Improved performance characteristics of the von Willebrand factor ristocetin cofactor activity assay using a novel automated assay protocol

An Automated pre-Dilution Setup for Von Willebrand Factor Activity Assays

Accurate quantification of von Willebrand factor ristocetin cofactor activity (VWF:RCo) is critical for the diagnosis and classification of von Willebrand disease, the most common hereditary and acquired bleeding disorder in humans. Moreover, it is important to accurately assess the function of von Willebrand factor (VWF) concentrates within the pharmaceutical industry to provide consistent and high-quality biopharmaceuticals. Although the performance of VWF:RCo assay has been improved by using coagulation analyzers, which are specialized devices for blood and blood plasma samples, scientists still report a high degree of intra- and inter-assay variation in clinical laboratories. Moreover, high, manual sample dilutions are required for VWF:RCo determination of VWF concentrates within the pharmaceutical industry, which are a major source for assay imprecision. For the first time, we present a precise and accurate method to determine VWF:RCo, where all critical pipetting and mixing steps are automated. A pre-dilution setup was established on CyBio FeliX (Analytik-Jena) liquid handling system, and an adapted VWF:RCo method on BCS-XP analyzer (Siemens) is used. The automated pre-dilution method was executed on three different, most frequently used coagulation analyzers and compared to manual pre-dilutions performed by an experienced operator. Comparative sample testing revealed a similar assay precision (coefficient of variation = 5.9% automated, 3.1% manual pre-dilution) and no significant differences between the automated approach and manual dilutions of an expert in this method. While no outliers were generated with the automated procedure, the manual pre-dilution resulted in an error rate of 8.3%. Overall, this operator-independent protocol enables standardization and offers an efficient way of fully automating VWF activity assays, while maintaining the precision and accuracy of an expert analyst. Key features • Automated pre-dilution setup for von Willebrand factor concentrates of various natures. • Combination of a liquid handling system (CyBio FeliX) with a coagulation analyzer (BCS-XP). • Simplifies method transfer to other laboratories. • Basic training for CyBio FeliX and BCS-XP is required. Graphical overview VWF:RCo assay principle and measurement setup. Platelets (yellow ellipsoids) with negative surface charge (- - -) are treated with formaldehyde, which partly denatures the cell surface and thus stabilizes platelets for use as assay reagents. Stabilized platelets (dark-yellow-framed yellow ellipsoids) are then brought in contact with ristocetin A (chemical structure shown; black dots), which binds to the platelet surface and facilitates binding of VWF (green circles). The graphs show an example of quantitative determination of platelet agglutination by measurement of light transmission, where increasing amounts of VWF increase light transmission over time. The photo in the left-bottom corner shows the CyBio FeliX setup for VWF sample dilution and the photo in the right-bottom corner displays the BCS-XP system, which is used for VWF:RCo measurements.

A Comparative Evaluation of an Automated Functional Assay for Von Willebrand Factor Activity in Type 1 Von Willebrand Disease

Background

von Willebrand factor ristocetin cofactor activity (VWF:RCo) is the standard functional assay used for von Willebrand disease (VWD) diagnosis. However, it has some drawbacks including being time consuming and labor intensive and having high inter-laboratory variability. The HemosIL VWF activity assay has the advantages of both high speed and automation. The purpose of this study was to prospectively compare these two functional assays for type 1 VWD detection.

Methods

Plasma samples from 108 subjects were assessed in this study. HemosIL VWF activity was measured with the HemosIL latex immunoturbidimetric commercial kits by the ACL TOP coagulation analyzer. VWF:RCo was measured by platelet aggregation method. Pearson correlation analyses were performed to estimate the correlation of HemosIL VWF activity with VWF:RCo. Receiver-operator characteristic (ROC) curve analysis was used to evaluate the performance of the two diagnostic tests.

Results

The correlation coefficient between VWF:RCo and HemosIL VWF activity was 0.874 overall and was 0.761 and 0.811 in the cohorts of type 1 VWD and non-VWD, respectively. The sensitivity and specificity of HemosIL VWF activity assay for type 1 VWD identification were 94.7% and 80.0%, respectively, and the ROC curve of HemosIL VWF activity was larger than that of VWF:RCo (0.928 vs 0.863, p=0.0138). Finally, the positive and negative predictive values of the HemosIL VWF activity assay for type 1 VWD detection were 72.0% and 96.6%, respectively.

Conclusion

Our results demonstrate that the HemosIL VWF activity assay was an effective method for type 1 VWD screening and diagnosis. It carried good sensitivity and specificity and had a higher ROC curve than VWF:RCo besides showing good correlation with VWF:RCo. With its advantages of greater speed and automated performance, these results suggest that the HemosIL VWF activity assay was reliable and precise in the clinical setting.

Functional Assessment of Platelet Dense Granule ATP Release

OBJECTIVES

This study was undertaken to explore the feasibility of assessing platelet dense granule release in response to platelet stimuli, using less than 1 mL of whole blood (WB).

METHODS

Optimization of the luciferin-luciferase (LL) assay for ATP release, together with additional modifications, was applied to 1:10 diluted WB.

RESULTS

LL assay optimization using nonstirred 1:10 diluted WB resulted in dense granule ATP release in response to thrombin receptor-activating peptide (TRAP) of similar magnitude to that observed using stirred platelet-rich plasma. Stirring of the 1:10 diluted WB restored collagen-induced dense granule secretion. Addition of lyophilized, formalin-fixed platelets, together with stirring, restored dense granule secretion responsiveness to ADP. TRAP, ADP, and collagen all stimulated ATP release in 1:10 diluted WB under the optimized conditions of this study at levels close to those observed using platelet-rich plasma. Blood sample reconstitution experiments offer hope that this assay may prove robust down to WB platelet counts as low as 50 × 103/μL.

CONCLUSIONS

Platelet dense granule release in response to a number of classic stimuli, including ADP, was accomplished from less than 1 mL WB with minimal specimen processing, using widely available reagents and instrumentation.

Potential misdiagnosis of von Willebrand disease caused by insufficient mixing after thawing frozen plasma

INTRODUCTION

von Willebrand disease (vWD) is a common inherited bleeding disorder caused by a deficiency in von Willebrand factor (vWF), but many laboratories and clinicians continue to struggle with diagnosing or excluding vWD. Its diagnosis requires laboratory testing, which may be compromised by preanalytical events, including poor specimen quality. This study assessed 17 different preanalytical conditions as potential causes of vWD misdiagnosis.

METHODS

Specimens from healthy controls (N = 21) were obtained. vWF antigen and vWF activity were analyzed using a newly developed automatic coagulation analyzer according to various preanalytic conditions such as centrifugation conditions, storage room temperature before centrifugation, cold storage temperature after centrifugation, thawing conditions, and inadequate mixing of thawed citrated plasma following the recommendations of the Clinical and Laboratory Standards Institute (CLSI) H21-A5 guidelines.

RESULTS

The only condition that was significantly different from the reference condition was lack of mixing after thawing frozen citrated plasma (vWF activity and antigen were reduced by 58.7% and 49.6%, respectively). Our study showed that mixing after thawing was more important than the chosen method of mixing.

CONCLUSION

Thawed plasma should be mixed because of the risk of misdiagnosing vWD. Further education regarding the importance of appropriate mixing is warranted to achieve results comparable to those of freshly centrifuged samples.

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.

Laboratory Testing for von Willebrand Factor Ristocetin Cofactor (VWF:RCo)

von Willebrand disease (VWD) is reportedly the most common inherited bleeding disorder and can also arise as an acquired syndrome (AVWS). These disorders develop due to defects and/or deficiency of the plasma protein von Willebrand factor (VWF). Laboratory testing for these VWF-related disorders requires assessment of both VWF level and VWF activity, the latter requiring multiple assays because of the many functions carried out by VWF to help prevent bleeding. The current paper describes several protocols for assessment of VWF activity by means of VWF ristocetin cofactor (VWF:RCo). These assays identify VWF activity by quantitative assessment of VWF protein adhesion to platelets or other particles and subsequent detection of the adhered VWF as facilitated by inclusion of ristocetin. The most commonly performed assays for VWF:RCo comprise platelet agglutination assays, latex agglutination assays, and chemiluminescent assay (CLIA), with three of these described in this chapter.

Developments in the diagnostic procedures for von Willebrand disease

Von Willebrand disease (VWD) is the most common inherited bleeding disorder but its diagnosis can be challenging due to the heterogeneity of the disease. VWD is mainly associated with mild mucocutaneous bleeding, although there are more severe phenotypes with bleeding from the gastrointestinal tract or even the joints. Also, surgical interventions and trauma may lead to critical bleeding events. These bleeding episodes are all related to quantitative or qualitative defects of von Willebrand factor (VWF), a multimeric glycoprotein produced by endothelial cells and megakaryocytes, which mediates platelet adhesion and aggregation and binds factor VIII (FVIII) in the circulation. This review describes the diagnostic procedures required for correct diagnosis. Accurate diagnosis and classification is required for proper treatment and counseling. Assessment of bleeding starts with the medical history. After a positive bleeding or family history, subsequent laboratory investigations will start with a panel of standard screening tests for hemostatic defects. Patients suspected of having VWD will be tested for plasma VWF antigen levels, the ability of VWF to bind platelets and FVIII activity. When VWD is confirmed, a set of subtyping tests can classify the patients as VWD types 1, 2 (A, B, M or N) or 3. The performance of some additional assays and analyses, such as VWF propeptide measurement or genetic analysis, may help in identifying the pathological mechanism behind certain defects or can guide in the choice of treatment.

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.

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.

Diagnosis of inherited von Willebrand disease: comparison of two methodologies and analysis of the discrepancies

Diagnostics of von Willebrand disease (VWD) includes assessment of factor VIII (FVIII) coagulant activity, von Willebrand factor (VWF) antigen (VWF:Ag) and VWF ristocetin cofactor activity (VWF:RCo), and more specific tests as multimeric and genetic analyses are necessary for the correct VWD classification. The ACL AcuStar analyzer introduces chemiluminescence (CL) technology in detection of VWD with automated VWF:Ag and VWF:RCo assays. Compare VWF:Ag-ELISA and VWF:RCo by aggregometry conventional assays with new CL VWF:Ag-IL and VWF:RCo-IL assays, investigate the ability to make accurate VWD diagnosis and concordance with multimeric and genetic analyses. 146 patients with congenital VWD (51 Type 1; 34 Type 2A; 16 Type 2B; 31 Type 2M; 5 Type 2N; 9 Type 3) and 30 healthy normal subjects were included. A comparison was made between CL and conventional methods. Diagnostic evaluation included: VWF:RCo/VWF:Ag ratio, multimeric distribution (sodium dodecyl sulfate [SDS]-agarose gel) of VWF and genetic analysis in 110 of 146 patients. CL and conventional methods revealed good correlation. Kappa test agreement diagnosis was >0.8. CL diagnostic sensitivity was 100% and specificity 97%. Multimeric and genetic analysis were of help in clarifying 13 discrepancies of diagnosis between methods, of which six discrepancies were explained by lack of conventional methods′ sensibility. CL methodology can detect VWD and discriminate between type 1, 3 and variant forms and offers an automated, faster, sensitive and less cumbersome method when compared to conventional assays, in particular VWF:RCo by aggregometry. In some cases, even with all phenotype and genetic analyses, discrepancies exist in the classification of VWD.

Differential sensitivity of von Willebrand factor (VWF) 'activity' assays to large and small VWF molecular weight forms: a cross-laboratory study comparing ristocetin cofactor, collagen-binding and mAb-based assays

BACKGROUND

von Willebrand disease (VWD), the most common inherited bleeding disorder, is caused by deficiencies and/or defects in von Willebrand factor (VWF). An effective diagnostic and VWD typing strategy requires plasma testing for factor VIII, and VWF antigen plus one or more VWF 'activity' assays. VWF activity is classically assessed by using VWF ristocetin cofactor activity (VWF:RCo), although VWF collagen-binding (VWF:CB) and VWF mAb-based (VWF activity [VWF:Act]) assays are used by some laboratories.

OBJECTIVE

To perform a cross-laboratory study to specifically evaluate these three VWF activity assays for comparative sensitivity to loss of high molecular weight (HMW) VWF, representing the form of VWF that is most functionally active and that is absent in some types of VWD, namely 2A and 2B.

METHODS

A set of eight samples, including six selectively representing stepwise reduction in HMW VWF, were tested by 51 different laboratories using a variety of assays.

RESULTS

The combined data showed that the VWF:CB and VWF:RCo assays had higher sensitivity to the loss of HMW VWF than did the VWF:Act assay. Moreover, within-method analysis identified better HMW VWF sensitivity of some VWF:CB assays than of others, with all VWF:CB assays still showing better sensitivity than the VWF:Act assay. Differences were also identified between VWF:RCo methodologies on the basis of either platelet aggregometry or as performed on automated analyzers.

CONCLUSIONS

We believe that these results have significant clinical implications for the diagnosis of VWD and monitoring of its therapy, as well as for the future diagnosis and therapy monitoring of thrombotic thrombocytopenic purpura.

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

BACKGROUND

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The pharmacokinetic diversity of two von Willebrand factor (VWF)/ factor VIII (FVIII) concentrates in subjects with congenital von Willebrand disease. Results from a prospective, randomised crossover study

The pharmacokinetic (PK) profiles of von Willebrand factor (VWF) /factor VIII (FVIII) concentrates are important for treatment efficacy and safety of von Willebrand disease (VWD) patients. This prospective, head-to-head, randomised crossover study compared the PK profile of a new, high purity, human plasma-derived (pd)VWF/FVIII concentrate, Wilate, with the PK profile of an intermediate purity (pd)VWF/FVIII concentrate, Humate-P, in VWD patients. Subjects with inherited VWD were randomised to a single intravenous dose (40 IU/kg VWF ristocetin cofactor activity [VWF:RCo]) of Wilate or Humate-P in Period 1, and switched to the other study drug in Period 2. Each period was preceded by a washout time of ≥ 7 days. Coagulation factor parameters were analysed at multiple time-points. Of 22 randomised subjects, 20 had evaluable PK profiles, which indicated comparability for VWF antigen and VWF:RCo between Wilate and Humate-P. The reported VWF:RCo average and terminal t1/2 of 10.4 and 15.8 hours (h), respectively, for Wilate and 9.3 h and 12.8 h for Humate-P, were not statistically different. Also, the mean VWF:RCo in vivo recoveries (Wilate 1.89, Humate-P 1.99 IU/dl per IU/kg) were similar between the two replacement therapies. Wilate showed parallel decay curves for VWF:RCo and FVIII clotting activity (FVIII:C) over time, while FVIII:C of Humate-P displayed a plateau between 0 and 12-24 h. This study demonstrated bioequivalent PK properties for VWF between Wilate and Humate-P. The PK profile of Wilate, combined with the 1:1 VWF/FVIII ratio, theoretically should facilitate dosing and laboratory monitoring of VWF replacement to prevent bleeding in individuals with VWD.