Laboratory diagnosis of von Willebrand disorder (vWD) and monitoring of DDAVP therapy: efficacy of the PFA-100® and vWF:CBA as combined diagnostic strategies

We have coevaluated a combination of test processes for diagnosing von Willebrand disease (vWD) and monitoring deamino-delta-D-arginine vasopressin (DDAVP) therapy. Using normal controls (n = 23), closure time (CT) ranges measured by PFA-100(R) were (mean +/- 2SD): (i) collagen/ADP cartridge (C/ADP): 67-127 s (ii) collagen/epinephrine (C/Epi): 94-162 s. From a panel of 125 patients undergoing evaluation for clinical haemostatic defects, 29/30 samples from patients with vWD [17/18 type 1, 1/1 type 3, 3/3 type 2A, 7/7 type 2B and 1/1 pseudo-vWD] gave prolonged CTs using C/Epi. The C/ADP was less sensitive, being normal in 7/18 of the type 1 vWD individuals, with higher sensitivity to more severe vWD. Individuals with haemophilia (six factor VIII-deficient, one factor XI-deficient) gave normal CTs, while those with clinical thrombocytopenia (n=13) gave normal or prolonged CTs, somewhat dependent on platelet count. The PFA-100 was also evaluated as a part of the laboratory monitoring procedure in patients with either vWD or haemophilia undergoing a DDAVP trial as a therapeutic management process. For vWD, correction of an initially prolonged CT by DDAVP, accompanied by normalization of von Willebrand factor (vWF) measurable by von Willebrand factor antigen, vWF collagen binding activity and vWF ristocetin cofactor assays (vWF:Ag, vWF:CBA and vWF:RCof), was achieved in type 1 vWD (n=5). In an individual with type 2A vWD, DDAVP normalized vWF:Ag and vWF:RCof, but had no apparent effect on the baseline maximally prolonged CT. In an individual with type 2B vWD, factor VIII/vWF concentrate also normalized vWF:Ag and vWF:RCof, but similarly had no apparent effect on the baseline maximally prolonged CT. vWF:CBA did not normalize for either of these individuals, potentially suggesting that normalization of vWF:CBA might be required for normalization of CT. This concept is supported by correlation analysis undertaken between CT and various vWF parameters. Among these, vWF:CBA held the strongest relationship in our data set, which showed an inverse progressive rise in CT for falling vWF:CBA. Based on these results, we would conclude that the PFA-100 is highly sensitive to the presence of vWD, and may thus provide a valuable screening test for vWD. Furthermore, the combined utility of the PFA-100 and vWF:CBA as markers of DDAVP responsiveness may prove to be simple, quick but powerful, predictors for its clinical efficacy.     

The von Willebrand factor collagen-binding activity assay: clinical application

A collagen type III based collagen-binding assay was developed for measuring the functional activity of the von Willebrand factor. The assay had a low coefficient of variance (4.8%) for normal values under optimized conditions. The results of the collagen-binding activity (CBA) assay correlated with ristocetin cofactor activity tested in normal plasma samples (n=29). We found that the CBA of blood group O is lower than that of other blood groups. The test was used for the diagnosis of von Willebrand's disease (VWD) and for estimating the response to treatment with DDAVP (1-deamino-D-arginine-8 vasopressin) and factor VIII concentrate. A mean ratio of VWF antigen (VWF:Ag) to CBA of 1.5 indicated type 1 and of 2.7 indicated type 2 VWD. The increase in the collagen-binding activity of VWF released in type 1 VWD patients (n=7) after treatment with DDAVP was higher than the increase in the VWF antigen; this is characteristic of very high multimers with greater functional activity. Factor VIII concentrate Koate-HP (Bayer) administered to a patient with VWD type 3 had a mean residence time of 12.6 h for VWF:Ag and 11.2 h for CBA. These findings suggest that the collagen-binding assay is a useful test for measuring the functional activity of VWF in plasma samples, factor VIII concentrates, as well as for estimating the outcome of treatment.     

Von Willebrand factor collagen-binding (activity) assay in the diagnosis of von Willebrand disease: a 15-year journey

The correct diagnosis and classification of von Willebrand disease (vWD) is crucial because the presenting biological activity of von Willebrand factor (vWF) determines both the hemorrhagic risk and the subsequent clinical management. A variety of laboratory assays may be employed, not necessarily restricted to assessments of vWF. This article discusses the relative strengths and limitations of various functional or discriminatory vWF assays with a special focus on the vWF:collagen-binding activity (vWF:CBA) assay. This is a functional vWF assay that relies on the property of vWF adhesion to collagen. The vWF:CBA was first described approximately 15 years ago. The journey from that time point has been an interesting one, and the vWF:CBA is now gaining more widespread acceptance. There are now many published studies confirming the superiority of the vWF:CBA over the vWF ristocetin cofactor (vWF:RCof) activity as a functional screening diagnostic test process for vWD. However, both tests may be required in order to appropriately diagnose all forms of vWD. The relationship of these assays with multimer analysis is also discussed. In summary, an optimized vWF:CBA detects primarily high-molecular-weight (HMW) vWF forms and probably only about 30% of the total plasma vWF pool detected by vWF antigen (vWF:Ag). Because these HMW vWF forms are missing in types 2A and 2B vWD, the vWF:CBA is extremely useful in the detection of these qualitative vWD subtypes. In addition, however, concordance of vWF:CBA with vWF:Ag in unison with low vWF levels may alternatively suggest a type 1 vWD, and an absence of both vWF:Ag and vWF:CBA will suggest type 3 vWD. The vWF:CBA is also being investigated in various disease states, as is its usefulness as an effective marker of functional HMW vWF in both desmopressin (DDAVP) and factor-concentrate therapy in vWD.     

Laboratory monitoring of replacement therapy for major surgery in von Willebrand disease

Von Willebrand disease (VWD) is an inherited haemorrhagic disorder caused by a quantitative or qualitative defect of von Willebrand factor (VWF), a multimeric plasma glycoprotein that plays a key role in platelet adhesion to the subendothelium and acts as a carrier of factor VIII (FVIII) in blood. Patients with VWD experience bleeding symptoms that are mainly localized in mucous membranes and soft tissues, and their severity depends on the degree of the primary reduction in VWF and the secondary deficiency of FVIII in plasma. Because VWD patients are also at increased risk of perioperative bleeding, a prophylactic treatment aimed to correct the dual haemostatic defect (i.e. VWF and FVIII) is warranted. This review summarizes knowledge on the current management of patients undergoing major surgery, focusing on the peri‐surgical laboratory monitoring of replacement therapy with VWF/FVIII concentrates. We suggest to monitor plasma levels of FVIII coagulant activity in the postoperative period rather than a surrogate maker of platelet‐binding VWF activity as the ristocetin cofactor assay and its recent modifications.     

The utility of the PFA-100 in the identification of von Willebrand disease: a concise review

The PFA-100 (platelet function analyser; Dade-Behring, Marburg, Germany) is a relatively new tool for the investigation of primary hemostasis. Recent studies have shown its utility as a screening tool for investigating various platelet disorders and possible von Willebrand disorder (vWD), both in the initial investigation and in subsequent therapeutic monitoring of desmopressin therapy. This article reviews current findings with respect to the identification of vWD, and highlights both the benefits and the limitations of its clinical utility. In brief, sensitivity to vWD types 2A, 2B, 2M, and 3 is > 98%, but overall sensitivity to vWD (types 1, 2A, 2B, 2M, and 3 combined) is ~85 to 90%. Ultimately, the high sensitivity of the PFA-100 to vWD and its simplicity of use provide its greatest strengths. However, because it is a global test system, and also sensitive to low hematocrit, low platelet counts, and platelet dysfunction (both congenital and acquired; e.g., secondary to medication such as aspirin) it must be recognized that the PFA-100 is neither specific for, nor predictive of, any particular disorder (inclusive of vWD). Nevertheless, used appropriately, the PFA-100 can be considered a worthwhile addition to the hemostasis laboratory involved in the diagnosis or therapeutic monitoring of vWD, and a normal PFA-100 result can be used with some confidence to exclude severe vWD.     

The Platelet Function Analyzer (PFA-100) may not be suitable for monitoring the therapeutic efficiency of von Willebrand concentrate in type III von Willebrand disease

 We describe a type-III von Willebrand patient who was admitted to the hospital with severe deformity and functional deficit of the left knee joint due to recurrent hemarthrosis. Orthopedic intervention was necessary. To prevent bleeding episodes, von Willebrand factor (vWF) replacement therapy was given during and after surgery. APTT, plasma FVIII activity (FVIIIc), vWF antigen (vWF Ag), and vWF ristocetin cofactor (vWF Rco) were measured. Primary hemostasis was monitored using the PFA-100. This "Platelet Function Analyzer" is designed to measure platelet adhesion and aggregation capacities. Whole blood is aspirated through a capillary and is forced to flow through the central hole of a membrane coated with collagen and epinephrine (COL/EPI) or ADP (COL/ADP) as platelet activators. Irreversible platelet aggregation results in the formation of a stable platelet plug, closing the central hole. The result is expressed as "closure time" (CT), i.e., time necessary to stop the blood flow, and is a measure of platelet hemostasis capacity. Laboratory investigations during substitution therapy revealed no shortening of closure times with both COL/EPI and COL/ADP cartridges despite normalization of plasma vWF Ag, vWF Rco, and FVIIIc levels. These observations suggest that intraplatelet vWF, which is totally absent in type-III von Willebrand disease, plays an important function in the adhesion of platelets to the collagen-coated membrane of the PFA-100 system, simulating an injured vessel wall. Consequently, we conclude that the PFA-100 may not be suitable for monitoring the therapeutic efficacy of von Willebrand concentrate in type-III von Willebrand patients during substitution therapy. Received: February 1, 1999 / Accepted: April 30, 1999     

Optimizing therapy with factor VIII/von Willebrand factor concentrates in von Willebrand disease

In von Willebrand disease, the main goals of treatment are to correct the dual defect of haemostasis caused by a reduced or abnormal von Willebrand factor (vWF), i.e. the prolonged bleeding time (BT) and the deficiency of factor VIII coagulant activity (FVIII:C). The synthetic vasopressin analogue, desmopressin (DDAVP), has reduced the need for transfusions in most of the mild forms of von Willebrand disease but DDAVP is ineffective in type 3 and in other severe cases of types 1 and 2 von Willebrand disease. For many years cryoprecipitate has been the mainstay of replacement therapy but, after the introduction of virucidal methods, concentrates containing FVIII/vWF have been considered much safer than cryoprecipitate and proposed in von Willebrand disease management. FVIII/vWF concentrates have been produced and tested by many authors but there is only one report describing four virus-inactivated FVIII/vWF concentrates evaluated in a cross-over randomized trial. According to these in vitro and pharmacokinetic data, the following information can be derived: (a) no FVIII/vWF concentrate had an intact multimeric structure similar to that of normal plasma or of cryoprecipitate; (b) all FVIII/vWF concentrates were equally effective in attaining normal and sustained levels of FVIII:C postinfusion, although peak levels were more delayed in the concentrate devoid of FVIII:C; (c) no FVIII/vWF concentrate consistently normalized the BT in a sustained fashion. On the other hand, clinical haemostasis can be achieved in the management of bleeding episodes and of surgery for most of von Willebrand disease cases regardless of whether the BT is corrected; in the few rare cases with mucosal bleeding not controlled by FVIII/vWF concentrates, infusion of DDAVP or platelet concentrates can be administered in addition.     

Ultrastructural analysis of the distribution of von Willebrand factor and fibrinogen in platelet aggregates formed in the PFA-100

The PFA-100 is a new apparatus used to detect platelet dysfunction in vitro . Anticoagulated blood flows under constant pressure through a capillary, and across an aperture that pierces a membrane coated with collagen and either epinephrine or ADP. Through their ability to adhere and aggregate, platelets occlude the orifice and the closure time is a test of platelet function. Using electron microscopy and immunogold staining, we have analyzed the ultrastructure of platelet aggregates formed within the aperture and that are responsible for the occlusion. Standard electron microscopy showed that the aggregates formed on both collagen-epinephrine and collagen-ADP cartridges presented the same morphological features. The aggregates were exclusively composed of platelets, some of which were degranulated. Degranulation was particularly intense at the periphery of the aggregate where platelets were often totally devoid of secretory organelles. Immunogold staining on ultrathin frozen sections with polyclonal antibodies, allowed us to evaluate the distribution of adhesive proteins such as fibrinogen and von Willebrand factor (vWF) within the aggregate. The latter was found to be abundant in the intercellular spaces between adjoining platelets. Although fibrinogen was also present, its labeling was less intense suggesting that vWF is the major protein implicated in the platelet-platelet interactions in the aggregates formed in the PFA-100 system. This may be because of the high shear rate that occurs across the aperture which suggests that the PFA-100 is particularly sensitive for detecting abnormalities of vWF-platelet interactions.     

Changes in von Willebrand factor level and von Willebrand activity with age in type 1 von Willebrand disease

In a normal population, VWF plasma levels (VWF:Ag) and VWF activity (VWF:RCo) increase by approximately 0.17 and 0.15 IU mL^?1 per decade, but the influence of age is unknown in patients with type 1 von Willebrand disease (VWD). In a retrospective cohort study, the medical records of 31 type 1 VWD patients over the age of 30, who had been followed for ≥5 years, were reviewed for baseline clinical data and previously performed VWF:Ag, VWF:RCo and factor VIII levels (FVIII:C). VWF multimer analysis was normal in 28/31 cases performed. Mean age at diagnosis was 33 (range 16–60 years), and duration of follow‐up ranged from 5 to 26 years (mean 11 years). Patients had 2–10 time points of VWD testing (mean of 5.2). The mean VWF:Ag, VWF:RCo and FVIII:C at time of diagnosis were 0.44 IU mL^?1 0.34 IU mL^?1 and 0.75 IU mL^?1. At last follow‐up, the mean VWF:Ag, VWF:RCo and FVIII:C were significantly increased to 0.71 IU L^?1, 0.56 IU mL^?1 and 0.90 IU mL^?1 (P ≤ 0.001, <0.001, and 0.0081 respectively). Here 18/31 patients had VWF:Ag, VWF:RCo and FVIII: C levels that increased into the normal range. The rate of change in VWF:Ag, VWF:RCo and FVIII was 0.30 IU mL^?1 (0.21–0.39, CI 95%, P < 0.0001), 0.20 IU mL^?1 per decade (0.13–0.27, CI 95%, P = 0.0001) and 0.20 IU mL^?1 (0.11–0.29, CI 95%, P = 0.0011). Patients with type 1 VWD experience age‐related increases to VWF:Ag and VWF:RCo which can result in normalization of VWF levels. Further studies are required to determine if the bleeding phenotype resolves with the increases in VWF:Ag and VWF:RCo levels.     

In vitro aspirin resistance detected by PFA-100 closure time: pivotal role of plasma von Willebrand factor

The in vitro closure time (CT), determined by the Platelet Function Analyzer (PFA-100), is used to monitor patients treated with aspirin. A relatively high percentage of in vitro aspirin resistance was reported despite an adequate inhibition of platelet response to arachidonic acid and we investigated whether high plasma levels of von Willebrand factor ristocetin cofactor activity (vWF:RCo) may contribute to this profile. Platelet aggregation test, CT [collagen adrenaline (CEPI-CT) and collagen adenosine 5'-diphosphate (ADP) (CADP-CT)], and vWF:RCo levels were evaluated in 55 consecutive patients receiving aspirin (75-250 mg/d) versus 32 untreated control subjects. All the aspirin-treated patients showed platelet aggregation responses that reflected the aspirin intake. However, CT data analysis enabled aspirin good-responder (GR) and aspirin bad-responder (BR) patients to be identified. All GR group subjects (n = 27), had a CEPI-CT and a CADP-CT longer than 300 s and 96 s respectively. The BR group (n = 28) had CEPI-CT values below 200 s and all CADP-CT were in the normal range (77 +/- 19 s). Interestingly, the BR plasma vWF:RCo levels were significantly higher (159 +/- 43%) than those of the GR group (121 +/- 34%) (P < 0.01), which were similar to control values (114 +/- 31%). A negative correlation between vWF:RCo and CT values was established. We demonstrate that in vitro aspirin-resistance, revealed by PFA-100 CT prolongation failure, is correlated to increased plasmatic vWF:RCo levels, reinforcing its particular importance in PFA-100 cartridges performance.     

Rational use of the PFA-100 device for screening of platelet function disorders and von Willebrand disease.

Two hundred and five patients referred for evaluation of platelet functions and 126 healthy controls were tested with the PFA-100 instrument. A cut-off value of 150 s for collagen/epinephrine (CEPI) closure time (CT) produced most acceptable sensitivity (90%), specificity (85.2%), and positive (82.6%) and negative (91.6%) predictivity values for screening of platelet function disorders and von Willebrand disease (vWD). All patients with vWD and Glanzmann thrombasthenia could be detected by PFA-100. Both CEPI and collagen/adenosine diphosphate (CADP) CTs were elevated in all of these cases. Sensitivity of the device was 81.6% for patients with platelet secretion defects. CADP CT was normal in 63.9% of the patients in this subgroup. Specificity (47%) and positive predictivity (57%) of the instrument were diminished in patients with low hemoglobin concentrations. Depending on the results, an algorithm was developed for screening of platelet function disorders and vWD with PFA-100.     

Use of the collagen-binding assay for von Willebrand factor in the analysis of type 2M von Willebrand disease: a comparison with the ristocetin cofactor assay

This study compares the utility of two functional assays for von Willebrand factor (VWF), the ristocetin cofactor assay (VWF:RCo) and the collagen-binding assay (VWF:CBA). We analysed a group of 32 patients with type 2 von Willebrand disease (VWD) (25 patients with type 2M, six with type 2A and one with type 2B) and 22 normal control subjects. VWF:RCo/VWF antigen (VWF:Ag) ratios and VWF:CBA/VWF:Ag ratios were compared between the patient and control groups. In the six patients with type 2A VWD, both VWF:RCo/VWF:Ag ratios and VWF:CBA/VWF:Ag ratios were discordant (< or = 0.7). In the 25 type 2M VWD patients, the VWF:CBA/VWF:Ag ratios were concordant (> 0.7), but the VWF:RCo/VWF:CBA ratios were discordant (< or = 0.7) (P = 0.001) compared with control subjects. Thus, VWF:RCo/VWF:Ag ratios were discordant in both type 2M and 2A VWD patient groups indicating a functional abnormality. However, VWF:CBA/VWF:Ag ratios were discordant in the type 2A VWD group but not in the type 2M VWD group. Our study showed that VWF:CBA is sensitive to functional variants associated with the loss of high-molecular-weight multimers, i.e. type 2A and 2B in VWD, but the assay was unable to discriminate defective platelet-binding VWD variants with normal multimeric patterns such as type 2M VWD. It was concluded that the VWF:CBA assay should be used in association with rather than as a replacement for the VWF:RCo assay.     

Screening for von Willebrand disease: contribution of an automated assay for von Willebrand factor activity

Measuring von Willebrand factor (VWF) activity is essential to the diagnosis of von Willebrand disease (VWD). The VWF activity is usually assessed based on measurement of the ristocetin cofactor (VWF:RCo). However, that test is technically challenging and has high intra- and inter-assay variabilities. The HemosIL VWF activity (VWF:AC) is a fully automated assay, recently proposed as a good alternative to VWF:RCo for VWD diagnosis. This study was undertaken to assess this new method. First, the analytical performance of VWF:AC on an automated coagulo-meter (ACLTop) was determined, and then this new method was compared with VWF:RCo and the platelet function analyzer (PFA100) for 160 patients referred for VWD screening. The VWF:AC achieved acceptable precision with within-run and between-run coefficients of variation ranging from 2.3% to 14.1%, and linearity from 10% to 100%. Despite some marked differences between VWF:AC and VWF:RCo for 10 plasmas tested, their agreement for VWD diagnosis was good. The VWF:AC had sensitivity similar to that of PFA100 (close to 100%), but better specificity (97.7% vs. 66% or 60%, depending on the cartridge used). The good analytical performance, and the sensitivity and specificity of VWF:AC to detect VWF deficiency renders it a suitable method for VWD screening. Our findings support VWF:AC use for the diagnostic work-up of VWD, paying close attention to concomitant clinical signs and bleeding score, as recommended for VWD.     

Effect of ABO blood group on the collagen-binding assay for von Willebrand factor

von Willebrand disease (VWD) is the most common congenital bleeding disorder and is caused by a quantitative or qualitative abnormality of von Willebrand factor (VWF). Ristocetin cofactor (RCoF) assay is used to evaluate VWF activity, but it does not assess collagen-binding activity. Normal values of RCoF and VWF antigen vary with ABO blood group type. The collagen-binding assay (CBA) measures VWF activity; however, its relationship with ABO blood group has not been completely explored. We performed CBA on plasma samples from 131 healthy volunteers to determine if CBA values correlated with blood type. Individuals with blood group O had a mean CBA value of 94 +/- 28%, which was significantly different from the mean of 117 +/- 33% in persons with non-O blood groups (P = 0.0001). Thus, CBA values appear to correlate with ABO blood type in a manner similar to RCoF.     

Laboratory diagnostic approach of the parents-children relationship in differentiating low-level von Willebrand factor from mild type 1 von Willebrand disease

The present study aimed to evaluate the parent-child relationship in differentiating between unaffected healthy individuals and those with von Willebrand disease (VWD). This study was performed on 15 children between the ages of 5 and 15 years and parents with personal and familial evidence of bleeding. Diagnosis of VWD as considered 'low von Willebrand factor (VWF) level or mild type 1 VWD' in the following children: those with low VWF levels (VWF:RCo and VWF:Ag between 30 and 50 U/dl), at least one bleeding symptom and a family member with at least one bleeding symptom. Laboratory values in the parents of families 1-7 were VWF:Ag 65-90, VWF:RCo 54-87, and FVIII:C 74-110, versus VWF:Ag 33-47, VWF:RCo 30-42, and FVIII:C 36-67 in their children. The normal laboratory values in the parents of families 1-7 suggested that their children would probably have low VWF levels. Our findings are that VWF levels are increasing with age. Laboratory values in the parents of families 8-15 were VWF:Ag 30-59, VWF:RCo 32-55, and FVIII:C 44-66, versus VWF:Ag 32-48, VWF:RCo 30-54, and FVIII:C 38-55 in their children. The laboratory values in the children from families 8-15 were close to the minimum range of normal or below normal, which suggested that it was possible that the parents and children in families 8-15 could be diagnosed as having mild type 1 VWD. The present study's findings show that comparison of the VWF levels in parents and their children may be helpful in differentiating children with low VWF levels and mild type 1 VWD from children that only have low VWF levels.     

Pharmacokinetics of von Willebrand factor and factor VIII coagulant activity in patients with von Willebrand disease type 3 and type 2

Nine patients with von Willebrand disease type 3, six with type 2B, one with type 2A, and one patient with type 1/2N were infused with one dose of ≈50 or 100 IU ristocetin cofactor activity (RCoF) per kg body weight of von Willebrand factor (vWF) (Human), a product with a very low content of factor VIII (FVIII). Blood samples were collected over 96 h. The data for RCoF and vWF antigen (vWF:Ag) were fitted to a 1-compartment model decay. The data for FVIII:C were fitted to a model with a linear time 'synthesis' term and a 1-compartment decay. Results in von Willebrand disease type 3 patients (nine patients; 10 infusions) indicated a volume of distribution of 39.9 and 39.8 mL kg⁻¹ for RCoF and vWF:Ag, respectively. The FVIII:C rate of synthesis was 6.4 U dL⁻¹ h⁻¹ (range: 4.4–8.8). The decay rates for FVIII:C, RCoF, and vWF:Ag were 0.041 (h⁻¹) [ t _1/2: 16.9 h]; 0.061 (h⁻¹) [ t _1/2: 11.3 h] and 0.006 (h⁻¹) [ t _1/2: 12.4 h], respectively. In patients with von Willebrand disease type 2 ( n  = 8) the RCoF mean volume of distribution was 46 mL kg⁻¹. The factor VIIIC mean rate of synthesis was 5.5 U dL⁻¹h⁻¹ and the decay rate 0.043 (h⁻¹) [ t _1/2: 16.1 h]. The rate of decay for RCoF and vWF:Ag were 0.050 (h⁻¹) [ t _1/2: 13.9 h] and 0.044 (h⁻¹) [ t _1/2: 15.7 h], respectively.     

Correlation between von Willebrand factor antigen,von Willebrand factor ristocetin cofactor activity and factor VIII activity in plasma

Background The laboratory diagnosis of von Willebrand Factor (VWF) deficiencies includes qualitative and quantitative measurements of VWF and clotting factor VIII (FVIII). Since the FVIII activity is frequently normal in patients with mild type 1 or 2 von Willebrand disease (VWD), there is controversy whether FVIII testing should accompany VWF Antigen (VWF:Ag) assay. Methods The aim of this study was to explore the correlation between VWF:Ag, VWF ristocetin cofactor activity (VWF:RCo) and FVIII in 213 consecutive patients undergoing screening for VWD. Results Forty-six patients were identified with VWF:Ag levels lower than the diagnostic threshold (54 IU/dl). A significant correlation was observed between VWF:Ag and VWF:RCo (r = 0.892; p < 0.001), VWF:Ag and FVIII (r = 0.834; p < 0.001), VWF:RCo and FVIII (r = 0.758; p < 0.001). Receiver operating characteristic curve analysis of the VWF:Ag assay revealed an area under the curve of 0.978 and 0.957 for detecting life-threatening values of FVIII (<30 IU/dl) and VWF:RCo (<40 IU/dl), respectively. The negative and positive predictive values at the VWF:Ag threshold value of 54 IU/dl were 100% and 33% for detecting life-threatening FVIII deficiencies, 94% and 80% for identifying abnormal values of VWF:RCo. Conclusions Due to the excellent correlation between VWF:Ag and FVIII and to the diagnostic efficiency of VWF:Ag for identifying abnormal FVIII levels in patients with VWF deficiency, routine measurement of FVIII may not be necessary in the initial screening of patients with suspected VWD. However, the limited negative predictive value of VWF:Ag for identifying type 2 VWD does not allow to eliminate VWF:RCo or VWF:FVIIIB assays from the diagnostic workout.     

Proteolytic processing of von Willebrand factor subunit: Heterogeneity in type-IIA von Willebrand disease

Summary Type IIA von Willebrand disease (vWD) is a heterogeneous disorder for which two different pathogenetic mechanisms have been proposed: increased proteolytic susceptibility of von Willebrand factor (vWF), and/or interference of its post-translational processing. Subunit analysis of vWF in type-IIA vWD has revealed an increased relative proportion of the 176- and 140-kDa subunit-derived fragments, suggesting an augmented fragmentation of vWF, even in the resting state. We analyzed the subunit pattern of vWF in plasma from five previously described patients with type-IIA vWD. All of them showed the above-mentioned pattern. In addition, the presence of a new band with an apparent molecular mass of 200 kDa, not described in normal individuals or in patients with vWD, was repeatedly observed in one of these patients. This patient also exhibited an abnormal vWF multimeric structure in platelets and in plasma, before and after desmopressin administration, when the blood was collected either in the presence or in the absence of proteinase inhibitors. We believe that an abnormal primary structure of vWF could be responsible for this abnormal proteolytic fragmentation pattern, as well as for the abnormal multimerization of vWF. Moreover, an abnormal susceptibility to proteolysis appears to be present, as suggested by the increase in the relative proportion of the 176-kDa fragment observed in the same patient. Future sequencing studies and genetic analysis may clarify whether there are one or two different defects related to the vWF of that patient. Our results indicate that the subunit analysis of vWF may reveal additional defects present in type-IIA vWD that may help our understanding of the pathogenesis of such disease.Supported in part by grants 90/3229, 91-92/0372, 94/1509 (FIS, INSALUD, Spain).