The Subcommittee met Sunday, August 15, in Room 40 in the Washington Convention Center, Washington, USA, from 08:00 to 12:00. Presiding chair was Dr. Francesco Rodeghiero. Attendance was approximately 350.

FINAL REPORT OF THE International Registry on Acquired von Willebrand Syndrome: Guidelines FOR DIAGNOSIS AND TREATMENT (A.B. Federici, Chair)

Dr. A. Federici (Italy) presented the final report of the International Registry on Acquired von Willebrand Syndrome (AvWS). Information about 221 patients from 52 Centers worldwide was collected and analyzed. AvWS was associated with lymphoproliferative (LPD, 47%) or myeloproliferative (MPD, 19%) disorders, cardiovascular diseases (CVD, 13%), neoplasia (NEO, 7%) and others diseases (OTH, 14%). The results (as % or mean values) are as follows:

AvWS (total 221):             LPD(98)     MPD(40)     CVD(27)     NEO(14 )     OTH(32)

sex (% of male)                     59             38                56             50             46

age at onset (yrs)                   63            46                 57             61             62

bleeders (%)                          70            38                 11             50             50

in follow-up (%)                    58            18                 59             50             67

VWF:Ag (U/dL, mean)            25            68               120             32           31
 
VWF:RCof(U/dL,mean)            8            22                 68             17             7

FVIII:C (U/dL,mean)              21            33               131             23             25

pos anti-FVIII/VWF (%)         14             2                 n.t.             14            12
 

Effective therapy with:

ddavp (%)                            31             15                7              21             19

fVIII/VWF conc (%)             38                5                7             43              22

immunoglobulin (%)             16                0                0             14                3
 

The data from this International Registry suggest that AvWS is highly associated with certain disorders such as LPD, MPD, CVD. A practical flowchart for the diagnosis and the management of AvWS in these underlying disorders was discussed and approved during the meeting. An official document of the SSC-VWF will be prepared by Federici, Rand, Mannucci, Budde, Mohri, van Genderen, Bucciarelli and Rodeghiero. This report will be submitted to Thrombosis and Haemostasis.

REPORT FROM THE JOINT ISTH/WHO MEETING IN LONDON

(J.E. Sadler, Chair)

Under the sponsorship of WHO and ISTH, a joint committee meeting was held in at the Royal Free Hospital in London, October 12-14, 1998, to consider the impact, prevention and control of von Willebrand disease (VWD). The committee members were Erik Berntorp, Nikolai Bochkov, David Ginsburg, Dominique Meyer, Ian Peake, Francesco Rodeghiero, J. Evan Sadler, and Alok Srivastava. The meeting was chaired by Pier M. Mannucci, and the WHO Secretariat was Victor Boulyjenkov. Observers were Christine Lee, Vince Jenkins, Dale Owens, John Pasi, and Flora Peyvandi. The goal of the meeting was to develop recommendations for actions that ISTH and WHO could take to improve the diagnosis and treatment of VWD, particularly in developing countries.

Dr. J.E. Sadler (U.S.A.) presented a summary of the WHO/ISTH meeting proceedings, conclusions and recommendations. Dr. Srivastavaís survey data on VWD in the developing world was a focal point of the proceedings, and he presented these results separately to the Subcommittee. Several of the joint committeeís recommendations are congruent with the mandate of the Subcommittee, focusing on standards for classification, diagnosis, and treatment. One purpose of Dr. Sadlerís presentation was to consider the role the Subcommittee could reasonably perform to assist in the implementation of these recommendations, which are summarized below.

Assessment and Monitoring: National Patient Registries

Data on disease prevalence are important for making decisions about the allocation of health care resources. The number of persons at risk for VWD was estimated to be 1,480 to 3,580 persons per million. The number of persons with symptomatic VWD was estimated to be at least 100 per million. Among all symptomatic patients, intensive and frequent medical therapy is required mainly by patients with VWD type 3, who constitute 0.55 to 3.2 per million. These values were derived from data of both developed and developing countries, suggesting that the prevalence of VWD may be similar worldwide. However, additional data would be useful to substantiate these very rough prevalence estimates.

Based on a world population of approximately 5.8 billion, there are 8.6 to 21 million persons at risk for VWD and at least 580,000 persons with symptomatic VWD worldwide. Approximately 80% of these persons live in developing countries. Accurate knowledge of VWD prevalence, symptoms, treatment, and outcomes is required to evaluate the need for additional resources. Longitudinal data will be critical to evaluate the efficacy of interventions. During the meeting, ISTH and WHO agreed on the development of strategies for establishing national VWD patient registries and data collection methods applicable to developing countries. Guidelines for this activity may be derived from previous successful efforts for other diseases in developing countries, and for VWD in certain developed countries such as France and Italy.

The Subcommittee on VWF has been engaged in studies of VWD prevalence for some time and these efforts might be extended to encompass regions of the developing world.

Laboratory Methods and Quality Assurance

Patients with VWD and their physicians need access to the laboratory tests required for proper diagnosis, either locally or by prompt referral to qualified central diagnostic laboratories. The quality and accessibility of laboratory resources in the developing world varies considerably. Optimal strategies for laboratory testing in VWD have not been defined, and may vary depending on the medical resources that are available and their distribution. ISTH and WHO agreed on that ISTH should promote an analysis of laboratory testing for the diagnosis of VWD and for the discrimination between type 1 and type 2 VWD. The existing Subcommittee on VWF of the Scientific and Standardization Committee of the ISTH may be a suitable working group to pursue this objective. ISTH and WHO agreed that WHO should evaluate its External Quality Assurance schemes for participating diagnostic laboratories, with particular reference to coagulation laboratory services, to determine whether modifications are necessary to address quality control measures for laboratory testing in VWD. ISTH and WHO together recommended that WHO survey and inventory the laboratory services relevant to blood coagulation testing among developing countries, to facilitate the identification and correction of deficiencies that may currently limit the diagnosis and therapy of VWD and other bleeding disorders.

The Subcommittee on VWF has a natural interest in these specific issues and will consider methods to obtain more data on VWD prevalence, criteria for diagnosis and strategies for treatment in the developing countries.

VWD in the Developing Countries

Dr. A. Srivastava (India) reported on the magnitude of the problem of VWD in developing countries, which is undefined due to the lack of awareness of this disorder, inadequate facilities for diagnosis and limited resources for treatment. In preparation for the joint WHO/ISTH meeting on this disorder in 1998, a questionnaire survey was conducted by Dr. Srivastava to record the available information on the prevalence, diagnosis and treatment of VWD in as many developing countries as possible. Responses were received from 22/56 countries (39%) that were approached. The results are shown in the table. The salient features are: 1.) Majority of patients are unidentified; 2.) There is a higher proportion of patients with severe disease, particularly in those countries with significant levels of consanguinity; 3.) At least one center capable of performing VWF:Ag and/or RiCof exists in most countries but facilities for multimer analysis are uncommon; 4.) Entire range of products from FFP to purified factor concentrates are used for replacement; 5.) Countries where treatment is supported by government/insurance, "on-demand" replacement is provided, while in others only major bleeds are treated with factor replacement; 6.) Surgeries, both minor and major, are performed in most countries.

While this survey provided a glimpse of VWD in developing countries, more detailed and precise data would convey better the impact of VWD in these countries. At the joint WHO/ISTH meeting in London, it was suggested that a system be established to collect data on VWD from more developing countries. In collaboration with Dr. F. Rodeghiero, the previous questionnaire was modified to include data on mortality and on severe hemophilia from the same referral area to assist in estimation of the true prevalence of VWD in that population. This survey will be conducted over the next 12-18 months. ISTH could help the cause of VWD in developing countries with a training program and by promoting joint collaborative projects of mutual interest between centers in developed and developing countries. (See Table, attached.)

MOLECULAR DIAGNOSIS OF VWD (I. Peake, Chair)

Dr. A. Goodeve (UK) reported on the consensus on genetic and molecular terminology for VWF and VWD. Nomenclature schemes proposed for VWF nucleotides and amino acids in 1994 are still not in use by all investigators. Now that examination of the entire VWF gene for mutations, particularly in type 1 VWD, is becoming more common this is of greater importance. A reminder of the nomenclature convention was presented (Beaudet AL, Tsui LC, Hum Mutat 1993;2:245-8). This has nucleotides in the cDNA starting from the mRNA cap site as +1, intronic sequence numbered after Mancuso et al (1989) and amino acid numbering having the initiator methionine as +1. Using (c) to denote propeptide numbers should no longer be practiced. Dr. Sadler reminded that the presence of a mutation in the VWF gene is required as part of the definition of VWD according to the 1994 ISTH criteria. However, this definition excludes possible locus heterogeneity and could be therefore undesirable. Proposals for a renewed consensus on genetic and molecular terminology of VWF/VWD would require the involvement of the major centers working on the field.

Dr. J.E. Sadler (U.S.A.) discussed on behalf of Dr. D. Ginsburg the current status of database of VWF mutations and polymorphisms. A database of mutations in VWD and polymorphisms in the VWF gene was published in two articles during 1993. This depended on the contributions of many members of the Subcommittee on VWF as part of the "Consortium on VWF Mutations and Polymorphisms". Since then, the database has been maintained on a server at the University of Michigan and is accessible at http://mmg2.im.med.umich.edu/VWF. New data can be submitted online using either Netscape or Internet Explorer browser software. Submissions are reviewed and confirmed before posting. The current database contains approximately 200 mutation entries and approximately 50 polymorphism entries. Updating of the database relies on voluntary submissions from scientists, and there is no mechanism for ongoing review of Medline or published abstracts for new mutations or polymorphisms. Consequently, some published information is not included in the database. There were 13 volunteer submissions in 1997, 33 in 1998, and 13 in 1999 as of July. The database continues to be used heavily and usage is increasing. The database was accessed by users an average of 84 times per week in 1997, 116 times per week in 1998, and 161 times per week in 1999. The current rate of access is approximately 8,400 hits per year.

Other Internet resources also contain information on VWD mutations. The Human Gene Mutation Database Cardiff at the location http://www.uwcm.ac.uk/uwcm/mg/hgmd0.html lists approximately 115 VWD mutations.

The Genome Database (GDB) contains approximately 28 VWF polymorphism entries. Online Mendelian Inheritance in Man (OMIM) includes VWD as MIM #193400 and lists 28 mutations. These resources overlap to some extent but each contains some unique information.

The international genetics community is attempting to encourage collaboration among databases to develop standard nomenclature, procedures for quality control, and a common database format and software. These efforts so far have not proceeded to the point that they can influence the format or management of the current VWD databases. However, standards for nomenclature have been proposed and these probably should be adopted for general use. For example, the abbreviation for the von Willebrand factor protein would be "VWF" (all capitalized). The abbreviation for the human gene would be "VWF" (capitalized italics). The abbreviation for the mouse gene would be "Vwf" (initial capital, italicized). The abbreviation for von Willebrand disease would be "VWD" (all capitalized). Detailed recommendations for the description of mutations have been published in Antonarakis. Hum Mut 11:1-3,1998. Salient features include numbering of nucleotides beginning with the first nucleotide of the initiation codon, and numbering amino acid residues beginning with the initiation codon. These changes will be incorporated into the VWD databases as time permits.

Dr I. Peake (UK) reported on the molecular diagnosis of type 1 VWD. Since about 70% of VWD is phenotypically diagnosed as type 1, understanding the molecular basis of this condition is important diagnostically. In the dominant form, only a few mutations have been described. In order to further understand this condition, full VWF gene analysis in index cases from type 1 VWD families combined with precise phenotypic assays, family linkage studies, ABO blood grouping, VWF gene/promoter haplotype analysis and expression of detected mutations in mammalian expression systems, are considered necessary. Results from this type of study would form the basis of proposed molecular diagnostic recommendations to be put before this Subcommittee.

WORKING PARTY ON MEASUREMENT OF VWF CONTENT IN THERAPEUTIC PRODUCTS (C. Mazurier, Chair)

Dr. C. Mazurier (France) introduced the session by emphasizing that the evaluation of replacement therapy for VWD is complicated by the heterogeneity of the different concentrates and also by the adoption of different treatment strategies. There is a clear need for standardisation in terms of VWF potency estimation, and hence labeling of these concentrates, in order to promote improved harmonisation of treatment.

Dr. T. Barrowcliffe (UK) described the final results of the collaborative study designed to establish the 4th International Standard (IS) for FVIII and VWF plasma. For VWF:Ag and VWF:RiCof, mean potency estimates against the previous IS exceeded those against fresh plasma pools by 14 and 20 % respectively, indicating a possible drift of the International Unit away from normal plasma. It was proposed to make a partial correction for this drift by assigning potencies as the means of the two estimates. This proposal was supported by members of the Subcommittee to whom the report had been circulated, and was accepted by the Expert Committee on Biological Standardisation of WHO, which accordingly established plasma 97/586 as the 4th IS for FVIII/VWF plasma.

Dr. R. Seitz (Germany) asserted that a reliable assay of VWF potency in concentrates is a prerequisite for their clinical use for treatment of VWD. The European Pharmacopeia Expert Group 6B currently evaluated two candidate VWF collagen binding methods: (1) commercial method (Baxter), using pepsin-treated soluble type III collagen of human placenta, and (2) in-house method (Paul Ehrilch Institut, modified after Thomas et al, 1993), using fibrillary type I collagen of equine tendon. Method (1) is robust and reproducible and is sensitive also to low and medium MW multimers, while method (2) gives lower values in concentrates containing predominantly low MW multimers. The question is which method would more adequately reflect the clinically relevant VWF activity. The Expert Group 6B will go on with their evaluation of the methods and would appreciate the advice of the SSC on this matter.

Dr. A.R. Hubbard (UK) and Dr. A. Chang (USA) reported on the collaborative study between NIBSC (T. Barrowcliffe), FDA (M. Weinstein) and SSC VWF subcommittee (C. Mazurier) to produce an International Standard consisting of a stable freeze-dried concentrate. Trial fills of five concentrates, currently used to treat VWD, have been prepared for evaluation. Preliminary characterisation, in three laboratories, focus on stability, multimer composition and the validity of VWF potency estimation relative to the WHO International plasma standard and other concentrates. This will be followed by a multi-centre collaborative study that will include a variety of current assay methodologies. Based on these results, one or two suitable materials will then be selected as candidate preparations for inclusion in an international collaborative study which will lead to the establishment of the 1st IS for VWF concentrates.

WORKING PARTY ON LABORATORY DIAGNOSIS OF VWD

(R. Montgomery, Chair)

Dr. K. Friedman (U.S.A.) reported the results of an international multicenter study on interlaboratory variability in VWD testing. Ten fresh-frozen samples including normal plasma and plasmas from type 1, 2 and 3 VWD were sent to Centers in Europe and US for measurement of FVIII:C, VWF:Ag and VWF:RiCof.

On the basis of the encouraging results of this study, Dr. R. Montgomery (U.S.A.) made specific recommendations on behalf of the Working Party on Diagnosis of VWD. A number of different international laboratories should study a group of molecularly defined VWD variants with ratios of VWF:RiCof/VWF:Ag, VWF:CBA/VWF:Ag, and FVIII/VWF:Ag to determine agreement of assays. These assays should be compared to internal standards within that laboratory and between the study laboratories. A cohort of type 1, heterozygous type 3, and type 1 patients with different ABO blood groups should be compared by a small number of multinational laboratories. Multiple methods for ristocetin cofactor and collagen binding should be compared using multiple participating laboratories. All of above should be directly compared to the 4th International Standard. Commercially available standards should be compared to the 4th International Standard in multiple laboratories. Using the 4th International Standard, several laboratories should determine VWF antigen levels in 50 unselected individuals to determine the mean and standard deviation of normal individuals with this standard.

Shipping costs should be borne equally by participating laboratories.

Dr. A.B. Federici (Italy) reported VWF:RCof in comparison to two commercially available assays: functional ELISA and collagen binding assay (VWF:CBA). Dr. Federici showed the results of the comparison of two commercially available proposed functional assays with the home-made Ristocetin cofactor (VWF:RCof) activity in 81 VWD cases, diagnosed accordingly to the recommendations of the ISTH-SSC on VWF. The ELISA kit commercially available (VWF "ACTIVITY") proposed by Goodhall et al [TEST A] was compared to the home-made VWF:RiCof [TEST B] and the kit commercially available (IMMUNOZYM VWF:CBA), a two-step-ELISA which is made with collagen type III of human placenta [TEST C]. All the three assays were compared by expressing their absolute values (U/dl) by the ratio with the VWF:Ag measured by ELISA. Mean values (SD) of the ratios between each functional assay and the VWF:Ag are shown. Paired data [A vs. B and C vs. B] were analyzed by the paired Studentís t test and significant differences reported as follows:

Individuals(n):[A] VWF:Act/Ag     [AvsB]     B] VWF:RCof/Ag     C] VWF: CBA/Ag     [CvsB]

Normals (25)     0.90 (016)             ns                 0.97 (0.20)         1.01 (0.16)             ns

1 PN (22)         1,03 (0.24)             ns                 0.99 (0.17)         0.99 (0.16)             ns

1 PL (13)           0.88 (065)             ns                 0.67 (0.25)         0.78 (0.29)             ns

1 PD (7)             0.79 (0.40)         0.015              0.34 (0.17)         0.97 (0.17)         <0.001

2 A (10)             0.89 (0.34)         <0.001             0.31 (0.18)         0.47 (0.29)           0.07

2 B (16)             0.96 (0.31)         <0.001             0.56 (0.20)         0.75 (0.22)         0.004

2 M (13)             1.11 (0.63)            ns                 0.94 (0.50)         0.86 (0.44)            ns

(Vicenza)

The home-made VWF:RCof [TEST B] is more sensitive than VWF:ACTIVITY kit [TEST A] and VWF:CBA [TEST C] to identify defects due to abnormal VWF multimers (1PD, 2A and 2B).

Dr. A.B. Federici (Italy) reported VWF:(RCo/Ag) ratio in the different types of VWD. According to the study of different VWF functional assays previously shown, Dr. Federici pointed out that a presumptive diagnosis of type 2 VWD can be performed just by the VWF:RiCof/Ag ratio considering multimeric analysis a second-step test. In fact values of ratio > 0.7 are always present in normal subjects and in type 1 VWD with normal VWF while values < 0.65 correlate very well with the loss of higher molecular weight multimers as those shown in type 1 PD, 2 A, 2 B. This approach can be very useful in case of laboratories where the multimeric analysis is not available.

Dr. E. Fressinaud (France), also on behalf of Dr. C. Mazurier and Dr. D. Meyer presented data on the ratio of von Willebrand factor ristocetin cofactor activity to antigen in types 2A, 2B, 2M von Willebrand disease and ratio of factor VIII to von Willebrand factor antigen in type 2N VWD. VWF:RiCof/VWF:Ag ratio was studied in 93 patients, included in the French INSERM Network on molecular abnormalities in VWD. In all patients, the gene defect was characterized. Ninety-six healthy volunteers served as control group and exhibited a mean ratio of 0.99 ± 0.24 (2SD). Values were 0.32 ± 0.18 (mean ± SD) in 38 patients with type 2A and were very similar (0.36 ± 0.13) in 20 patients with type 2M (that is with a gene defect in the A1 loop). In contrast, the ratio was much higher (0.6 to 1) in 4 patients with the Vicenza variant that is with a candidate mutation in exon 27 of the gene. In 31 patients with type 2B, the mean ratio was 0.54 with extreme values from 0.25 to 0.9. Thus, the VWF:RCo/VWF:Ag ratio may be normal in patients with type 2B or the Vicenza variant.

Factor VIII:C/VWF:Ag ratio was studied in 47 patients with type 2N VWD. In the control group the ratio was 0.56 ± 0.24 (mean ± 2 SD). In 20 patients with compound heterozygous mutations and in 20 homozygous patients with the Arg 91 Gln mutation, values were 0.30 ± 0.13 (mean ± SD) and 0.31 ± 0.08 respectively. In 7 homozygous patients with another mutation, the mean ratio was particularly low: 0.10 ± 0.09.

Dr. C. Miller (U.S.A.) presented data showing disproportionately high factor VIII levels using the 3rd international plasma standard under clinical conditions. A population of 115 normal women studied recently at the CDC differed dramatically in factor VIII activity from a previously studied population (mean 150 vs. mean 106). FVIII levels were also significantly higher than VWF levels in the recent study and in 40 patients studied over 2 years. The 3rd IS was used in the recent studies. The 2nd IS was used for the earlier one, in which FVIII and VWF levels were equivalent. Adjustment of FVIII levels to the 2nd IS (60 IU/ml) rather than the 3rd (0.80 IU/ml) eliminated the discrepancies. Dr. Miller hypothesized that the potency of the third IS may be overestimated or that the preparation may behave differently with some instrument/reagent systems in current clinical use. Overestimation of FVIII levels in a clinical setting may lead to undertreatment of hemophilia patients and can complicate the diagnosis of von Willebrand disease. This issue continues to be of importance because commercial suppliers of reference materials may continue to use the 3rd IS for a long period of time before adopting the 4th IS. An ancillary concern is that many companies no longer specify against which standard their materials are calibrated, while clinical labs need that information.

Dr. C. Miller (U.S.A.) reported on difficulties with ristocetin cofactor measurement using commercial kits. A number of laboratories in the U.S. have had difficulties with the ristocetin cofactor assay over the past 2-3 years when using commercial reagents. The problems consist of inability to produce an acceptable standard curve, erratic results for the same sample during a single assay, and poor reproducibility of results from day to day. Quality control was unacceptable for patient care, causing many labs to abandon the assay. Reagents from three companies gave similar results, in two different brands of aggregometer. Dr. Miller has tracked the problem and found the common factor to be ristocetin, all of which comes from a single source. The U.S. companies have been reconstituting and then relyophilizing the material to be packaged in their kits. The ristocetin as originally supplied performed well in our hands, with over 250 tests on study subjects all with excellent results. The problem apparently lies with the quality of the processed ristocetin. Recommendation was made that lots of ristocetin be characterized for activity and stability prior to packaging to assure that the consumer gets a reliable product.

Dr. E. Favaloro (Australia) presented data on the role of CBA in the laboratory diagnosis of VWD produced by a multicenter evaluation. A multi-laboratory evaluation for VWD testing was undertaken in Australasia. 25 labs were sent 10 plasmas (normal plasma pool (PNP), PNP diluted to 50%, normal individual, and typed VWD samples: severe 1 [x1], 2B [x2], 3 [x1] and 2A [x1]), and were asked to perform all available tests used to diagnose VWD, and to comment on results. All laboratories tested for FVIII:C, VWF:Ag and functional VWF (VWF:RCof and/or VWF:CBA); only three labs performed VWF:Multimers. Assay methodologies varied widely (e.g. VWF:Ag, ELISA, EID, LIA, VIDAS; VWF:RCof: aggregometry vs. 'functional' ELISA). VWF:RCof gave highest inter-laboratory variation, and poorest sensitivity to low levels of VWF; VWF:CBA was better able to detect Type 2 VWD. VWF:RiCof by aggregometry gave greater variation & poorer sensitivity to low VWF levels compared to 'functional' ELISA, but was better for identifying Type 2 VWD. Most diagnostic predictions were correct (i.e. normal vs. VWD samples correctly identified, VWD subtype correct). 'Misinterpretations' usually linked to test panels used (i.e. VWF:Ag & VWF:RiCof combination performed worse than VWF:Ag & VWF:CBA). Conclusion: VWF:CBA outperformed VWF:RCof as a functional VWF assay, and should be included in future multi-laboratory evaluations. During the following discussion, Dr. Z. Ruggeri (U.S.A.) stated that the term "functional activity" for any VWF measurement be avoided.

VWF-CLEAVING PROTEASE ASSAY

Dr. M. Furlan (Switzerland) addressed the issue of the deficiency of VWF-cleaving protease and its measurement. VWF-cleaving protease was found deficient in patients with TTP, whereas patients with hemolytic-uremic syndrome (HUS) had normal activity. The laborious assays of VWF-cleaving protease activity described hitherto include sodium dodecyl sulfate (SDS)-electrophoresis of digested VWF and analysis of degraded VWF multimers by immunoblotting. A new simple functional assay, based on previous observations that binding of VWF to collagen depends on the multimeric size of VWF, has been recently developed in Dr. Furlanís laboratory. VWF present in a human plasma pool that had been depleted of protease activity was digested by the VWF-cleaving protease of added test plasmas. Proteolytic degradation led to low molecular weight forms of VWF showing an impaired binding to microtiter plates coated with human collagen type III. This assay can be performed in a non-specialized laboratory and may be accomplished within 6 hours.

Dr. J.P. Girma (France) reported on a new two-step method to quantify the VWF-cleaving protease activity in plasma. In the first step proteolysis is performed using a constant amount of wild type recombinant VWF as substrate and serial dilutions of tested plasma as protease provider. In the second step, the degradation of VWFAg is estimated by a two-site IRMA using as coated antibody a monoclonal antibody (MoAb) directed against the C-terminal side of the cleavage site, and as labeled antibody a pool of MoAbs specific for the N-terminal side. Because the proteolytic process leads to the progressive separation of the C- and N-terminal portions of the VWF subunit the IRMA also shows a progressive apparent loss of VWF:Ag. The validity of the method was confirmed by showing a normal protease activity in relapsing thrombotic thrombocytopenic purpura as well as in normal plasma following addition of an anti-protease antibody raised in mice against partially purified VWF-cleaving protease.

CLOSING REMARKS

Dr. F. Rodeghiero (Italy) reported the current status of the International Multicenter Study for the Validation of the Diagnostic Criteria of Type 1 and Type 3 VWD. Twenty-one Centers had definitely agreed to participate and a sufficient number of families is expected to be enrolled and available for analysis. Preliminary data will be presented during the next SSC meeting and could provide the basis for formal criteria for VWD diagnosis. Hopefully, before the next SSC meeting most of the ongoing projects of the Subcommittee will be concluded and other issues, like clinical management of VWD and its impact in women, could be addressed.

SUMMARY OF SUBCOMMITTEE ACTIVITIES

Issues voted: