet binding assay to differentiate between qualitative and quantitative defects. Further subtyping requires additional testing and is needed to ensure appropriate treatment. Desmopressin, antifibrinolytics, hormonal treatments for heavy menstrual bleeding, and VWF concentrates are commonly used in the treatment of VWD.

Comparison of von Willebrand factor platelet-binding activity assays: ELISA overreads type 2B with loss of HMW multimers

BACKGROUND

A number of new assays with different measuring principles are available to measure von Willebrand factor (VWF) glycoprotein Ib (GPIb)-binding activity, but little is known about how these assays might behave differently for subtypes of von Willebrand disease (VWD).

OBJECTIVES

The Comparison of Assays to Measure VWF Activity (COMPASS-VWF) study was designed to compare all available VWF GPIb-binding activity assays for VWF. We specifically searched for particular assay behavior differences.

PATIENTS/METHODS

To sort out random differences from systematic assay behavior deviations, all assays were performed in different laboratories on the same samples in a blinded fashion. Samples from 53 normal controls and 42 well-characterized VWD patients were reanalyzed in this study to dissect assay-specific discrepancies.

RESULTS

No assay behavior differences were found for 53 normal controls. For VWD patients, we found the following systematic assay behavior patterns: (a) All ELISA assays for VWF:GPIbR as well as VWF:GPIbM are insensitive to detect the low VWF activity of VWD type 2B patients with loss of high molecular weight multimers; (b) VWF:Ab assay reports higher activity for the p.V1665E mutation than all other assays; and (c) all ristocetin-based assays (including VWF:RCo using fixed platelets) but the AcuStar assay report discrepantly low VWF activity for the p.P1467S polymorphism. No systematic assay-specific difference was observed for either the particle agglutination VWF:GPIbM assay or the AcuStar assay using magnetic beads.

CONCLUSIONS

Different assay principles may lead to discrepant results for certain VWD types or mutations. Therefore, a more extensive study for a large number of patients is needed to better characterize the incidence and relevance of such assay-specific differences.

Bleeding risks for uncharacterized platelet function disorders

BACKGROUND

The bleeding risks for nonsyndromic platelet function disorders (PFDs) that impair aggregation responses and/or cause dense granule deficiency (DGD) are uncertain.

OBJECTIVES

Our goal was to quantify bleeding risks for a cohort of consecutive cases with uncharacterized PFD.

METHODS

Sequential cases with uncharacterized PFDs that had reduced maximal aggregation (MA) with multiple agonists and/or nonsyndromic DGD were invited to participate along with additional family members to reduce bias. Index cases were further evaluated by exome sequencing, with analysis of RUNX1-dependent genes for cases with RUNX1 sequence variants. Bleeding assessment tools were used to estimate bleeding scores, with bleeding risks estimated as odds ratios (ORs) relative to general population controls. Relationships between symptoms and laboratory findings were also explored.

RESULTS

Participants with uncharacterized PFD (n = 37; 23 index cases) had impaired aggregation function (70%), nonsyndromic DGD (19%) or both (11%), unlike unaffected relatives. Probable pathogenic RUNX1 variants were found in 2 (9%) index cases/families, whereas others had PFD of unknown cause. Participants with PFD had increased bleeding scores compared to unaffected family members and general population controls, and increased risks for mucocutaneous (OR, 4-207) and challenge-related bleeding (OR, 12-43), and for receiving transfusions for bleeding (OR, 100). Reduced MA with collagen was associated with wound healing problems and bruising, and more severe DGD was associated with surgical bleeding (P < .04).

CONCLUSIONS

PFDs that impair MA and/or cause nonsyndromic DGD have significantly increased bleeding risks, and some symptoms are more common in those with more severe DGD or impaired collagen aggregation.

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.

Bleeding Symptoms and von Willebrand Factor Levels: 30-Year Experience in a Tertiary Care Center

Correlations between bleeding symptoms and von Willebrand factor (VWF) levels may help to predict hemorrhagic severity in the Westerners with von Willebrand disease (VWD), but data in Asians are lacking. In this study, Thai patients with VWF levels <50 IU/dL without any secondary causes were enrolled from 1988 to 2018 to determine the relationship between VWF levels and hemorrhagic manifestations. According to the current concept, we reclassified VWD and low VWF by VWF levels ≤30 and 30 to 50 IU/dL, respectively. Type 2 VWD was diagnosed if VWF activity to antigen ratio was ≤0.6. Bleeding severity was determined by the condensed MCMDM-1VWD bleeding score (BS). Among 83 patients, VWF activities showed negative correlations with BS (P = .001), which were higher in type 2 (median: 7, interquartile range [IQR]: 5-11) compared with type 1 VWD (median: 3, IQR: 2-4) and low VWF (median: 4, IQR: 2-8). Bleeding symptoms were indistinguishable between type 1 VWD and low VWF using the 30 IU/dL cutoff point. However, VWF ristocetin cofactor activity or gain-of-function mutant glycoprotein Ib binding activity <36.5 IU/dL and VWF collagen binding activity <34.5 IU/dL could predict increased bleeding risk (BS ≥3) by 92.3% specificity and 70.0% sensitivity (P < .0001).

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.

Clinically relevant differences between assays for von Willebrand factor activity

Essentials It is unclear whether there are differences between von Willebrand factor (VWF) activity assays. We compared the four most used VWF activity assays in 661 von Willebrand disease (VWD) patients. All assays correlated excellently, but a discrepant classification was seen in 20% of patients. Differences between VWF activity assays have a large impact on the classification of VWD. SUMMARY: Background Measuring the ability of von Willebrand factor (VWF) to bind to platelets is crucial for the diagnosis and classification of von Willebrand disease (VWD). Several assays that measure this VWF activity using different principles are available, but the clinical relevance of different assay principles is unclear. Objective To compare the four most widely used VWF activity assays in a large VWD patient population. Methods We measured VWF:RCo (ristocetin to activate VWF + whole platelets), VWF:GPIbR (ristocetin + platelet glycoprotein Ib receptor [GPIb] fragments), VWF:GPIbM (gain-of-function GPIb fragments that bind VWF spontaneously without ristocetin) and VWF:Ab (monoclonal antibody directed against the GPIb binding epitope of VWF to mimic platelets) in 661 VWD patients from the nationwide 'Willebrand in the Netherlands' (WiN) Study. Results All assays correlated excellently (Pearson r > 0.9), but discrepant results led to a different classification for up to one-fifth of VWD patients. VWF:RCo was not sensitive enough to classify 18% of patients and misclassified half of genotypic 2B VWD patients, especially those with p.Arg1306Trp. VWF:GPIbR was more sensitive, accurately classified the vast majority of patients, and was unaffected by the p.Asp1472His variant that causes artificially low VWF:RCo. VWF:GPIbM was the most precise assay but misclassified over a quarter of genotypic 2A, 2B and 3 patients. VWF:Ab, often not considered an actual VWF activity assay, performed at least equally to the other assays with regard to accurate VWD classification. Conclusion Although the different VWF activity assays are often considered similar, differences between assays have a large impact on the classification of VWD.

[Von Willebrand disease]

Von Willebrand factor is involved in primary hemostasis (adhesion of platelets to subendothelium and platelet aggregation) and acts as the carrier of coagulation factor VIII. Von Willebrand disease, resulting from a quantitative or qualitative defect of this factor, is the most frequent inherited bleeding disorder. It is mainly responsible for symptoms such as mucocutaneous bleeding and excessive bleeding after trauma or invasive procedures, but can also cause gastro-intestinal bleeding or hemarthrosis in the most severe forms of the disease. There are numerous causes of physiological variation of von Willebrand factor plasma levels which can be responsible for diagnostic difficulty or changes in symptoms over time. Diagnosis relies primarily on clinical symptoms but requires the use of several laboratory analyses: von Willebrand factor activity and antigen testing and factor VIII activity. More specialized assays allow classification of the disease in various types and subtypes which imply different management strategies (types 1, 2A, 2B, 2M, 2N, and 3). Treatment is based on desmopressin, responsible for an increase in plasma concentration of von Willebrand factor, and plasma-derived von Willebrand factor concentrates which can be combined with factor VIII.

How I investigate for bleeding disorders

INTRODUCTION

Laboratory investigations for bleeding disorders are warranted when an individual has a personal and/or family history of bleeding, and/or laboratory findings that suggest the possibility of an inherited or acquired bleeding disorder.

METHODS

This review summarizes author's experience with ordering and reporting on diagnostic investigations for common and rare bleeding disorders, with consideration of recent articles on diagnosing bleeding disorders. An updated strategy is presented for investigating common and rare, congenital and acquired bleeding disorders.

RESULTS

An investigation of a suspected bleeding disorder requires a practical strategy that considers the clinical problem to be investigated, the pretest probability of true-positive and false-positive findings, the investigations can be performed locally or in a reference laboratory and limit the number of blood samples required to establish a diagnosis. It is often advantageous to simultaneously test for von Willebrand disease and platelet function disorders, and for coagulation defects, including fibrinogen disorders. An investigation for rarer bleeding disorders, including those affecting factor XIII, α₂ antiplasmin, and plasminogen activator inhibitor-1, is appropriate when faced with a severe congenital or acquired bleeding problem that cannot be explained by the initial diagnostic investigations.

CONCLUSION

An organized strategy for investigating bleeding disorders that consider important issues, confirms abnormal findings, encourages proper interpretation of the results, and provides a helpful framework for assessing both common and rare causes of bleeding.

An international collaborative study to compare different von Willebrand factor glycoprotein Ib binding activity assays: the COMPASS-VWF study

Essentials New VWF activity assays are increasingly used but information on their comparability is limited. This is an ISTH SSC-organized study (expert labs, 5 countries) to compare all available assays. VWF activity by six assays correlated well with each other. The new assays show improved characteristics - minor differences are noted. SUMMARY: Background Several new assays have become available to measure von Willebrand factor (VWF) activity. The new assays appear to have improved performance characteristics compared with the old reference standard, ristocetin cofactor activity (VWF:RCo), but information is limited about how they compare with VWF:RCo and each other. Methods The von Willebrand factor Subcommittee of the International Society for Thrombosis and Haemostasis (ISTH) Scientific and Standardization Committee (SSC) designed a collaborative study involving expert laboratories from several countries to compare available tests with each other and with VWF:RCo. Eight laboratories from five countries were provided with blinded samples from normal healthy individuals and well-characterized clinical cases. Laboratories measured VWF activity using all tests available to them; data from six laboratories, not affected by thawing during transportation, are included in this study. Results All tests correlated well with VWF:RCo activity (r-values ranged from 0.963 to 0.989). Slightly steeper regression lines for VWF:Ab and VWF:GPIbM were clinically insignificant. The new assays showed improved performance characteristics. Of the commercially available assays, the VWF:GPIbR using the AcuStar system was the most sensitive and could reliably detect VWF activity below 1 IU dL-1 . The lower limit of the measuring interval for the VWF:GPIbM and the VWF:GPIbR assays was in the 3-4 and 3-6 IU dL-1 range, respectively. Inter-laboratory variation was also improved for most new assays. Conclusion All VWF activity assays correlated well with each other and the VWF:RCo assay. The slight differences in characteristics found in the COMPASS-VWF study will assist the VWF community in interpreting and comparing activity results.

Advances in the diagnosis and treatment of Von Willebrand disease

Von Willebrand disease (VWD) is the most common inherited bleeding disorder, yet diagnosis and management remain challenging. Development and use of bleeding assessment tools allows for improved stratification of which patients may require further assessment and which patients are most likely to require treatment of their VWD. New options for laboratory assessment of von Willebrand factor (VWF) activity include a new platelet-binding assay, the VWF:GPIbM, which is subject to less variability than the ristocetin cofactor activity assay, and collagen-binding assays that provide insight into a different function of VWF. Genetic testing may be helpful in some cases where a type 2 VWD variant is suspected but is usually not helpful in type 1 VWD. Finally, treatment options for VWD are reviewed, including the use of recombinant VWF. Despite these advances, still more work is required to improve diagnosis, treatment, and quality of life for affected patients.

Advances in the diagnosis and treatment of Von Willebrand disease

Von Willebrand disease (VWD) is the most common inherited bleeding disorder, yet diagnosis and management remain challenging. Development and use of bleeding assessment tools allows for improved stratification of which patients may require further assessment and which patients are most likely to require treatment of their VWD. New options for laboratory assessment of von Willebrand factor (VWF) activity include a new platelet-binding assay, the VWF:GPIbM, which is subject to less variability than the ristocetin cofactor activity assay, and collagen-binding assays that provide insight into a different function of VWF. Genetic testing may be helpful in some cases where a type 2 VWD variant is suspected but is usually not helpful in type 1 VWD. Finally, treatment options for VWD are reviewed, including the use of recombinant VWF. Despite these advances, still more work is required to improve diagnosis, treatment, and quality of life for affected patients.

Clinical and laboratory phenotype variability in type 2M von Willebrand disease

Essentials The pathophysiology of type 2M von Willebrand disease (VWD) is poorly understood. Sequence variations in type 2M VWD subjects were characterized. A high degree of clinical and laboratory variability exists within type 2M VWD variants. Some type 2M variants may share features of type 2A VWD.

SUMMARY

Background von Willebrand factor (VWF) is a multimeric coagulation factor that tethers platelets to injured subendothelium. Type 2M von Willebrand disease (VWD) is characterized by a qualitative defect in VWF with preserved multimer distribution. Objectives Through the Zimmerman Program for the Molecular and Clinical Biology for VWD, five VWF sequence variations were studied in subjects diagnosed with type 2M VWD. Methods Bleeding phenotype was assessed using the ISTH bleeding assessment tool. Full-length VWF gene sequencing was performed for each subject. Each variant was placed into a recombinant VWF vector using site-directed mutagenesis and expressed in HEK293T cells as homozygous or heterozygous VWF. Variant expression, collagen binding and platelet GPIbα binding were studied through ELISA assays. Multimer analysis was performed by gel electrophoresis. Results Bleeding scores were elevated for all subjects except for the p.P1162L and p.R1374C variants. Although all had reduced VWF ristocetin cofactor activity/VWF antigen ratios on plasma testing, recombinant VWF did not show a classic type 2M phenotype for any of the five variants. Homozygous expression of variants p.D1283Y, p.R1349C, p.R1374C and p.I1453N was consistent with type 2A VWD, although all had normal expression as heterozygous recombinant VWF. Variant p.P1162L had normal VWF expression and function, consistent with the lack of bleeding symptoms. Conclusions Although originally classified as type 2M VWD, these homozygous recombinant VWF variants do not fulfill complete 2M VWD diagnostic criteria. A better classification schema and improved testing for putative type 2M variants is needed in order to effectively diagnose and treat affected patients.

Perioperative onset of acquired von Willebrand syndrome: Comparison between HVAD, HeartMate II and on-pump coronary bypass surgery

OBJECTIVES

Acquired von Willebrand syndrome (AvWS) is associated with postoperative bleeding complications in patients with continuous flow left ventricular assist devices (CF-LVADs). The aim of this study is to analyze the perioperative vWF profile comparing an axial pump (HMII) to a centrifugal pump (HVAD) regarding the correlation between perioperative occurrence of AvWS, early- and late-postoperative bleeding events.

METHODS

From July 2013 until March 2015 blood samples of 33 patients (12 HMII/ 8 HVAD/ 13 controls) were prospectively collected at 12 different time points and analyzed for the vWF antigen (vWF:Ag), its activity (vWF:Ac) and the vWF:Ac/vWF:Ag-ratio (vWF:ratio). The follow up period for postoperative bleeding events was from July 2013 until July 2016.

RESULTS

Postoperatively, there was no difference in the vWF-profile between HVAD and HMII groups. However, a subgroup of patients already had significantly lower vWF:ratios preoperatively. Postoperatively, both CF-LVAD groups presented significantly lower vWF:ratios compared to the control group. Bleeding events per patient-year did not differ between the two groups (HMII vs. HVAD: 0.67 vs. 0.85, p = 0.685). We detected a correlation between vWF:ratio <0.7at LVAD-start (r = -0.583, p = 0.006) or at the end of surgery (r = -0.461, p = 0.035) and the occurrence of pericardial tamponade. In the control group, the drop in both vWF:Ag and vWF:Ac recovered immediately postoperatively above preoperative values.

CONCLUSION

A subgroup of patients with end-stage heart failure already suffers AvWS preoperatively. In both CF-LVAD groups, AvWS begins immediately after surgery. Intraoperative vWF:ratios <0.7 correlate with higher incidences of pericardial tamponade and re-operation. The presumably dilutive effect of the heart lung machine on vWF vanishes immediately at the end of surgery, possibly as part of an acute-phase response.

Laboratory Testing for von Willebrand Factor Activity by Glycoprotein Ib Binding Assays (VWF:GPIb)

In addition to assessment of von Willebrand factor (VWF) antigen (VWF:Ag), the first-line laboratory investigation of possible von Willebrand disease (VWD) often includes an assay to measure GPIb (glycoprotein Ib) binding activity of VWF. A decreased GPIb binding activity is characteristic for most of the VWD types. For many years, the most frequently used assay for measuring GPIb binding activity was the ristocetin cofactor assay (VWF:RCo), which measures the agglutination of fixed human platelets by VWF in the presence of ristocetin. Because of performance issues, including high assay variability and a lack of VWF sensitivity, this assay is currently being replaced or supplemented by assays based on the binding of VWF to recombinant GPIb. One published method (now abbreviated VWF:GPIbR) uses wild-type GPIb for triggering the binding reaction in the presence of ristocetin. Another more widely used method (now abbreviated VWF:GPIbM) uses gain-of-function GPIb without ristocetin; this permits spontaneous binding of VWF to GPIb and avoids problems associated with the nonphysiological substance ristocetin. The binding of VWF to GPIb can be quantified by using different principles, e.g., ELISA, particle agglutination, or chemiluminescence. The following chapter describes a ristocetin-free method based on particle agglutination in more detail.

Comparison of von Willebrand factor (VWF) activity levels determined by HemosIL AcuStar assay and HemosIL LIA assay with ristocetin cofactor assay by aggregometry

INTRODUCTION

Diagnosis of von Willebrand disease (VWD) requires quantitative as well as qualitative determination of von Willebrand factor (VWF) levels. For functional assessment of VWF, ristocetin cofactor assay by aggregometry is considered to be the gold standard. However, need for technical expertise, labour intensiveness, difficult standardization and high intra- and inter- assay variabilities are some of the limitations of this methodology. Various assays for determination of VWF adhesive function using different methodologies have been developed in recent years.

AIM

To evaluate the HemosIL AcuStar chemiluminescence assay (VWF:RCo[Acu]) and the HemosIL latex immunoassay (VWF:act) as diagnostic tests for VWD and identification of type 2 VWD in comparison with the ristocetin cofactor assay performed by aggregometry (VWF:RCo[Agg]).

METHODS

Results from 96 samples analysed by VWF:RCo[Acu] and 128 samples by VWF:act were compared with VWF:RCo[Agg]. Sixty of these samples (25 normal, 17 type 1 and 18 type 2) were analysed by all three assays.

RESULTS

VWF:RCo[Acu] showed excellent agreement with VWF:RCo[Agg], and readily identified all type 2 VWD samples tested. VWF:act showed reasonable agreement with VWF:RCo[Agg] for most patients, but had a slightly lower sensitivity for detection of type 2 VWD.

CONCLUSION

VWF:RCo[Acu] assay has the potential to replace VWF:RCo[Agg] for the diagnosis of VWD.

Clinical and laboratory variability in a cohort of patients diagnosed with type 1 VWD in the United States

von Willebrand disease (VWD) is the most common inherited bleeding disorder, and type 1 VWD is the most common VWD variant. Despite its frequency, diagnosis of type 1 VWD remains the subject of debate. In order to study the spectrum of type 1 VWD in the United States, the Zimmerman Program enrolled 482 subjects with a previous diagnosis of type 1 VWD without stringent laboratory diagnostic criteria. von Willebrand factor (VWF) laboratory testing and full-length VWF gene sequencing was performed for all index cases and healthy control subjects in a central laboratory. Bleeding phenotype was characterized using the International Society on Thrombosis and Haemostasis bleeding assessment tool. At study entry, 64% of subjects had VWF antigen (VWF:Ag) or VWF ristocetin cofactor activity below the lower limit of normal, whereas 36% had normal VWF levels. VWF sequence variations were most frequent in subjects with VWF:Ag <30 IU/dL (82%), whereas subjects with type 1 VWD and VWF:Ag ≥30 IU/dL had an intermediate frequency of variants (44%). Subjects whose VWF testing was normal at study entry had a similar rate of sequence variations as the healthy controls (14%). All subjects with severe type 1 VWD and VWF:Ag ≤5 IU/dL had an abnormal bleeding score (BS), but otherwise BS did not correlate with VWF:Ag. Subjects with a historical diagnosis of type 1 VWD had similar rates of abnormal BS compared with subjects with low VWF levels at study entry. Type 1 VWD in the United States is highly variable, and bleeding symptoms are frequent in this population.

Evaluation of an heterogeneous group of patients with von Willebrand disease using an assay alternative to ristocetin induced platelet agglutination

BACKGROUND

Diagnosis of von Willebrand disease (VWD) type 2 usually relies on the discrepancy between the von Willebrand factor (VWF) ristocetin cofactor activity (VWF:RCo) and VWF antigen (VWF:Ag). Type 2B patients can be discriminated from other qualitative VWD variants by using ristocetin-induced platelet agglutination (RIPA) test. The major limitation of RIPA is the requirement of fresh blood sample.

OBJECTIVES

In this study, we evaluated the VWF gain-of-function mutant GPIb binding (VWF:GPIbM) and VWF:RCo assays to investigate whether the VWF:GPIbM/VWF:RCo ratio was able to identify the type 2B variant among an heterogeneous VWD population, previously characterized following the ISTH-SSC guidelines.

PATIENTS/METHODS

Seventy-six VWD patients and 31 healthy subjects were evaluated by using VWF:Ag, VWF:RCo, and VWF:GPIbM assays.

RESULTS

The mean (minimum-maximum values) VWF:GPIbM/VWF:RCo ratio was higher in type 2B patients (2.53, 0.84-6.11) than in healthy controls (1.05, 0.87-1.34), type 1 (0.85, 0.51-1.15), 2A (1.20, 0.36-2.82), and 2M (1.07, 0.91-1.38) (P < 0.0001). Type 2B variants were divided into four groups (A, B, C, and D) according to their different multimeric patterns. The mean value of the VWF:GPIbM/VWF:RCo ratio in the four groups showed an increasing trend from group A (1.08) to D (3.69), proportional to the loss of high molecular weight multimers. Among 32 type 2B patients, previously diagnosed with RIPA, 8 (mainly with a type I New York/Malmö phenotype) were not confirmed using the VWF:GPIbM/VWF:RCo ratio.

CONCLUSIONS

Whenever the RIPA test is not feasible, the VWF:GPIbM/VWF:RCo ratio might help to identify severe type 2B VWD patients.