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

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

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

Consensus recommendations on flow cytometry for the assessment of inherited and acquired disorders of platelet number and function: Communication from the ISTH SSC Subcommittee on Platelet Physiology

Flow cytometry is increasingly used in the study of platelets in inherited and acquired disorders of platelet number and function. However, wide variation exists in specific reagents, methods, and equipment used, making interpretation and comparison of results difficult. The goal of the present study was to provide expert consensus guidance on the use of flow cytometry for the evaluation of platelet disorders. A modified RAND/UCLA survey method was used to obtain a consensus among 11 experts from 10 countries across four continents, on the appropriateness of statements relating to clinical utility, pre-analytical variables, instrument and reagent standardization, methods, reporting, and quality control for platelet flow cytometry. Feedback from the initial survey revealed that uncertainty was sometimes due to lack of expertise with a particular test condition rather than unavailable or ambiguous data. To address this, the RAND method was modified to allow experts to self-identify statements for which they could not provide expert input. There was uniform agreement among experts in the areas of instrument and reagent standardization, methods, reporting, and quality control and this agreement is used to suggest best practices in these areas. However, 25.9% and 50% of statements related to pre-analytical variables and clinical utility, respectively, were rated as uncertain. Thus, while citrate is the preferred anticoagulant for many flow cytometric platelet tests, expert opinions differed on the acceptability of other anticoagulants, particularly heparin. Lack of expert consensus on the clinical utility of many flow cytometric platelet tests indicates the need for rigorous multicenter clinical outcome studies.

Development of a novel flow cytometric immunobead array to quantify VWF: Ag and VWF: GPIbR and its application in acute myocardial infarction

OBJECTIVES

Both von Willebrand disease (VWD) and acute myocardial infarction (AMI) involve quantitative and qualitative changes in von Willebrand factor (VWF). Our objective was to develop a rapid and precise flow cytometric immunobead array (FCIA) to quantify VWF antigen (VWF:Ag) and ristocetin-triggered platelet glycoprotein Ib binding (VWF:GPIbR) and apply it in a clinical setting.

METHODS

Microbeads, coated with monoclonal antibodies for SZ29 or SZ151 IgG, were incubated with diluted plasma. VWF-binding microbeads were detected with FITC-conjugated sheep-anti-human VWF IgG by flow cytometry. Plasma VWF:Ag and VWF:GPIbR levels in normal controls (CTL; n=105), patients with VWD (n=21), and patients with AMI (n=146) were tested by FCIA and ELISA in parallel. ADAMTS13 activity and VWF multimer analyses were also implemented.

RESULTS

Our novel FCIA showed a strong correlation with the ELISA results (VWF:Ag, r=.855; VWF:GPIbR, r=.813). The intra-assay coefficient variations (CVs) of VWF:Ag-FCIA and VWF:GPIbR-FCIA were 9.2% and 7.7%, respectively, and the interassay CVs were 12.6% and 13.5%, respectively. Plasma VWF:Ag and VWF:GPIbR levels were significantly higher in patients with AMI than in CTL (P<.0001), whereas the ratios of ADAMTS13/VWF:Ag and ADAMTS13/VWF:GPIbR were significantly lower (P<.0001). Levels of plasma ultra-large VWF (UL-VWF) were dramatically increased in patients with AMI.

CONCLUSIONS

The novel VWF:Ag and VWF:GPIbR-FCIA assays were found to be simpler, more specific, and more accurate than the classical ELISA method. In addition, elevated VWF:GPIbR and UL-VWF may contribute to the pathogenesis of AMI.

Laboratory Testing for von Willebrand Factor Collagen Binding (VWF:CB)

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 the 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. This chapter describes several protocols for assessment of VWF activity by means of VWF collagen binding (VWF:CB). These assays identify VWF activity by quantitative assessment of VWF protein adhesion to collagen or collagen peptides and subsequent immunological detection of the adhered VWF. The most commonly performed assays for VWF:CB comprise enzyme-linked immunosorbent assays (ELISA) and chemiluminescent assay (CLIA), as described in this chapter.

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.

Conventional rapid latex agglutination in estimation of von Willebrand factor: method revisited and potential clinical applications

Measurement of von Willebrand factor antigen (VWF : Ag) levels is usually performed in a specialised laboratory which limits its application in routine clinical practice. So far, no commercial rapid test kit is available for VWF : Ag estimation. This paper discusses the technical aspect of latex agglutination method which was established to suit the purpose of estimating von Willebrand factor (VWF) levels in the plasma sample. The latex agglutination test can be performed qualitatively and semiquantitatively. Reproducibility, stability, linearity, limit of detection, interference, and method comparison studies were conducted to evaluate the performance of this test. Semiquantitative latex agglutination test was strongly correlated with the reference immunoturbidimetric assay (Spearman's rho = 0.946, P < 0.001, n = 132). A substantial agreement (κ = 0.77) was found between qualitative latex agglutination test and the reference assay. Using the scoring system for the rapid latex test, no agglutination is with 0% VWF : Ag (control negative), 1+ reaction is equivalent to <20% VWF : Ag, and 4+ reaction indicates >150% VWF : Ag (when comparing with immunoturbidimetric assay). The findings from evaluation studies suggest that latex agglutination method is suitable to be used as a rapid test kit for the estimation of VWF : Ag levels in various clinical conditions associated with high levels and low levels of VWF : Ag.

Perils, problems, and progress in laboratory diagnosis of von Willebrand disease

Diagnosis of von Willebrand disease (VWD) merits consideration of personal and family history of bleeding symptoms along with confirmatory laboratory testing. As the latter yields quantifiable results, overreliance on a laboratory diagnosis may occur. However, existing tests for VWD contain potential sources for error. Both intrinsic and extrinsic factors affecting these assays can contribute to either falsely normal or falsely abnormal results. This article will discuss the present available assays as well as new developments in diagnostic testing. A clear understanding of the limitations of VWD testing is helpful for ensuring the correct diagnosis of affected patients.