Hemostasis in patients with severe von Willebrand disease improves after normal platelet transfusion and normalizes with further correction of the plasma defect

BACKGROUND: A defective hemostatic effect of plasma concentrate infusion in patients with severe von Willebrand disease (vWD) has been ascribed to the absence of platelet von Willebrand factor (vWF) STUDY DESIGN AND METHODS: The role of platelet vWF in hemostasis of severe vWD was investigated. A plateletpheresis unit (4-5 × 10(11) platelets) from a normal compatible donor was transfused before any cryoprecipitate infusion to three type 3 vWD patients and to one patient with severe type 1 vWD with low levels of platelet vWF who required replacement therapy for bleeding episodes. Autologous platelets were transfused to one of the patients with type 3 vWD. RESULTS: Partial corrections of bleeding times (14-17 min vs. baseline>30 min) were observed in all patients after the transfusion of normal platelets. During cryoprecipitate infusion, bleeding times were normalized (<6 min), and bleeding episodes stopped when plasma levels of vWF activity ranged from 14 to 18 U per dL. Platelet interactions with the subendothelium increased in parallel with the correction of bleeding times. These results indicate that if approximately 20 percent of the total number of platelets have normal vWF antigen and if plasma vWF levels are at least 14 U per dL, then bleeding times will normalize and mucosal hemorrhages will stop. Transfusion of autologous platelets in one patient with type 3 vWD did not modify bleeding times or platelet adhesion on the subendothelium. CONCLUSION: The hemostatic effect of normal platelets in type 3 vWD seems to be related to the platelet vWF in the transfused platelets.     

Platelet membrane vesicles reduced microvascular bleeding times in thrombocytopenic rabbits

We investigated the possible role of platelet membrane vesicles on hemostatic function in vivo. Platelet membrane vesicles were prepared from rabbit platelets stored for up to 6 months at -65 degrees C and transfused into thrombocytopenic rabbits. Significant reductions in microvascular bleeding times were observed up to 24 hours after transfusion, with the greatest corrections at 4 hours. Measurements of factor V, factor VIII, fibrin degradation products, and fibrinogen in animals transfused with membrane ruled out intravascular coagulation and suggested a direct effect of platelet membrane vesicles at the bleeding sites. This conclusion was supported morphologically by identification of membrane vesicles in bleeding time lesions and radiologically by accumulation of 111In-labeled vesicles in lesions. Production of platelet membrane vesicles was simple, and freezing allowed long-term storage of a product capable of short-term hemostasis.     

Electrophoretic heterogeneity of normal factor VIII/Von Willebrand protein, and abnormal electrophoretic mobility in patients with Von Willebrand's disease.

Crossed antigen-antibody electrophoresis was used to determine the electrophoretic properties of factor VIII/Von Willebrand (F. VIII/VW) protein in normal plasma using a specific rabbit antiserum against purified human factor VIII. The electrohoretic patterns in seven patients with severe Von Willebrand's disease in two different families were studied. In these patients the prolonged bleeding time was related to a functionally abnormal F. VIII/VW protein which was unable to induce platelet aggregation in presence of ristocetin. F. VIII/VW protein had an increased electrophoretic mobility in all the patients. The electrophoretic pattern was similar in different members of the same family but differed from one family to the other. Plasma samples collected after transfusion of normal cryoprecipitate to one member of the first family showed an increasing amount of F. VIII/VW protein, but even one hour after transfusion only one peak migration in an abnormal position was found. However, when mixed in vitro with normal plasma, the plasmas of the same patient showed two peaks, one in position of normal F. VIII/VW protein and one with increased electrophoretic mobility. These results suggest that normal F. VIII/VW protein transfused to recipients with Von Willebrand's disease synthesizing a functionally deficient factor undergoes a rapid alteration.     

Cryoprecipitate transfusion: assessing appropriateness and dosing in trauma

Background: Originally developed for patients with congenital factor VIII deficiency, cryoprecipitate is currently largely used for acquired hypofibrinogenemia in the context of bleeding. However, scant evidence supports this indication and cryoprecipitate is commonly used outside guidelines. In trauma, the appropriate cryoprecipitate dose and its impact on plasma fibrinogen levels are unclear. Objectives: The aims were to evaluate (i) the appropriateness of cryoprecipitate transfusion in trauma and (ii) the plasma fibrinogen response to cryoprecipitate transfusion during massive transfusion in trauma. Methods: Retrospective review (January 1998–June 2008) of indications, dose and plasma fibrinogen response to cryoprecipitate transfusion at a large teaching hospital. A fibrinogen of <1·0 g L^?1 within 2 and 6 h of transfusion was used for evaluating appropriateness. Results: Ten thousand five hundred and forty cryoprecipitate units were transfused in 1004 patients. Thirty‐seven percent and 31% were used in cardiac surgery and trauma, respectively. In 394 events in trauma, 238 (60%) and 259 (66%) were considered appropriate using the 2‐ and 6‐h cut‐off criteria, respectively. In patients who did not receive plasma components 2 h prior to cryoprecipitate, a dose of 8·7 (±1·7) units caused a mean increase in fibrinogen levels of 0·55 (±0·24) g L^?1, or 0·06 g L^?1 per unit. Conclusions: In our hospital, where transfusion guidelines are overseen by transfusion medicine specialists and technologists, and policies for rapid blood component and laboratory turnaround times exist, it is possible to achieve high rates of appropriateness for cryoprecipitate transfusion in trauma. The current recommended dose causes a modest increase in fibrinogen levels (0·55 g L^?1).     

Effect of circulating immune complexes on transfusional therapy in patients with hemophilia or von Willebrand's disease

Circulating immune complexes were examined in patients with hemophilia or von Willebrand's disease in order to determine the immediate or long- term side effects after transfusion. The conglutinin binding assay which allows quantitation of C3bi-bearing immune complexes was used for 82 patients with hemophilia A. Immune complexes were detected in 37 (45%) of these cases prior to transfusion. Immune complexes also were detected in four of 11 patients with hemophilia A and factor VIII inhibitors, in five of 11 patients with hemophilia B, and in three of 10 patients with von Willebrand's disease. The levels of circulating immune complexes in 21 patients with hemophilia A and seven with von Willebrand's disease significantly increased 24 hours after concentrate or cryoprecipitate transfusions. Purified immune complexes from three patients with hemophilia A were shown to contain IgG, IgM, and complement components. No factor VIII coagulant or antigenic protein or fibrinogen was identified in the immune complexes using specific antisera. Side effects immediately after transfusion were not associated with immune complexes. The levels of factor VIII or IX after transfusion were not particularly decreased in relation to the presence of immune complexes. Finally, the presence of circulating immune complexes in the patients studied did not correlate with the number of transfusions, the units of concentrates injected, the presence of HBsAg or HbsAb, the levels of plasma aspartate transferase, or the presence of rheumatoid factor. Proteinuria was absent in all the patients studied.     

The response of an acquired Factor V inhibitor to activated Factor IX concentrate

A 91-year-old man developed a severe bleeding diathesis postoperatively. Laboratory studies showed an inhibitor to factor V which was identified as IgG. The patient failed to respond to fresh-frozen plasma and platelet transfusions, but demonstrated both clinical and laboratory improvement after transfusion with an activated prothrombin complex concentrate (Autoplex). Patients with refractory inhibitors to factors VIII or IX have been managed successfully with this concentrate; however, this case demonstrates that is also may be of value in managing patients with refractory inhibitors to factor V.     

Preoperative hemostasis and its association with bleeding and blood component transfusion requirements in cardiopulmonary bypass surgery

BACKGROUND: Variables of hemostasis before surgery might indicate an elevated risk of bleeding. We determined hemostasis tests and standardized bleeding history and their association with bleeding and transfusion requirements in cardiopulmonary bypass (CPB) surgery. STUDY DESIGN AND METHODS: In a prospective trial, variables from 104 patients were associated with postsurgical bleeding and with red blood cells (RBCs) and platelet concentrate (PC) transfusions. Variables included standardized bleeding history, prothrombin time (PT), fibrinogen, fibrin monomers, Factor VIII, von Willebrand factor (VWF), multiple electrode aggregation (MEA), and the day of aspirin or thienopyridine withdrawal before operation. RESULTS: Multiple linear regression revealed bleeding history score, ADP‐induced MEA, CPB time, and hemoglobin (Hb) independently associated with postoperative bleeding and bleeding history, arachidonic acid (AA)‐induced MEA, CPB time, and PT associated with RBC transfusions. The logistic regression model for the outcome of bleeding within 24 hours after operation indicated ADP‐induced MEA, the day of aspirin withdrawal before operation, and CPB time as predictors. AA‐induced MEA, CPB time, Hb, and PT were predictors of RBCs transfusion. ADP‐induced MEA, the day of aspirin withdrawal, PT, and VWF were associated with PC transfusion. CONCLUSIONS: A standardized bleeding history may help to identify patients undergoing CPB surgery whose risk of bleeding is elevated. ADP‐induced MEA appears to predict postoperative bleeding and PC transfusion requirements, while AA‐induced MEA and preoperative Hb indicate the need for RBCs. The time of aspirin withdrawal before surgery influences perioperative blood loss and PC transfusion.     

Double-antibody radioimmunoassay for factor VIII-related antigen.

A plasma protein required for the support of ristocetin-induced platelet aggregation was isolated from antihemophilic factor concentrate and radiolabeled with 125I. A double-antibody radioimmunoassay was developed, with use of specific rabbit anti-VIII related antigen serum and goat anti-rabbit globulin. The assay is sensitive, reproducible, and technically simple to perform. Values obtained in normal subjects ranged from 0.65 to 1.53 units, similar to our normal range for VIII coagulant activity (0.67-1.43 units). However, normal or increased values of VIII-related antigen were observed in VIII coagulant-deficient hemophiliacs. Also, concentrations of VII-related antigen significantly exceeded coagulant concentrations in several patients with liver disease or disseminated intravascular coagulation, or both. Of a broad selection of congenital coagulation disorders examined, only patients with von Willebrand's disease had decreased VIII-related antigen concentrations, and these corresponded to the lowered concentration of ristocetin cofactor in the patients. In three transfused patients, VII-related antigen values correlated with the concentration of the cofactor. Our results suggest that the radioimmunoassay of VIII-related antigen is a simple and valuable adjunct in the study of patients with clotting abnormalities.     

A heat-treated factor VIII concentrate prepared by controlled-pore glass adsorption chromatography

Four years' experience with a method for preparing a high-purity, low-fibrinogen, heat-treated factor VIII concentrate is reported. The process, batch adsorption of a cryoprecipitate extract with controlled-pore glass granules, removes 77 percent of the cryoprecipitate fibrinogen, resulting in a final concentrate-specific activity of 0.74 IU factor VIII per mg of protein at a yield of 194 IU factor VIII per kg of starting plasma. Heat treatment of the lyophilized concentrate for 72 hours at 60 degrees C results in less than 10 percent loss of factor VIII activity. This process does not require expensive fractionation equipment, is suitable for small-to medium-scale batch concentrate production and could be adopted by moderately well-equipped regional blood processing laboratories for the decentralized production of a high-quality, heat-treated factor VIII concentrate.     

Home treatment in hemophilia

88 patients with severe haemophilia (66 with haemophilia A without inhibitor, 12 with haemophilia A and inhibitor, 10 with haemophilia B) currently receive comprehensive care at the Vienna haemophilia centre. Data available at the centre and a questionnaire answered by the hemophiliacs were used in order to evaluate the current situation of home care. At present, 62 (70%) out of 88 patients receive home treatment (51 with haemophilia A without inhibitor, 5 patients with haemophilia A and inhibitor and 6 with haemophilia B). For treatment of joint and muscle bleeding mean dosages of 15.3 units/kg body weight of factor VIII concentrate, 17.0 units/kg of factor IX concentrate and 30 units/kg of FEIBA were administered by the hemophiliacs. Children and young patients required higher doses (30 and 17.4 units/kg F VIII, respectively). Two thirds of the bleeding episodes were successfully treated by a single infusion. No severe side effects were observed during home treatment. Home treatment has been widely accepted by the patients. It is regarded as a practical and safe therapy and has improved the life quality of haemophiliac patients.     

Inherited Bleeding Disorders in the Obstetric Patient

Inherited bleeding disorders increase the risk of bleeding in the obstetric patient. Randomized controlled trials to compare prophylactic or therapeutic interventions are rare, and guidance documents rely heavily on expert opinion. Here we report the results of a systematic review of the literature for the treatment and prevention of peripartum bleeding in women with an inherited bleeding disorder. The highest-quality evidence is for the use of tranexamic acid in postpartum hemorrhage, which has been shown to decrease bleeding-related mortality in women without bleeding disorders. There is limited evidence for prophylactic use of this agent in women with inherited bleeding disorders. Desmopressin has also been used in observational studies of patients with von Willebrand disease and carriers of hemophilia A with some success, although concerns about the risk of hyponatremia persist. In patients with deficiencies of specific factors, replacement is generally the preferred approach, and concentrates have been studied in deficiencies of VWF and factors VII, VIII, IX, XI, and XIII as well as in patients with fibrinogen deficiency. Because of the small size of these studies, neither safety nor efficacy is well established, although the literature suggests that bleeding history may be more predictive of outcomes than factor levels in many cases. Goal factor levels have not been studied or systematically established in any of these diseases, although observational data suggest that achieving normal levels may be inadequate, particularly for VWF and factor VIII, which are physiologically elevated in pregnancy. For factor deficiencies in which no specific concentrate is available, such as factors II (prothrombin) and V, prothrombin complex concentrate or fresh frozen plasma may be used, and for platelet defects or deficiencies, such as Glanzmann thrombasthenia or Bernard-Soulier syndrome, platelet transfusion is generally first line, although use of recombinant FVIIa has been reported in patients with Glanzmann thrombasthenia to avoid development of, or treat patients with, antibodies to platelet glycoprotein IIbIIIa. Ultimately, data are lacking to definitively support an evidence-based approach to management in any of these disorders, and prospective, controlled studies are desperately needed.     

Cryoprecipitate: an outmoded treatment?

Cryoprecipitate is an allogeneic blood product prepared from human plasma. It contains factors VIII, von Willebrand factor (vWF), fibrinogen, fibronectin and factor XIII. Its use was first described in the 1960s for treatment of patients with factor VIII deficiency. It has also been used to treat patients with congenital hypofibrinogenaemia. Now, the most common use of cryoprecipitate is fibrinogen replacement in patients with acquired hypofibrinogenaemia and bleeding. Despite almost 50 years of use, evidence of efficacy is limited. This review provides an overview of the history of cryoprecipitate use, the current debates on the use of this product and future developments.     

Use of Frozen Cryoprecipitate for the Preparation of Clinical Factor VIII Concentrate

Two years of experience is described in the preparation of factor VIII concentrate for clinical use, prepared from frozen cryoprecipitate by a modified polyethylene glycol procedure. The yield of factor VIII was fire plasma equivalents per gram of starting material with a 100‐fold purification over plasma. Anti‐A and B isoagglutinin levels were consistently higher in factor VIII concentrates derived from paid donors than from volunteer donors. The incidence and levels of hepatitis B antigen in the factor VIII concentrates from paid unscreened donors were significantly higher than that derived from volunteer donors. Factor VIII concentrates derived from screened paid or volunteer donors contained little or no detectable hepatitis B antigen. These highly soluble concentrates met all the Bureau of Biologics requirements and have been clinically effective.     

Fibrinogen concentrate for acquired hypofibrinogenaemic states

This study aims to assess the efficacy and safety of fibrinogen concentrate in acquired hypofibrinogenaemic states. Cryoprecipitate is standard treatment for replacement of fibrinogen in acquired hypofibrinogenaemia. A virally inactivated fibrinogen concentrate (Haemocomplettan((R)); CSL Behring, Marburg, Germany) is licensed in some European countries. Clinical data for its use in acquired hypofibrinogenaemic states are scarce. Demographic and pretreatment clinical data of patients treated with fibrinogen concentrate for acquired hypofibrinogenaemia were retrospectively reviewed. Pretreatment and posttreatment fibrinogen levels, transfusion requirements, outcomes and adverse events were recorded. Thirty adult patients who received fibrinogen concentrate for acquired hypofibrinogenaemia (fibrinogen <1.5 g L(-1)) were included in the study. Causes of hypofibrinogenaemia included placental abruption, disseminated intravascular coagulation as a result of massive blood loss and transfusion, liver failure and cardiac surgery. Following a median dose of 4 g fibrinogen concentrate, median Clauss fibrinogen level rose from 0.65 to 2.01 g L(-1), with a median fibrinogen increment of 0.25 g L(-1) per 1 g fibrinogen concentrate administered. Forty-six per cent of patients stopped bleeding with blood components and fibrinogen concentrate alone, and a further 29% stopped bleeding with surgical or endoscopic intervention. Inpatient mortality was 40%. No venous thromboses were observed. Four patients with massive perioperative haemorrhage and hypotension (including three postcardiothoracic surgery) had arterial ischaemic events, none of which was attributable to fibrinogen overreplacement. The cost of fibrinogen concentrate was comparable with that of cryoprecipitate. Purified, virally inactivated fibrinogen concentrate appears effective in the management of acquired hypofibrinogenaemic states and enables rapid administration of a known fibrinogen dose in an emergency setting.     

The Properties of Immune Complexes Formed by Human Antibodies to Factor VIII

Although human antibodies to Factor VIII inactivate its procoagulant activity, they do not form immunoprecipitates when tested with this antigen. To understand this observation, we have examined the interaction of normal human Factor VIII with four high-titer human anti-Factor VIII, two from transfused hemophiliacs and two “spontaneous” antibodies from nonhemophilic individuals. An estimate of the size of complexes formed by these antibodies has been obtained by agarose gel filtration of mixtures of anti-Factor VIII with cryoprecipitate. Complexed anti-Factor VIII was detected by the method of Allain and Frommel: acid dissociation of complexes at pH 3.5.     

Defective Ristocetin-Induced Platelet Aggregation in von Willebrand''s Disease and its Correction by Factor VIII

The antibiotic ristocetin, in concentrations of 1.0-1.5 mg/ml, aggregated normal platelets in citrated platelet-rich plasma by a mechanism in which the release reaction played only a minor role. Platelet aggregation by ristocetin in a concentration of 1.2 mg/ml was absent or markedly decreased in 10 patients with von Willebrand's disease. Lesser degrees of abnormality were obtained with a concentration of 1.5 mg/ml. The magnitude of the defect in ristocetin-induced platelet aggregation correlated well with the degree of abnormality of the bleeding time and the levels of antihemophilic factor (AHF, VIII(AHF)) procoagulant activity. In all patients, the defect in ristocetin-induced platelet aggregation was corrected in vitro by normal plasma. Correction was also obtained with a fraction of normal cryoprecipitate that eluted in the void volume with VIII(AHF) after chromatography on a gel that excludes molecules larger than 5 x 10(6). A similar fraction, devoid of VIII(AHF) activity, obtained from patients with von Willebrand's disease had no corrective effect, but fractions obtained from patients with hemophilia were just as effective as those obtained from normal subjects. The correction activity of plasma and partially purified factor VIII was inhibited by a rabbit antibody to human factor VIII but not by a human antibody against VIII(AHF) procoagulant activity. The studies provide further evidence that patients with von Willebrand's disease are deficient in a plasma factor that is necessary for normal platelet function. The activity of this factor appears to be associated with factor VIII but is unrelated to VIII(AHF) procoagulant activity.     

Modulation of fibrinogen content in cryoprecipitate by temperature manipulation during plasma processing

In routine blood bank production of single-donation cryoprecipitate, the introduction of a 16-hour hold at 4 degrees C, with the frozen plasma units packed into polystyrene containers, resulted in plasma prethaw temperatures of -4 degrees C to -8 degrees C. This in turn resulted in cryoprecipitate fibrinogen levels that were 214 percent of those obtained when units were thawed immediately after removal from -30 degrees C storage. In scale-model production of factor VIII concentrate, plasma warmed from -30 to -10 to -15 degrees C over 18 hours before pooling and thawing yielded cryoprecipitate fibrinogen levels that were 66 percent of those found in plasma warmed to -2 to -5 degrees C over the same period. Processing -30 degrees C plasma without a warming period led to cryoprecipitate fibrinogen levels that were 40 percent of those obtained from plasma warmed to -2 to -5 degrees C. These differences were accentuated after purification of the cryoprecipitates to an intermediate-purity factor VIII concentrate. These results suggest that simple modifications in production methods allow the fibrinogen content of cryoprecipitate to be tailored to specific uses.     

Disseminated intravascular coagulation (DIC).

DIC is a life-threatening complication of several disease states. It is characterized by systemic activation of the hemostasis system. In many instances the release of tissue factor (TF) from endothelial cells or other circulating cells triggers the system. Initially, the increased activation can be compensated for by the natural inhibitor systems, a state referred to as compensated DIC. As the trigger persists, inhibitors will be consumed leading to more coagulation. In this process many clotting factors, most notably fibrinogen and platelets are consumed, resulting eventually in a complete breakdown of the hemostasis system. This results in a profuse and diffuse bleeding tendency or decompensated DIC. The term consumptive coagulopathy denotes this process. Of crucial importance is the fate of fibrin that is formed from fibrinogen by thrombin. If the fibrinolytic system is insufficiently activated, fibrin will be deposited in the microcirculation leading to MODS. This will not occur if the fibrinolytic system is fully activated. The clinical suspicion of DIC must be confirmed by laboratory tests and decreasing fibrinogen levels and platelet counts support the diagnosis. The determination of D-dimer, fibrin(ogen) split products (FSP) and soluble fibrin monomer (FM) further support the diagnosis. FM suggest the presence of thrombin, FSP the generation of plasmin, and D-dimer, both thrombin and plasmin. While the tests are not specific for DIC, they can be helpful, in the proper clinical setting, to diagnose decompensated or acute DIC. The tests are not useful for the diagnosis of compensated DIC, except for D-dimer, FSP, and FM if elevated. Compensated DIC can be diagnosed by molecular markers of in vivo hemostasis activation, such as thrombin-antithrombin (TAT) complexes, prothrombin fragment 1 + 2 (F 1 + 2), or plasmin-antiplasmin (PAP) complexes. For the treatment of DIC it is imperative to remove the triggering underlying disease. The consumption of coagulation constituents can be corrected by cryoprecipitate, platelet concentrates, and fresh frozen plasma, if needed. This may reduce the bleeding tendency. Arrest of the activated hemostasis system by heparins, either subcutaneous in low doses or intravenous in therapeutic doses, is only recommended in patients with compensated DIC. If the patient bleeds, heparins should not be given. The administration of concentrates of natural anticoagulants, i.e., antithrombin, protein C, or tissue factor pathway inhibitor are safer than heparins since they do not exacerbate the bleeding tendency. These concentrates were found to be very effective in animal models of DIC; human experience is still limited. Generally, the earlier treatment is initiated, the better the patient's prognosis.     

A novel, automated method of temperature cycling to produce cryoprecipitate

BACKGROUND: Cryoprecipitate continues to find wide application in transfusion practice. Current AABB standards call for a minimum of 80 units (U) of factor VIII and 150 mg of fibrinogen per bag of cryoprecipitate. However, individual cryoprecipitates can vary greatly in content, with as many as 20 different factors known to affect the yield. STUDY DESIGN AND METHODS: Plasma was processed in a new, rapid, automated device (CryoSeal, Thermogenesis) with computer-controlled temperature cycling to produce cryoprecipitate. RESULTS: In repeat runs (n = 20), the automated procedure yielded a product containing 184 mg of fibrinogen and 158 U of factor VIII in 55 minutes. Additional studies using plasma pools to compare the quality of the machine-generated products to those of traditionally prepared cryoprecipitate showed comparative recoveries of 182 and 187 mg of fibrinogen and 172.1 and 129.7 U of factor VIII and no significant difference in the levels of plasminogen, protein C, or protein S. CONCLUSION: The new system offers an automated method of cryoprecipitate production in which the steps involved in temperature cycling are initiated sequentially, producing within 1 hour a preparation that is equivalent to standard cryoprecipitate.     

Platelet interaction with rabbit subendothelium in von Willebrand''s disease: altered thrombus formation distinct from defective platelet adhesion

Blood interaction with the subendothelium of rabbit aorta was investigated in an annular perfusion chamber using patients with von Willebrand's disease, hemophilia, and afibrinogenemia. The vessels were exposed to nonanticoagulated blood for a range of flow conditions (wall shear rates of 650-3,300 s-1) and exposure times (1.5-10 min). The resultant platelet and fibrin interaction was quantified by the use of several morphometric techniques, one of which was developed to measure more precisely the dimensions (height and volume) of platelet thrombi attached to the subendothelium. A major finding was that under flow conditions in which little or no defect in platelet adhesion was observed in von Willebrand's disease, platelet thrombus height and volume in this disorder were significantly reduced as compared with normal controls or patients with hemophilia. Thus, Factor VIII/von Willebrand factor (VIII/VWF) may mediate not only the adhesion of platelets to subendothelium but also platelet-platelet attachments necessary for normal thrombus development. The level of Factor VIII:coagulant activity (VIII: C) was also observed to influence the resultant thrombus height and volume deposited on subendothelium, presumably through the generation of thrombin or some other procoagulant factor preceding fibrin formation, since normal values of thrombus dimensions were always observed in a patient with a fibrinogen deficiency. The influence of VIII:C became greater as shear rate was reduced, whereas as shear rate was increased, VIII/VWF was more dominant in determining the resultant platelet deposition on subendothelium. Thus, the deficiencies of VIII:C and VIII/VWF in hemophilia and von Willebrand's disease can lead to various abnormalities in platelet and fibrin association with subendothelium. The importance of a particular deficiency will depend strongly on the local blood flow conditions.