Impact of procoagulant concentration on rate, peak and total thrombin generation in a model system

Tissue factor in thrombosis and hemorrhage

The research aims of our laboratory are to provide a realistic description of biologic processes involved in protection from hemorrhage and the evolution of thrombosis. To evaluate these processes, we use 4 models of coagulation ranging from 1) studies of blood exiting from microvascular wounds in humans through 2) minimally altered whole blood induced to clot by tissue factor (TF) to 3) reconstitution of the blood coagulation proteome with purified components and to 4) mathematical descriptions of the chemical processes and dynamics that occur. The integration of these 4 models permits comprehensive analyses of the blood coagulation system and predictions of its behavior under normal and pathologic conditions. Data accumulated thus far have led to advances in our understanding of 1) the processes occurring during the initiation and propagation phases of thrombin generation, 2) the roles for individual proteins involved in blood coagulation and its regulation, 3) defects in thrombin generation and clot formation in hemophilia, 4) actions and limitations of pharmacologic agents used to control hemorrhage, thrombosis, and chronic cardiovascular disease, and 5) the relationship between genotypic and phenotypic features of an individual's plasma proteome and his/her immediate and long-term thrombotic risk.

Tranexamic acid combined with recombinant factor VIII increases clot resistance to accelerated fibrinolysis in severe hemophilia A

BACKGROUND

Most patients with severe hemophilia A suffer from a profoundly compromised hemostatic response. In addition to both the delayed and slow development of a clot, previous studies have documented that severe hemophilia A is also associated with reduced clot stability.

OBJECTIVES

We examined whether the clot stability in hemophiliacs could be improved by treatment with tranexamic acid (TXA) in combination with recombinant factor VIII (rFVIII).

PATIENTS/METHODS

Baseline blood samples were obtained from eight males with severe hemophilia A. Thereafter, a bolus injection of rFVIII was administered to increase the functional level of FVIII to approximately 50%. After 10 min, blood was collected followed by an intravenous injection of TXA. A third blood sample was obtained after a further 10 min. Whole blood clotting profiles were determined by thrombelastography using minimal tissue factor activation. Clot formation was assessed by both clot initiation and clot propagation. At clot termination, the maximum clot firmness and area under the elasticity curve were used to illustrate clot stability. Tissue-plasminogen activator was included in those experiments designed to assess clot stability.

RESULTS

As expected, rFVIII increased clot formation, whereas TXA had no effect upon this parameter. Assays including tissue-plasminogen activator revealed that rFVIII increased the maximum clot firmness 3-fold; whereas the presence of TXA induced an additional 4-fold increase. The area under the elasticity curve increased 5-fold after rFVIII and 24-fold after addition of TXA.

CONCLUSIONS

The study demonstrates that simultaneous treatment with TXA and rFVIII significantly improves the clot stability in patients with hemophilia A.

Influence of factor IX on overall plasma coagulability and fibrinolytic potential as measured by global assay: monitoring in haemophilia B

We sought to determine the influence of factor IX (FIX) deficiency upon overall coagulative and fibrinolytic capacities in plasma using the clot formation and lysis (CloFAL) assay, and to investigate the role of this global assay as an adjunctive monitoring tool in haemophilia B. CloFAL assay parameters were measured in vitro in platelet-poor plasma in relation to FIX activity and antigen (FIX:Ag), and were determined ex vivo among FIX-deficient patients (n = 41) in comparison to healthy individuals (n = 48). Supplementation of FIX-deficient plasma with FIX in vitro demonstrated a non-linear concentration dependence of FIX upon overall plasma coagulability. Ex vivo, coagulability was significantly decreased in FIX-deficient vs. healthy subjects among adults [median coagulation index (CI): 4% vs. 104% respectively; P < 0.001] and children (median CI: 9% vs. 63%; P < 0.001). Fibrinolytic capacity was increased in adult FIX-deficient vs. healthy subjects (median fibrinolytic index: 216% vs. 125%, respectively, P < 0.001), and was supported by a trend in shortened euglobulin lysis time (ELT). Severe haemophilia B patients showed heterogeneity in aberrant CloFAL assay waveforms, influenced partly by FIX:Ag levels. Patients with relatively preserved FIX:Ag (i.e. dysfunctional FIX) exhibited a shorter time to maximal amplitude in clot formation than those with type I deficiency. During patient treatment monitoring, markedly hypocoagulable CloFAL assay waveforms normalized following 100% correction with infused FIX. The CloFAL global assay detects FIX deficiency, demonstrates differences in coagulability between dysfunctional FIX and type I deficiency, and appears useful as an adjunctive test to routine FIX measurement in monitoring haemophilia B treatment.

Beyond cell based models of coagulation: analyses of coagulation with clot "lifespan" resistance-time relationships

Cell based models of coagulation (CBM) have provided mechanistic insight into numerous hematological issues for nearly two decades. This review discusses another coagulation model system--the clot lifespan model (CLSM)--that has been designed to compliment the CBM-based approach to elucidating the mechanisms responsible for a variety of hemostatic disorders/phenomena. The CLSM is a thrombelastograph-based approach that utilizes a standardized clotting stimulus (e.g., celite, tissue factor) and a fibrinolytic stimulus (e.g., tissue type plasminogen activator) to assess clot growth and disintegration via changes in clot resistance. The CLSM utilizes parametric, elastic modulus-based parameters to document these phenomena. The CLSM has recently been employed to discern the effects of protamine and hydroxyethyl starch on key fibrinolytic-antifibrinolytic protein interactions, as well as demonstrating differences in fibrinolytic kinetics dependent on whether contact pathway proteins or tissue factor is used to initiate coagulation. The CLSM is presently being utilized to investigate the effects of ventricular assist device placement on fibrinolysis, and it is anticipated that this model system will be employed in both basic science and clinical investigations in the future.

Hemostatic Changes After Crystalloid or Colloid Fluid Administration During Major Orthopedic Surgery: The Role of Fibrinogen Administration

BACKGROUND

To explore whether disturbed fibrin polymerization is the main problem underlying dilutional coagulopathy and can be reversed by fibrinogen administration, we conducted a prospective study using modified thrombelastography (ROTEM).

METHODS

Sixty-six orthopedic patients randomly received modified gelatin solution, hydroxyethyl starch 130/0.4, or exclusively Ringer lactate solution. ROTEM analysis was performed, concentrations of coagulation factors and markers of thrombin generation were measured. Fibrinogen concentrate (Hemocomplettan) was administered (30 mg/kg) when thrombelastographically measured fibrinogen polymerization was critically decreased.

RESULTS

The alpha angle, clot firmness, and fibrinogen polymerization (median [min to max]) significantly decreased in the patients receiving hydroxyethyl starch (area under the curve minus baseline (-5 [-9 to -2]), followed by gelatin solution (-3 [-8 to 0]), with the least reductions seen for Ringer lactate solution (-2 [- 4 to 1]) (colloids versus Ringer lactate P < 0.0001). Thirteen patients in the colloid groups but none in the Ringer lactate group needed fibrinogen concentrate to maintain borderline clot firmness. Activity of FVII, FVIII, FIX, and von Willebrand ristocetin activity decreased significantly with colloids. Thrombelastographically measured coagulation time, molecular markers of thrombin generation, and activity of all other coagulation factors were comparable in all groups.

CONCLUSION

Disturbance of fibrinogen/fibrin polymerization is the primary problem triggering dilutional coagulopathy during major orthopedic surgery. The magnitude of clot firmness reduction is determined by the type of fluid used, with hydroxyethyl starch showing the most pronounced effects. These undesirable effects of intravascular volume therapy can be reversed by increasing fibrinogen concentration by administering fibrinogen concentrate, even during continuing blood loss and intravascular volume replacement.

The impact of major surgery on blood coagulation factors and thrombin generation

We studied the blood coagulation system of 14 patients with metastatic malignancies before and after they had undergone major surgery. In addition to measuring a battery of coagulation factors, we assessed the function of the system with assays of whole blood thrombin generation. With the exceptions of factor VIII (fVIII), which increased, and fibrinogen and fIX, which did not change, the activities of all the pro- and anticoagulant proteins were significantly lower postoperatively. However, the thrombin generating capacity of the system was relatively preserved. Although the integral of thrombin activity over time was lower after surgery, the mean peak thrombin concentration was unchanged and the time to clot formation was shortened. Similar changes could be reproduced by lowering the concentrations of pro- and anticoagulant factors together in control blood samples. Therefore, simultaneous reductions in pro- and anticoagulant proteins postoperatively worked to maintain the functional integrity of the blood coagulation system.

A novel approach to improving recombinant factor VIIa activity with a preserved platelet preparation

Recombinant activated factor VII (NovoSeven, rFVIIa) is used to abrogate bleeding in haemophiliacs with inhibitors and is hypothesised to work by increasing activated factor X generation on the platelet surface. We hypothesised that rFVIIa activity could be increased by the co-addition of platelet procoagulant surface. This study characterised the ability of a rehydrated, lyophilised (RL) platelet preparation to increase rFVIIa activity in haemophilic conditions. RL platelets supported thrombin generation in the presence of factors VIII and IX but, in the absence of factors VIII and IX, thrombin generation was significantly reduced. RL platelets supported rFVIIa-mediated thrombin generation in a rFVIIa-concentration dependent manner. In a cell-based in vitro model of haemophilia, the presence of RL platelets increased the rFVIIa-dependent thrombin generation rate 2.8-fold compared with rFVIIa alone. Similarly, the addition of RL platelets plus rFVIIa to the in vitro model of haemophilia and to haemophilic platelet-rich plasma shortened the onset of clot formation and increased clot stability in a fibrinolytic environment versus rFVIIa alone. These results suggest that RL platelets can support rFVIIa-mediated thrombin generation, and that co-administration of RL platelets with rFVIIa may increase the efficacy of rFVIIa in some patients.

An open non-randomized study of recombinant activated factor VII in major postpartum haemorrhage

BACKGROUND

Empirical off-label use of recombinant activated factor VII (rFVIIa) has been reported to be effective in some cases of severe postpartum haemorrhage (PPH). Successful management of these patients has lead to more wide-spread use of rFVIIa in less severe cases without any evidence for the advantages of its administration.

METHODS

Until November 2006, we had administered rFVIIa to 38 parturients. Based on our initial experience with the first 12 patients, we prepared guidelines for the use of rFVIIa. During the existence of these guidelines, we made a retrospective comparison of the 26 women who received rFVIIa with another 22 women who were treated during the same time period without using rFVIIa.

RESULTS

The total amount of blood loss was significantly higher (11.3 +/- 4.5 vs. 8.0 +/- 3.1 l), and the coagulation screen revealed significantly longer partial thromboplastin time (APTT) and prothrombin time (PT) values and significantly lower fibrinogen values in patients receiving rFVIIa. The need for red blood cells, platelets and fibrinogen concentrate was significantly higher in these women. Although the response was considered good in two-thirds of the women, several patients received rFVIIa with a poor or no response as a result of arterial bleeding.

CONCLUSION

The decision to use rFVIIa resulted from a more profound haemorrhage. We did not gain any evidence to extend the use of rFVIIa into less severe cases of PPH. Furthermore, this policy would result in a profound increase in the overall costs of the treatment. Randomized placebo-controlled trials are urgently needed to optimize the use of rFVIIa in obstetric haemorrhage.

Sequential therapy with activated prothrombin complex concentrates and recombinant FVIIa in patients with severe haemophilia and inhibitors: update of our previous experience

Haemophilia patients with inhibitors can develop bleeding episodes, which are refractory to monotherapy with either recombinant factor VIIa (rFVIIa) or activated prothrombin complex concentrates (APCC). Management of such bleeds is often difficult. We previously reported the safety of using a combination of rFVIIa and APCC given in sequential fashion. In this report, we update our experience with sequential therapy. A retrospective review of medical records was conducted including all reports of sequential therapy defined as receiving both rFVIIa and APCC within 6 h. Data extracted included demographic data, treatment prior to and following hospital admission, clinical data including type and location of bleed, response to therapy, physical examination and laboratory data. In addition, for some patients, thromboelastography was performed to document the effect of sequential therapy on clot formation characteristics. Four patients comprising 35 admissions, 209 hospital days and 115 days of sequential therapy were included in the updated dataset. No patient developed thrombosis or overt disseminated intravascular coagulation (DIC) although elevations in the D-dimer above 5 microg mL(-1) were noted in 42% of the courses that lasted >3 days. Efficacy is suggested by the fact that patients had resolution of their bleeds after a median of 3 days of sequential therapy after failing to respond to a median of 3 days of monotherapy. Thromboelastography demonstrated an additive effect. Sequential therapy is a safe, potentially efficacious approach in the management of refractory bleeding episodes in patients with haemophilia and inhibitors.

Preferential localization of recombinant factor VIIa to platelets activated with a combination of thrombin and a glycoprotein VI receptor agonist

BACKGROUND

Activation of platelets with a combination of collagen and thrombin generates a subpopulation of highly procoagulant 'coated' platelets characterized by high surface expression of fibrinogen and other procoagulant proteins.

OBJECTIVES

To analyze the interaction of recombinant factor VIIa (rFVIIa) with coated platelets.

METHODS AND RESULTS

rFVIIa localized to the coated platelets in flow cytometry experiments, while minimal rFVIIa was found on platelets activated with adenosine diphosphate, thrombin or via glycoprotein VI individually, and essentially no rFVIIa was found on non-stimulated platelets. Removal of the gamma-carboxyglutamic acid (Gla) domain of rFVIIa, and addition of EDTA, annexin V or excess prothrombin inhibited rFVIIa localization to the coated platelets, indicating that the interaction was mediated by the calcium-dependent conformation of the Gla domain and platelet exposure of negatively charged phospholipids. A reduced level of platelet fibrinogen exposure was observed at hemophilia A-like conditions in a model system of cell-based coagulation, indicating that coated platelet formation in hemophilia may be diminished. Addition of rFVIIa dose-dependently enhanced thrombin generation and partly restored platelet fibrinogen exposure.

CONCLUSIONS

The data suggest that rFVIIa localized preferentially on platelets activated with dual agonists, thereby ensuring enhanced thrombin generation localized at the site of injury where both collagen and tissue factor are exposed, the latter ensuring the formation of thrombin necessary for coated platelet formation.

Proactive administration of platelets and plasma for patients with a ruptured abdominal aortic aneurysm: evaluating a change in transfusion practice

BACKGROUND

Continued hemorrhage remains a major contributor of mortality in massively transfused patients and those who survive have a higher platelet (PLT) count and a shorter prothrombin time and activated partial thromboplastin time (APTT) than nonsurvivors. It was considered that early substitution with PLTs and fresh-frozen plasma (FFP) would prevent development of coagulopathy and thus improve survival.

STUDY DESIGN AND METHODS

Survival of patients undergoing surgery for a ruptured abdominal aortic aneurysm (rAAA) was compared after implementing a proactive transfusion therapy encompassing two pooled buffy-coat PLT concentrates (PBPCs) immediately when a rupture of the aorta was suspected and again 30 minutes before aortic unclamping together with FFP administered in a 1:1 ratio to the amount of red blood cells (RBCs) with that of a control group receiving transfusion therapy according to existing recommendations.

RESULTS

The intervention group (n = 50) had a higher PLT count at arrival at the intensive care unit compared to the control group (n = 82; 155 x 10(9)/L vs. 69 x 10(9)/L; p < 0.0001), shorter APTT (39 sec vs. 44 sec; p < 0.001), fewer postoperative transfusions (RBCs, 2 vs. 6; FFP, 2 vs. 4; and PBPCs, 0 vs. 1; p < 0.01), and a higher 30-day survival rate (66% vs. 44%; p = 0.02).

CONCLUSION

This study suggests that proactive administration of PLTs and FFP improves coagulation competence, reduces postoperative hemorrhage, and increases survival in massively bleeding rAAA patients.

Diagnostic approach to inherited bleeding disorders

The appropriate development of hemostasis encompasses a delicate equilibrium between anti- and prothrombotic forces developing during three distinct phases (primary hemostasis, coagulation and fibrinolysis) that are closely linked to each other and precisely regulated to close vessel wounds, promote vascular healing and maintain vessel patency. Imbalance in each of these systems produces either hemorrhagic or thrombotic disorders. Inherited bleeding disorders, caused by quantitative or qualitative alterations of either platelets or plasma proteins involved in blood coagulation and fibrinolysis, may lead to serious and lifelong bleeding conditions, the severity of which is inversely associated with the degree of the underlying defect. Rapid and reliable identification of these pathologies is worthy of focus to allow the adoption of appropriate substitutive or supportive antihemorrhagic therapies. Evaluation of the hemorrhage-prone patient requires careful recording of the medical history, attention to pertinent physical findings and the discretionary use of laboratory resources. Owing to the low diagnostic efficiency of clinical history and examination, an appropriate and reliable laboratory approach, encompassing first- and second-line testing, is essential to screen, diagnose and monitor patients with bleeding diatheses. As both the analytical sensitivity and responsiveness of traditional coagulation assays to different abnormalities differ widely, each laboratory should establish individual guidelines based on field experience and on reagent and instrument characteristics. Emerging evidence indicates that the implementation of global coagulation tests, such as the thrombin generation assay and clot waveform analysis, would provide additional information for clinical decision-making for patients with inherited bleeding disorders.

Thrombin generation and fibrin clot structure

Generation of a hemostatic clot requires thrombin-mediated conversion of fibrinogen to fibrin. Previous in vitro studies have demonstrated that the thrombin concentration present at the time of gelation profoundly influences fibrin clot structure. Clots formed in the presence of low thrombin concentrations are composed of thick fibrin fibers and are highly susceptible to fibrinolysis; while, clots formed in the presence of high thrombin concentrations are composed of thin fibers and are relatively resistant to fibrinolysis. While most studies of clot formation have been performed by adding a fixed amount of purified thrombin to fibrinogen, clot formation in vivo occurs in a context of continuous, dynamic changes in thrombin concentration. These changes depend on the local concentrations of pro- and anti-coagulants and cellular activities. Recent studies suggest that patterns of abnormal thrombin generation produce clots with altered fibrin structure and that these changes are associated with an increased risk of bleeding or thrombosis. Furthermore, it is likely that clot structure also contributes to cellular events during wound healing. These findings suggest that studies explicitly evaluating fibrin formation during in situ thrombin generation are warranted to explain and fully appreciate mechanisms of normal and abnormal fibrin clot formation in vivo.

A variant of recombinant factor VIIa with enhanced procoagulant and antifibrinolytic activities in an in vitro model of hemophilia

OBJECTIVE

Recombinant factor VIIa (rFVIIa, NovoSeven) has proven efficacy in treating bleeding in hemophilia patients with inhibitors. A rFVIIa analog with mutations V158D/E296V/M298Q (NN1731) exhibits increased procoagulant activity in in vitro and in vivo models. The aim of this work was to define the effects of NN1731 toward factor X activation, platelet activation, thrombin generation, and fibrin clot formation and stability.

METHODS AND RESULTS

In a cell-based in vitro model of hemophilia, rFVIIa and NN1731 similarly increased factor X activation on tissue factor-bearing cells; however, NN1731 exhibited 30-fold higher factor Xa generation on platelets than similar rFVIIa concentrations. NN1731-mediated thrombin generation depended on platelet activation, but NN1731 did not directly activate platelets. NN1731 produced 4- to 10-fold higher maximal thrombin generation rates than equal rFVIIa concentrations. Both rFVIIa and NN1731 shortened clotting times in the absence of factors IX and VIII; however, NN1731 did so at 50-fold lower concentrations than were required of rFVIIa. In fibrinolytic conditions, both rFVIIa and NN1731 increased fibrin formation and stability; however, NN1731 was effective at 50-fold lower concentrations than were required of rFVIIa.

CONCLUSIONS

By increasing factor Xa generation, NN1731 promotes the formation of thrombin and a stable clot to a greater degree than rFVIIa.

Basic aspects of bypassing agents

Bypassing agents consist of activated prothrombin complex concentrates (aPCC) and recombinant factor VIIa (rFVIIa). Their main utilization is for prevention and treatment of bleeding complications, which may occur in inhibitor-developing haemophiliacs, although new indications for rFVIIa (e.g. trauma-related and cerebral bleeds) are now under evaluation in clinical trials. The mechanisms of action for these agents are still not fully understood. The relative complexity of the composition of aPCC suggests the possibility of multiple modes of action for achieving haemostasis. Among those possibilities, the contributions of activated factor X and prothrombin have been demonstrated in recent years both in vitro and in animal models for the only aPCC which remains on the market. rFVIIa also exhibits a complex mode of action, improving coagulation through both tissue factor-dependent and -independent pathways. The various mechanisms that occur at the cellular surfaces, particularly on the outer leaflet of the platelet membrane, primarily contribute to Xase complex formation and thrombin generation. The ways in which these agents affect the complex kinetics of fibrin formation at the site of vascular damage need further clarification, although significant progress has been achieved in the last 10 years. In addition, the ex vivo monitoring that would reflect achievement of haemostasis in vivo is still not standardized, although several attempts using thromboelastography, thrombin generation and the kinetics of fibrin formation have been initiated.

Individualization of bypassing agent treatment for haemophilic patients with inhibitors utilizing thromboelastography

The treatment of bleeding for haemophilic patients with inhibitors relies on the use of the bypassing agents, recombinant factor VIIa and factor eight inhibitor bypass activity (FEIBA). While both therapies are effective in the majority of bleeding episodes, there is a significant amount of interindividual variability when it comes to the response to therapy. As of yet, there is no reliable laboratory parameter that can predict the response to therapy in the same manner that factor VIII and factor IX levels predict response in non-inhibitor patients. Developing such a laboratory parameter is vital in order to maximize the clinical efficacy of these agents. Thromboelastography (TEG) is a device, which assesses clot formation over time in whole blood and has several characteristics which suggest it may be an effective way to monitor bypass agent therapy. We studied the ability of TEG to individualize the treatment regimens of three patients with high titre inhibitors assessing the response to recombinant factor VIIa, FEIBA, and when both were used sequentially. The TEG allowed for individualization of treatment for each of the three patients and resulted in more effective, convenient and less expensive treatment regimens. We thus believe that TEG is a promising device for monitoring of bypass agent therapy and should be studied further.

Using aptamers as capture reagents in bead-based assay systems for diagnostics and hit identification

Most applications of xMAP (Luminex) bead-based assay technology in diagnostics and drug discovery use immobilized antigens or antibodies. Here the authors describe the development of novel assay systems in which synthetic oligonucleotides that specifically bind and inhibit other biomolecules--so-called aptamers--are directly immobilized on beads. The robustness, specificity, and sensitivity of aptamer-based assays were demonstrated in a test system that detected human alpha-thrombin in serum samples. xMAP technology was also adapted to competitive screening formats where an aptamer/protein complex was disrupted by a functionally analogous competitor. The results indicate that such assays are excellently suited for diagnostic applications or drug screening, where aptamers serve as competitive binding probes for the identification of small-molecule hits. These methods should be transferable to a large number of applications because specific aptamers can be rapidly generated for almost any protein target.

Manipulation of prothrombin concentration improves response to high-dose factor VIIa in a cell-based model of haemophilia

Clinical reports suggest that treatment regimens employing both activated prothrombin complex concentrates (aPCCs) and recombinant activated factor VII (rFVIIa) may control the bleeding in patients with haemophilia who fail to respond to either agent alone. We hypothesised that increased concentrations of prothrombin, as may be observed after the infusion of aPCCs, favourably influence parameters of thrombin generation in haemophilia treated with high-dose rFVIIa. We examined the effect of varied prothrombin and rFVIIa concentrations on thrombin generation in a model of haemophilia. At all concentrations of rFVIIa, increased prothrombin concentrations led to increases in the peak and rate of thrombin generation. In assays with the highest concentrations of prothrombin and rFVIIa, peak thrombin actually equalled that measured in the model of normal haemostasis. The significant impact of prothrombin concentration on the effect of rFVIIa in vitro may explain the improved haemostasis reported with concurrent use of aPCCs and rFVIIa. These results imply that persons with plasma prothrombin levels at either end of the 'normal' range could have significantly different responses to similar rFVIIa doses. Furthermore, these results suggest that increasing plasma prothrombin concentration prior to rFVIIa administration may offer advantages over the use of rFVIIa alone in the treatment of haemophilic bleeding.

The effect of uremia on platelet contractile force, clot elastic modulus and bleeding time in hemodialysis patients

INTRODUCTION

Uremic bleeding frequently occurs in dialysis patients. Although its mechanism is not well characterized, acquired platelet dysfunction has been implicated in its pathogenesis. Skin bleeding time has been used to characterize platelet dysfunction in this population. However, the bleeding time is prone to error. The goal of this study was to compare the bleeding time to the novel platelet function parameters platelet contractile force and clot elastic modulus as well as platelet aggregation studies in controls and patients receiving maintenance hemodialysis.

MATERIALS AND METHODS

Forty-five subjects completed this study (25 controls, 20 dialysis). All subjects had the Ivy skin bleeding time procedure performed, as well as the collection of whole blood samples for the determination of platelet contractile force, clot elastic modulus, % von Willebrand Factor antigen, and platelet aggregation studies. Pearson's correlation determined the relationships between skin bleeding time and platelet function and clot structure parameters and markers of renal dysfunction.

RESULTS

Bleeding time was significantly prolonged in the dialysis group relative to controls. The platelet function parameters were not significantly different between groups. There was a significant relationship between bleeding time and creatinine concentration, however, no relationship existed between bleeding time and platelet function parameters.

CONCLUSIONS

Skin bleeding time poorly correlates with measurements of platelet function. There were no significant differences noted in platelet function between the groups despite the prolongations in bleeding time in the dialysis group. These data may suggest that the bleeding time reflects perturbations in platelet adhesion or secretion, and not aggregation. Further study is needed to characterize platelet function in dialysis patients.

A critical appraisal of the use of recombinant factor VIIa in acquired bleeding conditions

Recombinant activated factor VII (rVIIa) was initially used for the treatment of inhibitors in patients with haemophilia. However, its localised mode of action at sites of damage to the vessel combined with dramatic clinical observations in exsanguinating patients have resulted in huge interest in its use as a global haemostatic agent. Although it appears safe, with no obvious excess of thrombotic events, its use in assorted acquired bleeding disorders, especially those associated with the development of complex coagulopathies, such as in the post-trauma state, has proved to be less dramatic than hoped.

Thrombin generation time is a novel parameter for monitoring enoxaparin therapy in patients with end-stage renal disease

BACKGROUND

Patients with end-stage renal disease (ESRD) who receive enoxaparin are at increased risk for adverse bleeding episodes. This phenomenon appears to occur despite judicious monitoring of antifactor Xa (aFXa) activity. Better monitoring parameters are needed to quantify the anticoagulant effects of enoxaparin in the ESRD population.

OBJECTIVES

The objective of this study was to determine the utility of using thrombin generation time (TGT), platelet contractile force (PCF) and clot elastic modulus (CEM) to monitor the degree of anticoagulation in ESRD subjects, and to compare these results to aFXa activity, the current gold-standard monitoring parameter.

METHODS

Eight healthy volunteers without renal dysfunction and eight ESRD subjects were enrolled into this study. Subjects received a single dose of enoxaparin 1 mg kg(-1) subcutaneously, and blood samples were obtained for the determination of aFXa activity, TGT, PCF and CEM at baseline, 4, 8, and 12 h postdose.

RESULTS

Baseline, 4, 8, and 12-h aFXa activity concentrations were not different between groups. However, the corresponding TGT at 8 and 12 h was significantly prolonged in the ESRD group (P = 0.04, and P = 0.008, respectively). The 4-h peak TGT trended toward significance (P = 0.06). There were no differences in PCF or CEM across time.

CONCLUSIONS

These data suggest that the parameter aFXa activity is a poor predictor of the anticoagulant effect of enoxaparin in patients with ESRD. Thrombin generation time appears to be more sensitive to the antithrombotic effects of enoxaparin in this population. Further large-scale trials are needed to corroborate these data.

High dose factor VIIa improves clot structure and stability in a model of haemophilia B

Factor IX (FIX) deficiency results in haemophilia B and high dose recombinant activated factor VII (rFVIIa) can decrease bleeding. Previously, we showed that FIX deficiency results in a reduced rate and peak of thrombin generation. We have now used plasma and an in vitro coagulation model to examine the effect of these changes in thrombin generation on fibrin clot structure and stability. Low FIX delayed the clot formation onset and reduced the fibrin polymerisation rate. Clots formed without FIX were composed of thicker fibrin fibres than normal. rFVIIa shortened the clot formation onset time and improved the fibre structure of haemophilic clots. We also examined clot formation in the presence of a fibrinolytic challenge by including tissue plasminogen activator or plasmin in the reaction milieu. In these assays, normal FIX levels supported clot formation; however, clots did not form in the absence of FIX. rFVIIa partially restored haemophilic clot formation. These results were independent of the effects of the thrombin-activatable fibrinolysis inhibitor. Our data suggest that rFVIIa enhances haemostasis in haemophiliacs by increasing the thrombin generation rate to both promote formation of a structurally normal clot and improve clot formation and stability at sites with high endogenous fibrinolytic activities.

Tailoring haemostatic treatment to patient requirements - an update on monitoring haemostatic response using thrombelastography

Currently, there is no single haemostasis laboratory test that has the capacity to accurately illustrate the clinical effects of procoagulant or anticoagulant interventions. Although the time course of thrombin generation in plasma and the endogenous thrombin potential (ETP) may be useful coagulation parameters, clotting involves components other than thrombin (e.g. platelets, fibrinogen). The continuous coagulation profiles of thrombelastography may provide a more accurate reflection of in vivo biology, covering initiation, development and final clot strength during whole blood clot formation. This method has helped to clarify the mechanism of action of whole blood clot formation, demonstrating the differences from clotting in plasma, and the importance of platelets and tissue factor titrations. It has also been used to investigate hypocoagulation (in haemophilia A, rare coagulation disorders, anticoagulant therapy and dilutional coagulopathy), hypercoagulation and the ex vivo testing of haemostatic interventions. Thrombelastography has been shown to reflect the clinical efficacy of activated prothrombin complex concentrate (aPCC) and recombinant activated factor VII (rFVIIa) in patients with haemophilia A with inhibitors and in patients with acquired haemophilia. Overall, tailoring laboratory assays to illustrate and correlate with clinical phenotypes is essential for effective coagulation monitoring. Applying an algorithm of preoperative, perioperative and postoperative tests, including thrombelastography, may enable physicians to achieve this.

What does it take to make the perfect clot?

The coagulation process has been conceptualized as being primarily dependent on adequate levels of the coagulation proteins. This concept was based on the clear relationship between the bleeding tendency and factor levels in hemophilia. The field is now evolving toward conceptualizing coagulation as being actively regulated by the specialized cellular components of the process. Rather than conceiving coagulation as only a "cascade" of proteolytic reactions, the coagulation reactions occur as overlapping steps on cell surfaces. Components of the old "extrinsic'" and "intrinsic" pathways of coagulation can be thought of as participating in the initiation and propagation of coagulation reactions, respectively. Thus, these pathways are not redundant as they are portrayed in the cascade model, but play distinct and complementary roles. Our understanding of how specific cellular features control the processes of hemostasis and thrombosis is developing rapidly. This review discusses some aspects of the cellular control of coagulation.

Towards a standardization of thrombin generation assessment: the influence of tissue factor, platelets and phospholipids concentration on the normal values of Thrombogram-Thrombinoscope assay

Background

Thrombin generation assay was developed several years ago to mimic physiological coagulation mechanisms but it had important limitations. Thrombogram-Thrombinoscope assay using a fluorogenic substrate, allows obtaining thrombin generation curves in non-defibrinated platelet rich plasma (PRP) in a fully automated manner.

Methods

We standardised the methodology of Thrombogram-Thrombinoscope and we evaluated the precision of thrombin generation parameters (lag-time, maximum concentration of thrombin [Cmax], time required to reach Cmax [Tmax] and endogenous thrombin potential ETP) using different concentrations of recombinant human tissue factor, platelets or phospholipids. Normal values of thrombin generation assay were established in optimal experimental conditions.

Results

In the presence of low TF concentrations (final dilution of thromboplastin in plasma: 1/1000–1/2000) the Thrombogram assay showed intra-assay and inter-assay coefficients of variation lower than 9%. Thrombin generation parameters showed an important inter-individual variability and the coefficients of variation ranged from 18% to 50%. In PRP the lag-time, Cmax and Tmax but not the ETP, were influenced by TF concentration. Thrombin generation parameters were not influenced by variations of platelet concentration from 50 × 10⁹/l to 400 × 10⁹/l. The addition of synthetic procoagulant phospholipids in PPP strongly influenced all the parameters of thrombogram. For all the parameters of thrombogram a threshold effect was observed in the presence of phspholipid concentrations equal or higher to 4 μM. In frozen-thawed PRP the lag-time and the Tmax were significantly reduced and the Cmax was increased compared to the fresh PRP, but the ETP, the intra assay and the inter-assay coefficients of variation were similar in both test-systems.

Conclusion

Thrombogram-Thrombinoscope assay performed in fresh or in frozen-thawed PRP has an acceptable precision, with low inter-assay and intra-assay coefficient of variations. The concentration of TF is determinant for the normal values of the studied parameters of thrombin generation. When the assay is performed in PPP, thrombin generation parameters are influenced by the concentration of procoagulant synthetic phospholipids. The optimal experimental conditions were obtained in the presence of 1/1000 final dilution of thromboplastin, a platelet count higher than 50 × 10⁹/l and a synthetic phospholipid concentration higher than 4 μM.

Thromboelastography measurements of whole blood from factor VIII-deficient mice supplemented with rFVIII

The rotational thromboelastography (ROTEG) assay system allows the real-time analysis of clot formation (fibrin formation) in a whole-blood assay format. The ROTEG system provides significant advantages over the current plasma-based assay systems as it includes the important interactions between cellular and plasmatic coagulation factors. We have employed the ROTEG system to characterize clot formation dynamics in factor VIII- deficient mouse whole blood and examined the ability of recombinant FVIII (rFVIII) supplementation to restore the normal phenotype. The ability to generate a clear dose-response relationship by adding rFVIII to FVIII-deficient murine whole blood (FVIII-/-) demonstrates the feasibility of this approach. A dose-response from 1 U to 0.00001 U mL(-1) demonstrates the enhanced sensitivity of the ROTEG system. Further characterization of this experimental approach may provide a potential tool for comparing the activity of FVIII concentrates and/or evaluating FVIII mutants.

Phenotyping the haemostatic system by thrombography--potential for the estimation of thrombotic risk

The aim of this paper is to review the thrombogram and its use for phenotyping the haemostatic system. The thrombogram can be readily obtained through Calibrated Automated Thrombography (CAT), using a commercially available fluorometer, dedicated software (Thrombinoscope) and a calibrator. Here we explore the possibility to use platelet-rich plasma (PRP) triggered with a low amount of recombinant human tissue factor (approximately 0.5 pM) and also explore the function of the protein C system by adding activated protein C (APC) or soluble recombinant thrombomodulin (TM). Examples are shown: inherited antithrombin (AT) and protein C deficiencies, and antiphospholipid antibodies.