The determination of von Willebrand factor activity by collagen binding assay

The Role of the von Willebrand Factor Collagen-Binding Assay (VWF:CB) in the Diagnosis and Treatment of von Willebrand Disease (VWD) and Way Beyond: A Comprehensive 36-Year History

The von Willebrand factor (VWF) collagen binding (VWF:CB) assay was first reported for use in von Willebrand diagnostics in 1986, by Brown and Bosak. Since then, the VWF:CB has continued to be used to help diagnose von Willebrand disease (VWD) (correctly) and also to help assign the correct subtype, as well as to assist in the monitoring of VWD therapy, especially desmopressin (DDAVP). However, it is important to recognize that the specific value of any VWF:CB is predicated on the use of an optimized VWF:CB, and that not all VWF:CB assays are so optimized. There are some good commercial assays available, but there are also some "not-so-good" commercial assays available, and these may continue to give the VWF:CB "a bad reputation." In addition to VWD diagnosis and management, the VWF:CB found purpose in a variety of other applications, from assessing ADAMTS13 activity, to investigation into acquired von Willebrand syndrome (especially as associated with use of mechanical circulatory support or cardiac assist devices), to assessment of VWF activity in disease states in where an excess of high-molecular-weight VWF may accumulate, and lead to increased (micro)thrombosis risk (e.g., coronavirus disease 2019, thrombotic thrombocytopenic purpura). The VWF:CB turns 37 in 2023. This review is a celebration of the utility of the VWF:CB over this nearly 40-year history.

Biochemical and cellular markers differentiate recovered, in-line filtered plasma, and plasma obtained by apheresis methods

BACKGROUND AND OBJECTIVES

Assessment of plasma quality often focuses on the common safety tests for minimizing the risk of transmitting blood-borne pathogens. Little attention is paid to the possible quality attributes that ensure a consistent biochemical composition of plasma for fractionation. We therefore investigated the suitability of selected biochemical and haematological attributes that could be used as markers of plasma quality obtained by different separation and pre-treatment procedures.

MATERIAL AND METHODS

We characterized six plasma types, including source plasma, plasma recovered by classic means and in-line filtered plasma, by determining the analytical attributes protein content, coagulation factors and markers of coagulation, contact and complement activation. Residual cell content and cell-specific variables were also measured.

RESULTS

We found relevant differences between the plasma types in complement activation, as indicated by C3a measurements, while thrombin antithrombin complex values and, to a minor extent, activated factor XII concentrations indicated only moderate differences in activation levels of coagulation and contact systems. The most striking differences, however, were detected in residual cell content and concentrations of the platelet-associated proteins, platelet factor 4 and β-thromboglobulin. We showed that leucocyte reduction filters disrupt cells. This includes platelets, thereby releasing the platelet-associated proteins platelet factor 4 and β-thromboglobulin, and leucocytes as demonstrated by the release of elastase from polymorphonuclear leucocytes. Furthermore, the filtration processing of whole blood can lead to activation of the complement system.

CONCLUSION

Our results show that biochemical and cellular surrogate markers are valuable discriminators of plasma types.

The contribution of von Willebrand factor-GPIbα interactions to persistent aggregate formation in apheresis platelet concentrates

BACKGROUND AND OBJECTIVES

Apheresis platelet concentrates sometimes contain persistent aggregates (PA). Because apheresis involves extracorporeal circulation, we hypothesized that interactions between GPIbα and von Willebrand factor (VWF) underlie their origin.

MATERIALS AND METHODS

Platelets in donations with PA were compared to aggregate-free (AF) controls. Flow cytometry was used to determine platelet bound VWF. Degranulation was measured using P-selectin expression in flow cytometry and cytokine release using immunosorbent assays. Platelet adhesion to VWF was assessed in hydrodynamic flow and real-time video microscopy.

RESULTS

Platelets in PA concentrates had significantly more (P = 0·009, n ≥ 8) bound VWF compared to AF platelets, but differences in VWF concentration, VWF collagen binding, activated VWF or GPIbα expression were not found. Degranulation was higher (P = 0·030, n = 7) in PA than AF concentrates on day 1 of storage, but adhesion to immobilized VWF under hydrodynamic flow conditions was normal at that moment. On day 6, however, significantly less VWF adhesion (P = 0·009, n ≥ 6) was found for PA platelets compared to AF, indicating accelerated storage lesion in PA products. In a model that mimicks PA formation by chemically induced binding of VWF to platelets, we found that degranulation, phosphatidylserine expression and metabolism did not differ with paired controls at any time during subsequent storage.

CONCLUSION

Accelerated storage lesion is found in concentrates with PA, but this cannot be explained solely by increased platelet bound VWF following apheresis. Therefore, additional stressors are probably responsible for the increases observed in platelet degranulation and storage lesion in products with PA.

Plasmatic ADAMTS-13 metalloprotease and von Willebrand factor in children with cyanotic congenital heart disease

Changes in plasma von Willebrand factor concentration (VWF:Ag) and ADAMTS-13 activity (the metalloprotease that cleaves VWF physiologically) have been reported in several cardiovascular disorders with prognostic implications. We therefore determined the level of these proteins in the plasma of children with cyanotic congenital heart disease (CCHD) undergoing surgical treatment. Forty-eight children were enrolled (age 0.83 to 7.58 years). Measurements were performed at baseline and 48 h after surgery. ELISA, collagen-binding assays and Western blotting were used to estimate antigenic and biological activities, and proteolysis of VWF multimers. Preoperatively, VWF:Ag and ADAMTS-13 activity were decreased (65 and 71% of normal levels considered as 113 (105-129) U/dL and 91 ± 24% respectively, P < 0.003) and correlated (r = 0.39, P = 0.0064). High molecular weight VWF multimers were not related, suggesting an interaction of VWF with cell membranes, followed by proteolytic cleavage. A low preoperative ADAMTS-13 activity, a longer activated partial thromboplastin time and the need for cardiopulmonary bypass correlated with postoperative bleeding (P < 0.05). Postoperatively, ADAMTS-13 activity increased but less extensively than VWF:Ag (respectively, 2.23 and 2.83 times baseline, P < 0.0001), resulting in an increased VWF:Ag/ADAMTS-13 activity ratio (1.20 to 1.54, respectively, pre- and postoperative median values, P = 0.0029). ADAMTS-13 consumption was further confirmed by decreased ADAMTS-13 antigenic concentration (0.91 ± 0.30 to 0.70 ± 0.25 µg/mL, P < 0.0001) and persistent proteolysis of VWF multimers. We conclude that, in pediatric CCHD, changes in circulating ADAMTS-13 suggest enzyme consumption, associated with abnormal structure and function of VWF.

Decreased plasma ADAMTS-13 activity as a predictor of postoperative bleeding in cyanotic congenital heart disease

OBJECTIVE

To analyze the preoperative plasma antigenic concentration and activity of von Willebrand factor and its main cleaving protease ADAMTS-13 in pediatric patients with cyanotic congenital heart disease undergoing surgical treatment and investigate possible correlations with postoperative bleeding.

METHODS

Plasma antigenic concentrations (von Willebrand factor:Ag and ADAMTS-13:Ag) were measured using enzyme-linked immunoassays. Collagen-binding assays were developed to measure biological activities (von Willebrand factor:collagen binding and ADAMTS-13 activity). The multimeric structure of von Willebrand factor was analyzed using Western immunoblotting. Demographic, diagnostic, and general and specific laboratory data and surgery-related variables were subjected to univariate, bivariate, and multivariate analysis for the prediction of postoperative bleeding.

RESULTS

Forty-eight patients were enrolled, with ages ranging from 9 months to 7.6 years (median 2.5 years). The plasma concentrations of von Willebrand factor:Ag and ADAMTS-13:Ag were decreased by 65 and 82%, respectively, in the patients compared with the controls (p<0.001). An increased density of low-molecular-weight fractions of von Willebrand factor, which are suggestive of proteolytic degradation (p = 0.0081), was associated with decreased ADAMTS-13 activity, which was likely due to ADAMTS-13 consumption (71% of controls, p = 0.0029) and decreased von Willebrand factor:collagen binding (76% of controls, p = 0.0004). Significant postoperative bleeding occurred in 13 patients. The preoperative ADAMTS-13 activity of <64.6% (mean level for the group), preoperative activated partial thromboplastin time, and the need for cardiopulmonary bypass were characterized as independent risk factors for postoperative bleeding, with respective hazard ratios of 22.35 (95% CI 1.69 to 294.79), 1.096 (95% CI 1.016 to 1.183), and 37.43 (95% CI 1.79 to 782.73).

CONCLUSION

Low plasma ADAMTS-13 activity is a risk factor for postoperative bleeding in children with cyanotic congenital heart disease, particularly in children undergoing cardiopulmonary bypass.

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

Introduction

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

Methods

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

Results

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

Conclusion

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

The perioperative von Willebrand factor activity and factor VIII levels among alcohol use disorder patients undergoing total knee or hip replacement

Alcohol use disorder patients have a five-fold higher risk of postoperative bleeding complications. We measured the perioperative von Willebrand factor and factor VIII levels in consecutive patients with alcohol use disorder. In one university hospital, 105 patients scheduled for arthroplasty were screened, and 25 fulfilled inclusion criteria. Postoperatively, we found significantly decreased von Willebrand factor ristocetin cofactor values over time among alcohol use disorder patients and significantly different time courses of factor VIII levels between patients with and without a diagnosed alcohol use disorder. Blood loss was significantly increased among alcohol use disorder patients on their first postoperative day.

ADAMTS-13 activity in the presence of elevated von Willebrand factor levels as a novel mechanism of residual platelet reactivity in high risk coronary patients on antiplatelet treatment

BACKGROUND

The pathophysiologic mechanisms leading to residual platelet reactivity (RPR) on antiplatelet therapy, a condition high prevalent in patients with acute coronary syndrome, are not yet elucidated. In the acute phase of coronary artery disease large amounts of ultra large VWF multimers (ULVWF) are released and cleaved by the activity of ADAMTS-13.

OBJECTIVE

Aim of this study was to evaluate the relationships between VWF antigen (VWF:Ag) levels, collagen binding activity of VWF (VWF:CB), ADAMTS-13 and interleukin-6 (IL-6) levels in affecting platelet response to dual antiplatelet treatment.

METHODS

In 159 acute coronary syndrome (ACS) patients undergoing percutaneous coronary interventions we measured platelet function by platelet aggregation with two agonists [1 mM arachidonic acid (AA) and 10 microM ADP]. We defined patients with RPR those with platelet aggregation by AA >20% and/or ADP (10 micromol) >70%.

RESULTS

We found significantly lower ADAMTS-13 activity, elevated IL-6, VWF:Ag and VWF:CB levels in patients with RPR. A lower ADAMTS-13 activity was present in patients with VWF:Ag and VWF:CB in the upper tertile. At the multivariate analysis ADAMTS-13 activity and IL-6 were independent risk factors for RPR.

CONCLUSION

Our results indicate that ADAMTS-13 activity and IL-6 levels independently affect RPR and suggest that, by different pathways, both are involved in the variable response to antiplatelet therapy.

von Willebrand factor/factor VIII concentrate (Haemate P) dosing based on pharmacokinetics: a prospective multicenter trial in elective surgery

BACKGROUND

While plasma-derived concentrates containing large amounts of von Willebrand factor (VWF) are effective in treating von Willebrand disease (VWD), optimal dosing remains to be fully characterized.

OBJECTIVES

To determine the feasibility of dosing Haemate P VWF/factor VIII (FVIII) concentrate based on pharmacokinetics (PK) in the management of surgical subjects with VWD.

METHODS

VWD subjects scheduled for elective surgery were enrolled in a prospective multicenter open-label cohort study. A pre-operative loading dose of VWF/FVIII concentrate based upon prior individual subject PK analysis was administered followed by postoperative therapeutic/maintenance infusions.

RESULTS

Twenty-eight subjects with types 1, 2A or 3 VWD and one with type 2 M were enrolled. Median in vivo recovery of VWF ristocetin cofactor (VWF:RCo) was 1.9 IU dL(-1) (IU kg(-1))(-1) with an interquartile range (IQR) of 1.6-2.5 IU dL(-1) (IU kg(-1))(-1). Median response, half-life and clearance were 74.0% (IQR, 55.5-100%), 15.6 h (IQR, 9.0-28.4 h) and 3.26 mL kg(-1) h(-1) (IQR, 2.29-5.21 mL kg(-1) h(-1)), respectively. A PK-guided median VWF:RCo loading dose of 62.4 IU kg(-1) (IQR, 50.1-87.0 IU kg(-1)) was administered. Postoperative mean trough VWF:RCo levels of 62-73 IU dL(-1) were sufficient to prevent bleeding. Investigators rated hemostasis excellent or good in 96.3% of subjects on the day of surgery and 100% on the next day and on day 14. A subject with multiple risk factors developed pulmonary embolism, which resolved without sequelae.

CONCLUSIONS

Haemate P provided effective and safe hemostasis in VWD subjects undergoing elective surgery. Selection of Haemate P loading dose on the basis of VWF PK proved feasible.

Performance and utility of a cost-effective collagen-binding assay for the laboratory diagnosis of Von Willebrand disease

BACKGROUND

The collagen-binding assay, a functional assay of Von Willebrand factor (VWF), discriminates the subtypes of Von Willebrand disease (VWD). Commercial collagen binding assays have the advantage of immediate use for fast results, but are expensive.

METHODS

In this study we evaluated an in-house collagen-binding assay using type III collagen. We included it in the diagnostic work-up of 44 patients with VWD and 40 normal subjects. Other assays included VWF antigen, ristocetin cofactor activity, ristocetin-induced platelet agglutination and VWF multimeric analysis.

RESULTS

The cost of this collagen-binding assay is 10-fold lower than that of commercial kits. The intra- and inter-assay coefficients of variation were <8% and <9% for normal values, respectively, and the normal reference range varies between 51% and 143%. This assay is sensitive to large VWF multimer representation, with a mean collagen-binding activity/antigen level (CB/Ag) ratio of 0.18 and 0.45 for type 2A and type 2B VWD, respectively, indicating functional discordance. It correlates with the antigen levels in type 2M and type 1 VWD, with mean CB/Ag ratios of 1.1 and 1, respectively.

CONCLUSIONS

Our cost-effective in-house collagen-binding assay produced reliable results. We recommend the use of this test together with the ristocetin cofactor test in the diagnostic work-up of VWD.

A rapid assay for ristocetin cofactor activity using an automated coagulometer (ACL 9000)

We have set up a rapid assay for measuring the activity of the von Willebrand factor ristocetin cofactor (VWF : RCo) using an automated coagulometer (ACL 9000; Instrumentation Laboratory, Lexington, Massachusetts, USA) and commercially available lyophilized platelet reagents (Dade-Behring, Marburg, Germany). Since VWF : RCo tested with the coagulometer (ACL-VWF : RCo) did not require loading, calculation of von Willebrand factor (VWF) activity or pretreatment of samples (calibration standard and tested plasmas), we thought it might be useful for rapid, automatic screening of VWF activity. To assess the precision and the potency of this ACL-VWF : RCo, we tested the assay in this laboratory's internal normal and abnormal controls, in a group of 67 healthy individuals and in 28 patients with different types of von Willebrand disease (VWD). We compared the ACL-VWF : RCo findings with those of the standard agglutination test (Agg-VWF : RCo), normalizing both assays against VWF antigen (VWF : Ag) measured by 'automatic' and standard enzyme-linked immunosorbent assay 'in-house' methods, to calculate the VWF : RCo/VWF Ag ratios. The within-assay and between-assay repeatability of the automatic ACL-VWF : RCo gave coefficients of variation < 5%, and reproducibility in normal and abnormal laboratory controls gave coefficients of variation of 7.9 and 9.3%, respectively. In VWD patients the results were equivalent to those of the standard Agg-VWF : RCo assay but, because of the good sensitivity, the ACL-VWF : RCo was more accurate in evaluating the VWF : RCo and establishing the VWF : RCo/VWF : Ag ratios in VWD patients with low VWF levels. Moreover, this ACL-VWF : RCo is faster than Agg-VWF : RCo, providing results of calibration curves and of 10 patient samples within 15 min. We can conclude that this automatic ACL-VWF : RCo gives a reliable and useful measure of VWF activity and can be proposed as a screening test for rapid diagnosis of VWD even in patients with low VWF levels in plasma.

Template bleeding time and PFA-100 have low sensitivity to screen patients with hereditary mucocutaneous hemorrhages: comparative study in 148 patients

OBJECTIVES AND PATIENTS

We compared the template bleeding time (BT) and closure time (CT) in the PFA-100 as screening tests in 148 consecutive patients with unequivocal mucocutaneous bleeding and positive family history.

EXCLUSION CRITERIA

drug intake, concomitant diseases including minor infections, low platelet count, diseases of secondary hemostasis.

RESULTS

Type 1 von Willebrand disease (VWD-1) was diagnosed in 26 patients, primary platelet secretion defect (PSD) in 33, VWD-1 + PSD in nine, whereas 80 patients did not comply with the criteria for known hemostatic disorders (UD, unknown diagnosis). BT and CT were prolonged in 35.8% and 29.7% of all the patients, respectively (P = 0.23). Sensitivity increased to 48% if an abnormality of BT and/or CT was considered. Same comparisons for BT and CT in each diagnostic category were, respectively: 42 vs. 61.5% in VWD-1 (P = 0.18), 42 vs. 24% in platelet secretion defects (P = 0.11), 67 vs. 89% in VWD-1 + PSD (P = 0.50), and 27.5 vs. 15% in UD (P = 0.06).

CONCLUSION

Both tests were relatively insensitive and not significantly different in detecting incoming patients with mucocutaneous hemorrhages. In patients with VWD-1, the PFA-100 performed slightly better, whereas the opposite occurred in those patients with platelet secretion defects. In the UD group, both tests lost sensitivity, but the BT detected 1.8 times more patients than the PFA-100. Given the large proportion of undiagnosed bleeders and the overall low sensitivity of these tests, clinical decisions still rely on the medical history and etiological diagnosis of the bleeding disorder.

Changes in von Willebrand factor-cleaving protease (ADAMTS13) activity after infusion of desmopressin

von Willebrand factor-cleaving protease (ADAMTS13) cleaves von Willebrand factor (VWF) and regulates its physiologic function. To investigate the relation between ADAMTS13 activity and VWF, we compared ADAMTS13 activity with the VWF-related parameters VWF antigen (VWF:Ag), VWF collagen-binding activity (VWF:CBA), VWF-propeptide, proVWF, and VWF multimeric composition in 10 healthy volunteers and 3 patients with type 1 von Willebrand disease before and after infusing 0.3 microg/kg desmopressin. The VWF-related parameters in the volunteers increased 60 minutes after start of infusion by 3.7-fold for VWF:Ag, 7.2-fold for propeptide, and 2.2-fold for VWF:CBA. Unusually large VWF multimers and traces of proVWF appeared. The ADAMTS13 activity decreased to about half the initial value. After 24 hours values returned to baseline. Patients with type 1 von Willebrand disease showed similar results. We conclude that the inverse correlation between ADAMTS13 and VWF-related parameters suggests a consumption of ADAMTS13 after the desmopressin-induced release of higher multimers of VWF.

Development and applications of surface plasmon resonance-based von Willebrand factor-collagen binding assay

Von Willebrand factor (vWf) functions both as a carrier of factor VIII (fVIII) in plasma and as an adhesive protein providing the primary link between collagen of the extracellular matrix and platelets sequestered from blood flow. The functional activity of vWf correlates with the level of its binding to collagen, which is commonly measured in the enzyme-linked immunosorbent assay (ELISA). We developed an automated collagen-binding assay employing the surface plasmon resonance (SPR) phenomenon, which allows one to quantitatively measure the binding of purified vWf and vWf-containing therapeutic fVIII concentrates to collagen type III immobilized on a biosensor chip. The results of the SPR-based assay highly correlated (r = 0.987) with collagen-binding ELISA. The advantages of the SPR-based assay are its higher accuracy and reproducibility in comparison with ELISA. We applied the developed assay for monitoring structural changes in the vWf component of plasma-derived fVIII/vWf concentrates during a virus inactivation procedure performed by heat treatment. We determined the critical residual moisture content of 2% that can be present in lyophilized concentrates during heat-treatment procedures without causing deteriorative changes in vWf properties. Our data suggest that the SPR-based assay is a useful tool in the development of industrial virus-inactivation procedures, allowing one to preserve vWf activity and achieve the maximal therapeutic efficacy of fVIII/vWf concentrates.

Deposition of whole blood platelets on extracellular matrix under flow conditions in preterm infants

BACKGROUND

A previous study showed greater adhesion by platelets of healthy full term infants to subendothelial extracellular matrix (ECM) under flow conditions compared with healthy adult platelets.

AIM

To investigate the adhesion and aggregation of platelets from preterm infants on ECM under defined shear conditions.

METHODS

In vitro platelet function was investigated in 106 preterm infants, 74 full term infants, and 26 healthy adults. Blood samples were obtained from all infants within 24 hours of birth, and weekly until discharge from preterm infants only. Citrated whole blood was placed in ECM precoated tissue culture plates and subjected to shear stress (1300 s-1) for two minutes using a rotating Teflon cone. Platelet adhesion (surface coverage) and aggregation (average size) to ECM were assayed using an image analyser. Assays for von Willebrand factor (vWF) antigen, ristocetin cofactor, and vWF collagen-binding activity were performed on samples from an additional 70 preterm infants, 23 healthy full term infants, and 24 healthy adults. Preterm infants with hyaline membrane disease (HMD) were analysed separately in both cohorts.

RESULTS

Platelets from preterm infants displayed significantly less platelet adhesion than those from full term infants but similar aggregation and levels of vWF antigen, ristocetin cofactor, and collagen binding activity. Mean surface coverage was 22.0 (8.4)% for preterm infants with HMD, 28.7 (8.0)% for healthy preterm infants, and 35.7 (7.9)% for full term infants. Surface coverage in the preterm infants correlated with gestational age during the first 24 hours only, and did not reach full term levels during 10 weeks of follow up.

CONCLUSION

Platelet adhesion to ECM is significantly poorer in preterm than in full term infants, and poorer in preterm infants with HMD than in healthy preterm infants. Intrinsic platelet properties rather than the concentration or activity of vWF may be responsible for this difference.

Control of von Willebrand factor multimer size by thrombospondin-1

Plasma von Willebrand factor (vWF) is a multimeric protein that mediates adhesion of platelets to sites of vascular injury. Only the very large vWF multimers are effective in promoting platelet adhesion in flowing blood. A protein disulfide bond reductase in plasma reduces the average multimer size of vWF secreted by endothelial cells. This activity has been isolated from human endothelial cell conditioned medium and shown to be the trimeric glycoprotein, thrombospondin-1 (TSP-1). Incubation of purified TSP-1 with vWF resulted in formation of thiol-dependent complexes of TSP-1 and vWF, generation of new thiols in vWF, and reduction in the average multimer size of vWF. The ratio of the concentrations of TSP-1 and vWF in plasma reflected with average multimer size of vWF. The higher the plasma TSP-1/vWF molar ratio, the smaller the average vWF multimer size. In addition, administration of TSP-1 to mice resulted in reduction in the average multimer size of plasma vWF. Interaction of TSP-1 with vWF is mediated by TSP-1 type 1 properdin domains and the vWF A3 domain. These results indicate that TSP-1 regulates the multimeric size and therefore hemostatic activity of vWF.

How I treat patients with von Willebrand disease

Von Willebrand disease (vWD) is a frequent inherited disorder of hemostasis that affects both sexes. Two abnormalities are characteristic of the disease, which is caused by a deficiency or a defect in the multimeric glycoprotein called von Willebrand factor: low platelet adhesion to injured blood vessels and defective intrinsic coagulation owing to low plasma levels of factor VIII. There are 2 main options available for the treatment of spontaneous bleeding episodes and for bleeding prophylaxis: desmopressin and transfusional therapy with plasma products. Desmopressin is the treatment of choice for most patients with type 1 vWD, who account for approximately 70% to 80% of cases. This nontransfusional hemostatic agent raises endogenous factor VIII and von Willebrand factor 3 to 5 times and thereby corrects both the intrinsic coagulation and the primary hemostasis defects. In patients with the more severe type 3 and in most patients with type 2 disease, desmopressin is ineffective or is contraindicated and it is usually necessary to resort to plasma concentrates containing both factor VIII and von Willebrand factor. Concentrates treated with virucidal methods should be preferred to cryoprecipitate because they are equally effective and are perceived as safer. (Blood. 2001;97:1915-1919)

Detection of von Willebrand disorder and identification of qualitative von Willebrand factor defects. Direct comparison of commercial ELISA-based von Willebrand factor activity options

Two von Willebrand factor (vWF):collagen binding (activity) assay (CBA) kit methods are commercially available. A monoclonal antibody (MAB)-based enzyme-linked immunosorbent assay (ELISA) system reported to correlate with a standard vWF:ristocetin cofactor (RCof) assay is also commercially available. It is marketed as a vWF:Activity assay and is available in 2 assay version formats. In the present study, these 4 vWF-activity options were compared directly with in-house vWF:CBA ELISAs for their ability to detect von Willebrand disease (vWD) and identify qualitative vWF defects. The 2 MAB-based systems detected vWD but could not specifically identify qualitative vWF defects, although the recently modified Mark II kit was more effective for the latter compared with the original Mark I kit. All vWF:CBA methods, including in-house and commercial, also effectively detected vWD but differed in their ability to identify qualitative vWF defects. Effectiveness was highest using the in-house reference vWF:CBA (using a type I/III collagen mix product from equine tendon), the Gradipore vWF:CBA (also uses equine tendon-derived collagen), or the in-house vWF:CBA methods using type III human collagen at a relatively low concentration (1 or 3 micrograms/mL, without covalent linkage). The IMMUNO vWF:CBA seemed to be the least effective among the vWF:CBA methods for detection of qualitative vWF defects.

Survey of the 1998 optical biosensor literature

The utilization of optical biosensors to study molecular interactions continues to expand. In 1998, 384 articles relating to the use of commercial biosensors were published in 130 different journals. While significant strides in new applications and methodology were made, a majority of the biosensor literature is of rather poor quality. Basic information about experimental conditions is often not presented and many publications fail to display the experimental data, bringing into question the credibility of the results. This review provides suggestions on how to collect, analyze and report biosensor data.