Von Willebrand Factor Activity Assays Research Articles

Point-of-Care Testing in Patients with Hereditary Disorders of Primary Hemostasis: A Narrative Review.

Inherited disorders of primary hemostasis, such as von Willebrand disease and congenital platelet disorders, can cause extensive, typically mucocutaneous bleeding. Assays to diagnose and monitor these disorders, such as von Willebrand factor activity assays and light transmission aggregometry, are performed in specialized hemostasis laboratories but are commonly not available in local hospitals. Due to the complexity and relative scarcity of these conventional assays, point-of-care tests (POCT) might be an attractive alternative in patients with hereditary bleeding disorders. POCTs, such as thromboelastography, are increasingly used to assess hemostasis in patients with acquired hemostatic defects, aiding clinical decision-making in critical situations, such as during surgery or childbirth. In comparison, the use of these assays in patients with hereditary hemostasis defects remains relatively unexplored. This review aims to give an overview of point-of-care hemostasis tests in patients with hereditary disorders of primary hemostasis. A summary of the literature reporting on the performance of currently available and experimental POCTs in these disorders is given, and the potential utility of the assays in various use scenarios is discussed. Altogether, the studies included in this review reveal that several POCTs are capable of identifying and monitoring severe defects in the primary hemostasis, while a POCT that can reliably detect milder defects of primary hemostasis is currently lacking. A better understanding of the strengths and limitations of POCTs in assessing hereditary defects of primary hemostasis is needed, after which these tests may become available for clinical practice, potentially targeting a large group of patients with milder defects of primary hemostasis.

Early Postoperative Changes in Von Willebrand Factor Activity Are Associated With Future Bleeding and Stroke in HeartMate 3 Patients.

Hemocompatibility-related adverse events (HRAEs), particularly gastrointestinal bleeding, remain a frequent complication after left ventricular assist device (LVAD) implantation. The current study sought to describe and analyze whether early (<60 days) postoperative von Willebrand factor (VWF) activity assays predict the risk of gastrointestinal bleeding and stroke. A prospective single-center study including 74 HeartMate 3 device recipients between 2016 and 2023 was undertaken. The postoperative trajectory of the VWF profile was analyzed using linear mixed-effect models and Cox models were used to quantify associations between an early postoperative dip (≤0.7) in VWF activity assay measurements and late outcomes. Preoperatively, the mean VWF:Activity (Act)/Antigen (Ag) and VWF:Collagen Binding (CB)/Ag ratios were 0.94 (95% confidence interval [CI] = 0.81-1.02) and 0.95 (95% CI = 0.80-1.03), respectively, decreasing to 0.66 (95% CI = 0.57-0.73) and 0.67 (95% CI = 0.58-0.74) within 40 days (p < 0.05). In patients with VWF:CB/Ag and VWF:Act/Ag ratios ≤0.7 significantly more gastrointestinal bleeding (hazard ratio [HR]: 2.53; 95% CI = 1.1-5.8, and HR: 3.7; 95% CI = 1.5-9.2, respectively) and hemorrhagic stroke events (HR: 3.5; 95% CI = 1.6-7.6 and HR: 4.9; 95% CI = 2.1-11.7, respectively) were observed throughout the entire late (>60 days) postoperative period. In patients with VWF:Act/Ag ratio ≤0.7 less ischemic stroke events were observed (HR: 0.11; 95% CI = 0.01-0.85). In conclusion, VWF:Act/Ag and VWF:CB/Ag ratios ≤0.7 in the early postoperative phase can be used as biomarkers to predict HRAEs during long-term LVAD support.

Nanobody activator improves sensitivity of the von Willebrand factor activity assay to multimer size

BackgroundThe activity of von Willebrand factor (VWF) in facilitating platelet adhesion and aggregation correlates with its multimer size. Traditional ristocetin-dependent functional assays lack sensitivity to multimer sizes. Recently, nanobodies targeting the autoinhibitory module and activating VWF were identified. ObjectivesTo develop an assay that can differentiate the platelet-binding activity of VWF multimers. MethodsA novel enzyme-linked immunosorbent assay (nanobody-triggered glycoprotein Ib binding assay [VWF:GPIbNab]) utilizing a VWF-activating nanobody was developed. Recombinant VWF, plasma-derived VWF (pdVWF), and selected gel-filtrated fractions of pdVWF were evaluated for VWF antigen and activity levels. A linear regression model was developed to estimate the specific activity of VWF multimers. ResultsOf the 3 activating nanobodies tested, 6C11 with the lowest activation effect exhibited the highest sensitivity for high-molecular-weight multimers (HMWMs) of VWF. VWF:GPIbNab utilizing 6C11 (VWF:GPIbNab6C11) produced significantly higher activity/antigen ratios for recombinant VWF (>2.0) and HMWM-enriched pdVWF fractions (>2.0) than for pdVWF (∼1.0) or fractions enriched with shorter multimers (<1.0). The differences were much larger than those produced by VWF:GPIbNab utilizing other nanobodies, VWF:GPIbM, VWF:GPIbR, or VWF:CB assays. Linear regression analysis of 5 pdVWF fractions of various multimer sizes produced an estimated specific activity of 2.7 for HMWMs. The analysis attributed >90% of the VWF activity measured by VWF:GPIbNab6C11 to that of HMWMs, which is significantly higher than all other activity assays tested. ConclusionThe VWF:GPIbNab6C11 assay exhibits higher sensitivity to HMWMs than ristocetin-based and collagen-binding assays. Future studies examining the application of this assay in clinical settings and any associated therapeutic benefit of doing so are warranted.

Evaluation of an automated von Willebrand factor glycoprotein IbM activity assay compared with 3 alternative von Willebrand factor activity assays

BackgroundTo overcome deficiencies of the traditional von Willebrand factor (VWF) ristocetin cofactor activity assay (VWF:RCo), several automated assays for VWF platelet-binding activity have been developed. Information on the performance of these assays and their diagnostic utility remains limited. ObjectivesTo validate the VWF:glycoprotein IbM assay INNOVANCE VWF Ac and compare it with an automated VWF:RCo assay as well as with an automated assay and a manual VWF:Ab assay and to generate reference ranges and analyze reproducibility of the VWF:glycoprotein IbM assay. MethodsClinical sites enrolled healthy subjects and patients representing the intended use population; VWF activity assays were performed, and results were analyzed. The performance of the INNOVANCE VWF Ac assay was also compared between the BCS XP System and the CS-2500 and CS-5100 analyzers. ResultsThe INNOVANCE VWF Ac assay correlated well with the VWF:RCo assay and the automated HemosIL VWF:Ab assay, with Pearson coefficients of >.9 and a predicted bias of ≤5.0 IU/dL at VWF levels of 30 IU/dL and ≤5.8 IU/dL at the levels of 50 IU/dL, but correlation and bias were not as good when compared with the REAADS manual VWF:Ab assay. Reference ranges observed for healthy subjects correlated well with previously published findings. Reproducibility of the INNOVANCE VWF Ac assay on the BCS XP System and the CS analyzers was excellent, as was correlation among devices. ConclusionThe characteristics of the INNOVANCE VWF Ac assay regarding comparability with other VWF activity assays, reference ranges, and precision support the use of this assay for evaluation of patients with concern for von Willebrand disease.

Analysis of College of American Pathologists von Willebrand Factor Proficiency Testing Program.

Von Willebrand factor (VWF) level and/or function is altered in von Willebrand disease (VWD), the most common heritable bleeding disorder worldwide. Laboratory assessment of VWF is continually evolving. Historically, the primary method for the assessment of VWF platelet-binding activity was the ristocetin cofactor assay (VWF:RCo). Contemporary alternative measures of VWF platelet-binding activity include VWF:GPIbR (recombinant; using ristocetin), VWF:GPIbM (recombinant; gain-of-function mutant), and monoclonal antibody. Recently, the American Society of Hematology, International Society on Thrombosis and Haemostasis, National Hemophilia Foundation, and World Federation of Hemophilia collaboration issued guidelines recommending the use of newer assays of VWF platelet-binding activity (VWF: GPIbM, VWF: GPIbR) over VWF:RCo, given known limitations of the VWF:RCo assay. Despite this recommendation, the newer VWF:GPIbM and VWF:GPIbR assays are not United States Food and Drug Administration cleared, limiting their availability in the United States. We sought to assess assay utilization trends, agreement of VWF testing methods, and imprecision of VWF testing (based on assigned sample type) from the College of American Pathologists Proficiency Testing Surveys. The analysis confirms that, while VWF antigen testing has low imprecision, the various VWF activity assays have significant interassay variability, with VWF:RCo showing greater imprecision than the newer GPIb-binding assays. The overall trends in assay utilization reflect the barriers to complete compliance with modern VWD diagnostic guidelines in North America.

External Quality Assessment Data for Investigation of von Willebrand Disease: Focus on Relative Utility of Contemporary Functional von Willebrand Factor Assays. The United Kingdom National External Quality Assessment Scheme (UK NEQAS) Experience.

Von Willebrand disease (VWD) is one of the most common hereditary bleeding disorders. Effective management of patients and their families depends on accurate diagnosis and subtype classification, and quality assurance including participation in proficiency testing programs is essential to ensure the accuracy of the panel of assays required to achieve this diagnosis. We report here findings from recent external quality assessment (EQA) exercises, as well as from a questionnaire about diagnostic practices employed by centers in the United Kingdom National Quality Assessment Scheme (UK NEQAS) performing von Willebrand factor (VWF) assays. Plasma samples from donors with VWD, "normal" donors, the International Society for Thrombosis and Haemostasis Scientific Subcommittee (ISTH SSC) plasma standard, and whole blood samples were sent to participants in the UK NEQAS BC program for VWF investigation. Calibration of lot#5 of the ISTH SSC plasma standard was shown to give improved comparability between the recovered value from an EQA exercise and the assigned potency for VWF activity assays. Diagnostic accuracy and precision amongst UK NEQAS participants was good, with an average 99% of centers reporting the correct interpretation for normal, type 1 and type 2 VWD samples, 100% diagnostic accuracy for centers performing FVIII binding assays, and good agreement amongst centers performing multimeric analysis. Genetic analysis of the VWF gene by specialist centers demonstrated errors in the genotyping process in one center, but also demonstrated failings in the interpretation of results in other centers. Despite evidence of good laboratory accuracy and precision in their assays, a questionnaire identified marked variation in diagnostic criteria employed, underlining the importance of guidelines to support the diagnosis of VWD.

Von Willebrand factor assays in patients with acquired immune thrombotic thrombocytopenia purpura treated with caplacizumab.

Acquired immune thrombotic thrombocytopenic purpura (iTTP) is a rare disease with a poor prognosis if undiagnosed. It is caused by autoantibody production to the von Willebrand factor (VWF) cleaving protease, A disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13). Caplacizumab, an immunoglobulin directed to the platelet glycoprotein Ibα receptor of VWF, has been reported to induce quicker resolution of iTTP compared to placebo. The laboratory measurement of VWF activity was significantly reduced in clinical trials of caplacizumab. Several VWF assays are available in the UK and this study investigated whether differences in VWF parameters were present in 11 patients diagnosed with iTTP and treated with daily caplacizumab. Chromogenic factor VIII activity, VWF antigen, collagen binding activity, VWF multimers and six VWF activity assays were measured prior to caplacizumab therapy and on several occasions during treatment. VWF antigen and collagen binding activity levels were normal or borderline normal in all patients. Ultra-large molecular weight multimers were present in all patients following treatment. VWF activity assays were normal or reduced during treatment, but this was reagent and patient dependant. In the unusual scenario of a caplacizumab-treated patient requiring measurement of VWF activity, it is important that laboratories understand how their local reagents perform as results cannot be predicted.

IX international curse of continuing formation in haemophilia and other congenital coagulopathies. The role of the Laboratory in coagulation disorders. Diagnosis of von Willebrand disease.

Von Willebrand disease (VWD) is the most frequent inherited bleeding disorder caused by quantitative or qualitative defects of von Willebrand factor (VWF). This protein far from simplicity constitutes a very complex molecular model, remaining unravelled yet many aspects of it, even though the VWF gene (VWF) was cloned already in 1985 and the structure of VWF well defined. VWD diagnosis is difficult to achieve in a significant proportion of patients due to both disease heterogeneity and limitations in existing test processes. The cornerstone of diagnosis relies on interpretation of VWF test results, the presence of clinical manifestations of bleeding, especially mucocutaneous, and (in most cases) a positive family history. However, even with a significant bleeding history, a family history may not be positive due to factors of incomplete penetrance and variable expressivity that affect genetic changes. 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 variants) VWD requires determination of VWF antigenic (VWF:Ag) levels and assaying of VWF ristocetin cofactor (VWF:RCo) activity. The latter is required for identifying and subtyping VWD, but the assay is poorly standardized. For that reason, novel VWF activity assays have been developed awaiting more extensive comparison data between different methodologies and requiring validation on larger patient series. The qualitative type 2 VWF deficiency can be further divided into four different subtypes (A, B, M and N) using specific assays that measure other activities or the size distribution of VWF multimers. However, frequently, it may be difficult to correctly classify the VWD phenotype, and genetic analysis is through mutation identification may provide a tool to clarify the disorder.

Monitoring the therapeutic effect of caplacizumab by von Willebrand Factor activity assays: Proceed with caution.

Monitoring the therapeutic effect of caplacizumab by von Willebrand Factor activity assays: Proceed with caution.

Active von Willebrand Factor in patients with a bleeding diathesis

IntroductionIncreased levels of circulating von Willebrand Factor (VWF) in its active, platelet-binding conformation have been implicated in the pathogenesis of several thrombotic conditions as well as bleeding conditions characterized by severe thrombocytopenia. However, it is unclear whether the proportion of activated VWF in the circulation also plays a role in patients with mild to moderate bleeding without thrombocytopenia. MethodsCitrated plasma samples were collected from 145 patients with a bleeding diathesis with unknown cause. Active VWF levels were measured with an in-house developed ELISA assay. In addition, VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo) and (flow-cytometric) platelet-VWF binding (Plt:VWF) were determined. ResultsActive VWF levels were on average mildly, but not significantly, lowered in patients with a bleeding diathesis compared to the reference interval (especially in individuals with non-O blood groups). Active VWF was not significantly different for subjects with (median 107.4%, IQR 18.3) versus without (median 111.1%, IQR 32.3%) an increased bleeding score, nor between subjects with suspected VWD (median 104%, IQR 20.6%) versus other suspected causes of bleeding diathesis (median 111.7%, IQR 33.3%). ConclusionIn this clinically heterogeneous population of patients with a mild bleeding phenotype, quantification of active VWF levels does not have added diagnostic value to VWF:Ag and VWF activity assays in the diagnosis of unexplained bleeding disorders.

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

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

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

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

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.

A decreased and less sustained desmopressin response in hemophilia A carriers contributes to bleeding

AbstractThe cause of hemophilia A carrier bleeding is not well established. Desmopressin (DDAVP), used clinically to treat or prevent bleeding, can also be used as a medical stress surrogate. This study's objective was to compare the response to DDAVP in hemophilia A carriers with that in normal control patients. Bleeding was assessed by the International Society on Thrombosis and Hemostasis Bleeding Assessment Tool (ISTH-BAT). DDAVP (0.3 μg/kg) was administered either IV or subcutaneously (SC), and blood was drawn at baseline and 1, 2, and 4 hours postadministration. Blood was assessed for factor VIII (FVIII) level, von Willebrand factor (VWF) antigen (VWF:Ag), VWF activity (VWF:RCo or VWF:GPIbM), thromboelastography (TEG), and thrombin generation assay (TGA) at all points, and for VWF propeptide (VWFpp):Ag ratio and ABO blood type at baseline. Carriers were older than control patients (median age, 34 and 21 years, respectively; P = .003) and had higher ISTH-BAT bleeding scores (median bleeding score, 8 and 0, respectively; P = .001). Carriers had a significantly reduced FVIII response to DDAVP compared with control patients (P ≤ .0001). When only carriers with normal baseline FVIII levels (n = 10) were included, carriers maintained a reduced FVIII response (P ≤ .0001). Furthermore, participants with abnormal bleeding scores had a significantly lower FVIII response to DDAVP compared with those with normal bleeding scores (P = .036). Hemophilia A carriers have a lower and less sustained FVIII response to DDAVP, suggesting an impaired ability to respond to hemostatic stress, which contributes to 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 1IUdL-1 . The lower limit of the measuring interval for the VWF:GPIbM and the VWF:GPIbR assays was in the 3-4 and 3-6IUdL-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.

Differential sensitivity of von Willebrand factor activity assays to reduced VWF molecular weight forms: A large international cross-laboratory study

Introductionvon Willebrand disease (VWD), the most common inherited bleeding disorder, is due to deficiencies/defects in von Willebrand factor (VWF). Effective diagnosis requires testing for FVIII, VWF antigen and one or more VWF ‘activity’ assays. Classically, ‘activity’ is assessed using ristocetin cofactor (VWF:RCo), but collagen binding (VWF:CB) and/or other assays are used by many laboratories. This extensive international cross-laboratory study has specifically evaluated contemporary VWF activity assays for comparative sensitivity to reduction in high molecular weight (HMW) VWF, and their ability to differentiate type 1 vs 2A VWD-like samples. Materials and methodsA set of four samples representing step wise reduction in HMW VWF were tested by over 400 laboratories worldwide using various assays. A second set of two samples representing type 1 or type 2A VWD-like plasma was tested by a subset of 251 laboratories. ResultsCombined data identified some differences between VWF activity assays, with sensitivity for reduction of HMW being highest for VWF:CB and VWF:GPIbM, intermediate for VWF:RCo and VWF:GPIbR, and lowest for VWF:Ab. ‘Within’ method analysis identified the Stago method as the most sensitive VWF:CB assay. A large variation in inter-laboratory CV (e.g., 7–24% for the normal sample) was also demonstrated for various methods. Although performance of various methods differed significantly, most laboratories correctly differentiated between type 1 and 2 samples, irrespective of VWF activity assay employed. ConclusionsThese results hold significant clinical implications for diagnosis and therapy monitoring of VWD, as well as potential future diagnosis and therapy monitoring of thrombotic thrombocytopenic purpura (TTP).

Von Willebrand disease—the Nordic perspective

Von Willebrand disease (VWD) is the most common bleeding disorder, showing a broad variation in prevalence of 0.2 to 4 cases per 100,000 inhabitants in type 2 and 0.2 to 1 in type 3 among the Nordic countries. Diagnosis and treatment of VWD in these countries primarily occur through comprehensive treatment centers by following guidelines outlined by the Nordic Haemophilia Council. Accordingly, assignment of VWD is based on comprehensive evaluation, including standardized bleeding score, traditional laboratory analysis and adoptive platelet function studies. The correct classification of VWD types is crucial in determining both the bleeding risk and clinical management, although clinical severity is not always related to biological activity of VWF. There are several developments in both diagnosis and clinical management of VWD. This review focuses on the novel aspects of VWD care in the Nordic countries, involving pure von Willebrand factor (VWF) concentrates as treatment options, as well as laboratory assay development, including the new VWF activity assays and comprehensive whole blood platelet function and global coagulation aspects.

Laboratory Assessment of Anti-Coagulant Properties of a Von Willebrand Factor Targeted Aptamer

Introduction: Many cardiovascular procedures including percutaneous coronary intervention, coronary bypass surgery, and peripheral vascular surgery require antithrombotic therapy during the procedure. Heparin is frequently used, due to low cost, ease of monitoring and reversibility with protamine. Antiplatelet agents such as aspirin and clopidogrel are also frequently used but these agents are not reversible. Alternative reversible antithrombotic strategies that interfere with platelet-mediated thrombosis are needed. We are investigating the use of a reversible agent that is specific for von Willebrand factor (VWF) for use during cardiovascular procedures.We previously developed an antidote-controlled 80 oligonucleotide RNA aptamer Ch-9.14-T10 (T10). (Nimjee, S.M., Lohrmann, Wang, H, et al. Molecular Therapy 2012; 2:391-397). T10 bound to VWF with high affinity and inhibited thrombosis in a murine carotid artery damage model. The aptamer also greatly increased bleeding in mice that were surgically challenged by tail transection and blood loss decreased when complementary antidote oligonucleotides were administered. In addition, T10 inhibited primary hemostasis in the PFA-100 assay and this inhibition was completely reversed by the antidote. This study further characterizes the T10 aptamer, as well as a truncated version of the T10 aptamer, using clinical laboratory assays that would be used to monitor antithrombotic therapies during cardiovascular procedures.Methods: We initiated our study with T10 and then developed a truncated version, T25, a 60 oligonucleotide with which we performed additional investigations. We recruited normal donors for analysis of aptamer and antidotes. All participants had normal platelet counts, VWF activity and antigen levels, and were not taking antiplatelet agents. Laboratory tests included: ristocetin induced platelet agglutination (RIPA), VWF activity, ristocetin cofactor activity, cone and plate(let) analyzer, thromboelastography (TEG) and thromboelastometry (ROTEM).Results: We first determined the effects of the VWF aptamers on RIPA, VWF activity, and ristocetin cofactor activity to confirm functional inhibition of VWF. We found that T10 (500-750 nM) and T25 (250-750nM) completely inhibited RIPA depending on the donor. T25 also inhibited VWF activity with a Ki of 160-320 nM and ristocetin cofactor activity with an IC50 of 3000 nM. Employing a cone and plate(let) analyzer we found that T25 (750 nm) reduced platelet adhesion by 33-49% on a polystyrene surface under arterial flow conditions (1800 s-1, for 2 minutes). The aggregate size was also reduced by 30%.As ACT and TEG are frequently used during cardiovascular procedures, we performed several measures with our VWF aptamer. T10 had no effect on the ACT (730-2000nM), TEG (1600 nM), or ROTEM (750 nM). Standard methods were used for the TEG (Kaolin activation followed by CaCl2) and ROTEM (EXTEM assay) Topf et al. reported a modified ROTEM protocol where they found that incubating citrated whole blood with ristocetin decreased the clot strength as measured by the maximum amplitude (Thromb.Haemost 2011; 105:1091). We reasoned that an aptamer that bound to VWF would inhibit the effect of ristocetin on the maximum amplitude. We found that T25 did inhibit the effect of ristocetin and titration studies showed that 200-400 nM T25 was needed to restore the maximum amplitude to at least 80% of its original value. An example of the ROTEM tracings is shown in figure 1.Conclusion: The VWF-specific aptamer T25 can be detected by several clinical assays, including the RIPA, VWF activity assays, and the cone and plate(let) analyzer. It is not detectable by an ACT or standard TEG or ROTEM, however. We are able to detect the aptamer using a modified ROTEM and are further defining the adaptability of this assay to facilitate monitoring during cardiovascular procedures.Citrated whole blood from was incubated with buffer or ristocetin (0.7 mg/ml) or ristocetin and T25 (300 nM) as indicated. Clotting was initiated with EXTEM (tissue factor pathway activation) and CaCl2. [Display omitted] DisclosuresBecker:Regado Biosciences: Membership on an entity’s Board of Directors or advisory committees.

Performance evaluation and multicentre study of a von Willebrand factor activity assay based on GPIb binding in the absence of ristocetin.

The functional activity of von Willebrand factor (VWF) is most frequently measured by using the ristocetin cofactor assay (VWF:RCo). However, the method's drawbacks include unsatisfactory precision, sensitivity and availability of automated system applications. We have developed an alternative assay (INNOVANCE VWF Ac) that is based on the binding of VWF to recombinant glycoprotein Ib (GPIb). Two gain-of-function mutations were introduced into a GPIb fragment, allowing an assay format without ristocetin. Fully automated assay applications are available for the BCS/BCS XP systems and the Sysmex CS-2000i, Sysmex CA-7000, Sysmex CA-1500 and Sysmex CA-560 systems.The INNOVANCE VWF Ac assay measuring range extends from 4 to 600% VWF for all systems except the Sysmex CA-560 system. Within-device precision values were found to be between 2 and 7%. The limit of detection was below 2.2% VWF. In a study on the BCS XP system, a total number of 580 sample results yielded a correlation to the VWF:RCo assay of r equal to 0.99 (slope = 0.96). Very similar results were observed when von Willebrand disease samples type 1, 2A, 2B, 2M, 2N and 3 were investigated with the new assay and the VWF:RCo assay. The excellent performance data and comparability to VWF:RCo, together with the ease of use, led us to the conclusion that the ristocetin cofactor assay can be replaced by the new GPIb-binding assay to reliably diagnosing patients with von Willebrand disease.

Towards improved diagnosis of von Willebrand disease: Comparative evaluations of several automated von Willebrand factor antigen and activity assays

Introductionvon Willebrand disease (VWD) is reportedly the most common bleeding disorder and arises from deficiency and/or defects of von Willebrand factor (VWF). Laboratory diagnosis and typing has important management implications and requires a wide range of tests, including VWF activity and antigen, and involves differential identification of qualitative vs quantitative defects. MethodsWe have assessed several VWF antigen and activity assays (collagen binding [VWF:CB], ristocetin cofactor [VWF:RCo] and the new Siemens INNOVANCE assay [VWF:Ac], employing latex particles and gain of function recombinant glycoprotein Ib to facilitate VWF binding and agglutination without need for ristocetin) using different instrumentation, including the new Sysmex CS-5100, with a large sample test set (n=600). We included retrospective plus prospective study designs, and also evaluated desmopressin responsiveness plus differential sensitivity to high molecular weight VWF. ResultsVWF:Ag and VWF:RCo results from different methods were respectively largely comparable, although some notable differences were evident, including one high false normal VWF:Ag value (105 U/dL) on a type 3 VWD sample, possibly due to heterophile antibody interference in the latex-based CS-5100 methodology. VWF:Ac was largely comparable to VWF:RCo, but VWF:CB showed discrepant findings to both VWF:RCo and VWF:Ac with some patients, most notably patients with type 2M VWD. Conclusions(a) VWF:Ag on different platforms are largely interchangeable, as are VWF:RCo on different platforms, except for occasional (some potentially important) differences, and manufacturer recommended methods may otherwise require some assay optimization; (b) VWF:RCo and VWF:Ac are largely interchangeable, except for occasional differences that may also relate to assay design (differing optimizations); (c) VWF:CB provides an additional activity to supplement VWF:RCo or VWF:Ac activity assays, and is not interchangeable with either.