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Background: Reliable assays to measure direct oral anticoagulant (DOAC) levels and their activity in critical situations are needed. Drug levels alone are not representative of the effect of DOACs on an individual's coagulation. We developed a technique that provides direct assessment of the global effect of rivaroxaban on the individual's coagulation in addition to plasma concentrations.Methods: DOAC concentrations were determined in fifty patients using rivaroxaban, with the new assay, Xross-CAT. The effect of rivaroxaban on coagulation (activity) was measured with thrombin generation (TG) in platelet poor plasma using 5 pM tissue factor on the same device. The levels were validated with the Biophen DiXal assay. The prothrombin time (PT) and dilute Russell viper venom time (dRVVT) were performed to estimate the effect on coagulation.Results: The variability of Xross-CAT was below 12%. Xross-CAT correlates well with Biophen DiXaI (r(s) = 0.885). The bias, determined by Bland-Altman analysis, was 4.9% and the Passing-Bablok equation was y = 1.1x - 2.1. The correlation of plasma levels with TG was moderate (ETP r(s) = -0.548; Peak r(s) = -0.559), as for the PT (r(s) = 0.739) and the dRVVT (r(s) = 0.692).Conclusions: Xross-CAT shows a good correlation with Biophen DiXaI that was previously confirmed to accurately assess rivaroxaban levels. Bleeding and thrombotic complications are not necessarily associated with drug levels and could be influenced by concomitant risk factors. The main benefit of Xross-CAT is that it can be performed simultaneously with thrombin generation, providing an overview of the global anticoagulation status of a patient in relation to circulating DOAC levels.

Background Bleeding is a feared adverse event during anticoagulant treatment. In patients on vitamin K antagonists, most bleedings occur with the international normalized ratio (INR) in the therapeutic range. Currently, identification of high-risk patients via laboratory methods is not reliable. In this systematic review, we assessed the ability of calibrated automated thrombin generation (CAT-TG) to predict bleeding in patients on anticoagulant treatment. Methods A systematic search was executed in three databases: Medline, Embase, and Cochrane. Results Seven studies were included; two were of good methodological quality. One study showed that patients on warfarin with INRs in range (2–3) admitted for hemorrhage (n = 28), had lower CAT-TG levels (endogenous thrombin potential [ETP]: 333 ± 89 nM/min) than patients on warfarin admitted for other reasons (ETP: 436 ± 207 nM/min; p Conclusion Studies investigating the direct association between decreased CAT-TG values and hemorrhagic events are rare. Therefore, the clinical consequences of low CAT-TG values remain to be further investigated.

Antiphospholipid syndrome (APS) is a condition in which the presence of antibodies against phospholipid-binding proteins is associated with thrombophilia and/or pregnancy morbidity. Although antiphospholipid antibodies have anticoagulant characteristics in vitro, they are associated with thromboembolic complications. Thrombin generation (TG) is a sensitive global test of coagulation, and elevated TG is associated with thrombosis. Increased TG can be caused by increased prothrombin conversion, decreased thrombin inactivation, or a combination of both. In this study, we measured TG in APS patients and healthy controls with and without vitamin K antagonist (VKA) treatment at 1 and 5 pM tissue factor and with thrombomodulin. Prothrombin conversion and thrombin inactivation were determined by thrombin dynamics analysis. The TG peak was increased in nontreated APS patients at 1 pM TF compared with nontreated controls. Prothrombin conversion was significantly increased in nontreated APS patients. In contrast, prothrombin conversion did not differ in controls and patients that were on VKA therapy. Thrombin inactivation was comparable between controls and APS patients in the presence and absence of VKAs. Both TG (peak and ETP) and prothrombin conversion were significantly higher in APS patients with prior thrombosis compared with patients without a history of thrombosis. In this study, we demonstrate that in APS, the hemostatic balance shifts toward a more prothrombotic phenotype due to elevated prothrombin conversion but unchanged thrombin inactivation rates. Within the group of APS patients, increased TG and prothrombin conversion are associated with a history of thrombosis.

Introduction Epidemiological studies suggest that hypobaric hypoxia at high altitude poses a risk for developing venous thromboembolism. The cause of this observed hypercoagulability remains unclear. Therefore, this study aimed to investigate the effect of hypobaric hypoxia at 3,883 m above sea level on thrombin generation and platelet activation. Methods After complying with medical ethical procedures, 18 participants were recruited, of whom 1 had to leave the study prematurely due to mild acute mountain sickness. Blood was drawn first at 50 m above sea level and second at 3,883 m altitude after gradual acclimatization for 6 days. Thrombin generation was measured in whole blood, platelet-rich plasma and platelet-poor plasma. Platelet activation was assessed using a whole blood flow-cytometric assay. Coagulation factor levels, D-dimer levels and markers of dehydration and inflammation were measured. Results Hypobaric hypoxia at 3,883 m altitude caused increased thrombin generation, measured as peak height and endogenous thrombin potential, in whole blood, platelet-rich and platelet-poor plasma without or at low tissue factor concentration. The elevated thrombin generation was mediated by increased factor VIII levels and not caused by dehydration or inflammation. In contrast, spontaneous and agonist-induced platelet activation was decreased at high altitude. Conclusion Hypobaric hypoxia causes increased factor VIII-mediated thrombin generation. The hypercoagulability was balanced by decreased platelet activation. These findings may explain why venous, and not arterial thrombotic events occur more frequently at high altitude.

Until recently, vitamin K antagonists (VKAs) were the mainstay of oral anticoagulant treatment with bleeding as the most prevalent adverse effect. One to four percent of patients experience major bleeding episodes, while clinically relevant bleeding occurs in up to 20%. At this moment no laboratory assays are available to identify patients at risk for bleeding. With this study we aimed to investigate whether thrombin generation tests might identify a bleeding risk in patients taking VKAs. This prospective cohort study included 129 patients taking VKAs for more than three months. Calibrated automated thrombinography (CAT) was performed in whole blood, platelet rich and platelet poor plasma. Hematocrit, hemoglobin concentrations and the International Normalized Ratio (INR) were defined and coagulation factor levels were measured. Forty clinically relevant bleeding episodes were registered in 26 patients during follow-up. No differences were found in plasma CAT parameters or INR values. Bleeding was not associated with age, sex, hematocrit, hemoglobin levels or coagulation factor levels. In whole blood a significantly lower endogenous thrombin potential (ETP) and peak were found in patients with bleeding (median ETP: 182.5 versus 256.2 nM. min, p = 0.002; peak: 23.9 versus 39.1 nM, p = 0.029). Additionally, the area under the receiver operating curve (AUC ROC) was significantly associated with bleeding (ETP: 0.700, p = 0.002; peak: 0.642, p = 0.029). HAS-BLED scores were also significantly higher in bleeding patients (3 versus 2, p = 0.003), with an AUC ROC 0.682 (p = 0.004). In conclusion, bleeding in patients taking VKAs is associated with a decreased whole blood ETP and peak as well as with an increased HAS-BLED score.

SummaryPhysical exercise is recommended for a healthy lifestyle. Strenuous exercise, however, may trigger the haemostatic system, increasing the risk of vascular thrombotic events and the incidence of primary cardiac arrest. Our goal was to study the effects of strenuous exercise on risk factors of cardiovascular disease. Blood was collected from 92 healthy volunteers who participated in the amateur version of the protour Amstel Gold cycling race, before and directly after the race. Thrombin generation showed a shortening of the lag time and time to peak and an increase of the velocity index. Interestingly, the endogenous thrombin potential measured in plasma decreased due to reduced prothrombin conversion. Platelet reactivity increased and this effect was stronger in men than in women. Lower fibrinogen and higher D-dimer levels after exercise indicated higher fibrin formation. On the other hand, fibrinolysis was also elevated as indicated by a shortening of the clot lysis time. Exercise activated the endothelium (von Willebrand factor (VWF) and active VWF levels were elevated) and the immune system (concentrations IL-6, IL-8, MCP-1, RANTES and PDGF increased). Additionally, an increased cardiac troponin T level was measured post-exercise. Strenuous exercise induces a temporary hyperreactive state in the body with enhanced proand anticoagulant responses. As strenuous exercise has a more pronounced effect on platelet function in male subjects, this gives a possible explanation for the higher incidence of sudden cardiac death during exercise compared to women. This trial is registered at www.clinicaltrials.gov as NCT02048462.

Hypoxia (oxygen deprivation) is known to be associated with deep vein thrombosis and venous thromboembolism. We attempted to get a better comprehension of its mechanism by going to high altitude, thereby including the potential contributing role of physical activity. Two groups of 15 healthy individuals were exposed to hypoxia by going to an altitude of 3900 meters, either by climbing actively (active group) or transported passively by cable car (passive group). Both groups were tested for plasma fibrinogen, von Willebrand factor and factor VIII levels, fibrinolysis, thrombin generating capacity, heart rate, oxygen saturation levels and blood pressure. As a control for the passive group, 7 healthy volunteers stayed immobile in bed for 7 days at normoxic conditions. The heart rate increased and oxygen saturation levels decreased with increasing altitude. Fibrinolysis and fibrinogen levels were not affected. Factor VIII and von Willebrand factor levels levels increased significantly in the active group, but not in the passive group. Plasma thrombin generation remained unchanged in both the active and passive group with increasing altitude and during 7 days of immobility in healthy subjects. However, by applying whole blood thrombin generation, we found an increased peak height and endogenous thrombin potential, and a decreased lagtime and time-to-peak with increasing levels of hypoxia in both groups. In conclusion, by applying whole blood thrombin generation we demonstrated that hypoxia causes a prothrombotic state. As thrombin generation in plasma did not increase, our results suggest that the cellular part of the blood is involved in the prothrombotic phenotype induced by hypoxia.

Introduction Infants and children have a lower incidence of thrombosis compared with adults. Yet, the mechanism of blood clot formation and structure in infants and children, as the end product of coagulation, has not been studied. This study aimed to establish differences in the mechanism of thrombin generation, fibrin clot formation and response to thrombolysis in infants and children compared with adults. Materials and methods We studied thrombin generation, fibrin clot formation, structure and fibrinolysis in healthy infants, children and adults. Results Younger populations had a decreased potential to generate thrombin, at a slower velocity compared with adults, correlating positively with age. Clot formation at venous shear rate was decreased in infants and children compared with adults, with increased time for fibrin formation, decreased fibrin formation velocity, resulting in decreased tendency for fibrin formation in younger populations. These differences were less pronounced at arterial shear rate. Studies of the fibrin clot structure in paediatric age groups showed a significantly larger pore size compared with adults, suggestive of a clot that is less resistant to fibrinolysis. The presence of tissue plasminogen activator (tPA) resulted in a significant decrease in the pore size of infants and children, but not in adults. Conclusions This is the first study to suggest that the mechanism of blood clot formation and nanostructure, as well as response to thrombolytic therapy is different in infants and children compared with adults.

Current practice in the prevention of venous thromboembolism (VTE) is either controlled dosage of vitamin K antagonists (VKA) or standard dosage of a low molecular weight heparin (LMWH) or an orally administrated direct inhibitor of thrombin or factor Xa (DOACs). This is justified by clinical trials showing the non-inferiority of standard dosage to VKA treatment - disregarding that the latter could be improved upon. Whether standard dosage is optimal for the individual patient depends upon how well (s)he is represented by the "average" patient that meets the inclusion criteria of the trial. Here we show that in a set of normal plasmas the individual thrombin generating power (the endogenous thrombin potential: ETP), after spiking with a fixed concentration (~ IC50) of different anticoagulants show such a wide variation that, even when plasma levels would be identical, a considerable percentage of patients could be over- or under-anticoagulated. Consequently adapted doses must be considered if better results than those with VKA are our aim. We recall that the ETP is close to constant in the individual person but varies in the population with a broad log-normal distribution (CV 16%). The ETP is highly correlated to the risk of thrombosis: The relative risk of VTE in the upper quartile being 5 - 7 times higher than that in the lower one (Winckers K., et al., poster PO617, ISTH 2015, Toronto). Reducing the ETP is the common feature of all antithrombotic therapy. The desired range of reduction is not exactly known. In congenital bleeding disorders bleeding risks increase sharply at ETP below 33%. In monitoring VKA treatment an INR below 2, which corresponds to an ETP > 66% of normal, is generally considered inadequate. We therefore arbitrarily choose the range 33 - 66% of normal, as a target range (TR). Experimental: Individual plasmas from 60 normal healthy volunteers were spiked with a fixed amount of either unfractionated heparin (UFH), low molecular weight heparin (LMWH), antithrombin binding pentasaccharide (penta), DOAC acting on thrombin or DOAC acting on factor Xa. We found the residual ETP to be highly variable (table column 3), obviously because the variation in susceptibility to the inhibitor superimposes upon the natural variation. Using these CVs we calculated what % falls either above or below the target range. The total variance of the ETP under treatment is the combined effect of natural variance, the pharmacokinetic- and the pharmacodynamic -variance. Here we determined pharmacodynamic effects only and calculated the % of ETP values that would fall outside the target range A: if there would be no pharmacokinetic variance (table column 4 & 5) and B: If the pharmacokinetic variance would equal the pharmacodynamic variance (table column 6 & 7). It is seen that in any case over 15% of the population will be outside the target range and that in the more likely case that pharmacokinetic variation counts as much as pharmacodynamic variation does around half of all patients will be outside the safe range. This could be avoided by measuring the effect of a standard dose of anticoagulant on the ETP once and increasing the dose in those with an ETP > 66% and decreasing it in those with ETP < 33%. The present work is meant to provide the rationale for starting clinical studies on the actual variation of the ETP attained under standard dosage of different anticoagulants and the effects thereon of personalised dose adjustment. Table Table. Disclosures Hemker: Diagnostica Stago: Consultancy.

Background: The antiphospholipid syndrome (APS) is characterized by the presence of antiphospholipid antibodies directed mainly against prothrombin and β2-glycoprotein I. The syndrome is associated with an increased risk of thrombosis. The global hemostatic state in a patient can be tested by measuring thrombin generation (TG). Recently, we developed a method to study the main pro- and anticoagulant processes at the basis of TG, called the thrombin dynamics method. Aim: In this study we investigated the dynamics of thrombin generation in healthy subjects and APS patients. Materials and methods: Healthy subjects (n=129) and antiphospholipid syndrome (APS) patients (n=31) were included in the study. Sixty-eight percent of the APS patients were lupus anticoagulant positive, anti-cardiolipin antibodies were detected in 84% of the patients, and 52% presented with anti-β2-glycoprotein I antibodies. Patients on anticoagulant therapy were excluded from the study. Thrombin generation was measured at 1 pM tissue factor (TF) and activated protein C (APC) system sensitivity was tested by measuring TG in the presence and absence of 20 nM thrombomodulin (TM). Results: Thrombin generation was measured in platelet poor plasma at 1 pM tissue factor. The lag time and time-to-peak were significantly prolonged in APS patients compared to healthy subjects (lag time: 3.30 ± 0.59 vs. 6.69 ± 4.26 min, p The pro- and anticoagulant processes underlying thrombin generation were studied separately. The total amount of prothrombin converted during thrombin generation (PCtot) did not differ between healthy subjects and patients (1234 vs. 1165 nM). However, the maximum rate of prothrombin conversion (PCmax) was significantly elevated in APS patients (291 vs 425 nM/min; p Thrombomodulin reduced the ETP by 45% in healthy subjects, but had significantly less effect in APS patients (10%). The addition of TM decreased total prothrombin conversion by 40% and the maximum prothrombin conversion rate by 50% in healthy subjects. In patients, TM only slightly reduced total prothrombin conversion (8%) and the maximum prothrombin conversion rate (7%). Discussion: The thrombin generation results indicate a predisposition to thrombosis in APS patients, as the TG parameters peak height and the velocity index are increased. Examination of the underlying pro- and anticoagulant processes of prothrombin conversion and thrombin inactivation revealed that although the same amount of prothrombin is converted in patients, the maximum activity of the prothrombinase complex is higher, indicating that patients can generate thrombin faster. In addition, APS patients have a dysfunctional APC system, as prothrombin conversion and thrombin generation could be only slightly inhibited by the addition of thrombomodulin. Disclosures No relevant conflicts of interest to declare.

Introduction: Thrombin generation (TG) assesses the hemostatic capacity of an individual and is indicative of a person's bleeding and thrombosis risk. TG dependson the balance between the pro- and anticoagulant processes in clotting plasma. Recently, we developed a method to study the main pro- and anticoagulant processes at the basis of TG, called the thrombin dynamics method. Vitamin K antagonist (VKA) treatment is known to reduce TG and that the reduction of TG correlates with the international normalized ratio (INR). In this study, we aim to investigate the effect of VKA therapy on the dynamics of TG by examining prothrombin conversion and thrombin inactivation. Materials and methods: TG was measured in platelet poor plasma at 1 pM tissue factor in 129 healthy subjects and 129 patients treated with VKA. The patients were classified according to the INR value: INR4 (n=19). Thrombin dynamics was calculated from TG curves and plasma antithrombin (AT), α2-macroglobulin (α2M) and fibrinogen levels. Three thrombin dynamics parameters were determined: the total amount of prothrombin conversion (PCtot), the maximum rate of prothrombin conversion (PCmax) and the thrombin decay capacity (TDC) of each plasma sample. Results: Vitamin K antagonist treatment reduced the ETP and peak height significantly at all INR values (p As previously reported, the thrombin decay capacity is mainly dependent on the plasma antithrombin and fibrinogen level. Plasma antithrombin levels were comparable between healthy subjects and patients on VKA (figure 2A) and showed a clear positive correlation with the thrombin decay constant in the patient group (R2=0.72, p Conclusion: Thrombin dynamics analysis shows that prothrombin conversion is reduced in patients treated with VKA and that thrombin inactivation is significantly attenuated. We demonstrate that the latter effect can be attributed to the elevated fibrinogen levels that accompany VKA treatment. Figure 1 Thrombin dynamics in healthy subjects and VKA patients. (A) Prothrombin conversion curves. (B) PCtot: total amount of prothrombin converted; (C) PCmax: maximum rate of prothrombin conversion; (D) TDC: thrombin decay capacity. *p Disclosures Hemker: Diagnostica Stago: Consultancy.

Background: In this study the value of thrombin generation parameters measured by the Calibrated Automated Thrombography for prediction of blood loss after cardiac surgery with cardiopulmonary bypass was investigated. Methods: Thirty male patients undergoing first-time coronary artery bypass grafting were enrolled. Blood samples were taken pre-bypass before heparinisation (T1) and 5 min after protamine administration (T2). Thrombin generation was measured both in platelet-rich plasma and in platelet-poor plasma. Besides thrombin generation measurements, activated clotting time, haematocrit, haemoglobin, platelet number, fibrinogen, antithrombin, D-dimers, prothrombin time and activated partial thromboplastin time were determined. Blood loss was measured and the amount of transfusion products was recorded postoperatively until 20 hours after surgery. Patients were divided into two groups based on the median volume of postoperative blood loss (group 1: patients with median blood loss

Introduction Cardiac surgery with cardiopulmonary bypass (CPB) causes hemodilution and coagulation factor consumption. The administration of unfractionated heparin accelerates thrombin inactivation and leads to elevated consumption of antithrombin (AT). The heparin-mediated release of tissue factor pathway inhibitor (TFPI) from the vessel wall reduces prothrombin activation. CPB surgery thus affects both pro- and anticoagulant processes, which are reflected in changes in a patient's thrombin generation potential. In this study we investigated how cardiac surgery changes thrombin generation (TG), prothrombin conversion and thrombin inactivation. In addition, we studied the consequences of these changes in pro- and anticoagulant processes for transfusion management using in silico experimentation. Methods Thirty patients undergoing cardiac surgery with CPB were included and samples were collected before and after surgery and protamine administration. AT and prothrombin levels were determined and TG was measured at 5 pM tissue factor by calibrated automated thrombinography. Prothrombin conversion and thrombin inactivation were quantified from each TG curve. The effect of transfusion of prothrombin complex concentrate (PCC) was simulated by an in silico increase of prothrombin conversion. The effect of PCC administration in combination with AT was modeled as an increase of prothrombin conversion and antithrombin levels. The effect of in silico transfusion of PCC on coagulation was quantified by TG parameters. Results CPB surgery causes a reduction of plasma coagulation factor levels: AT (47%, p We modeled the effect of the transfusion of prothrombin complex concentrate with or without added antithrombin on TG. If the prothrombin conversion capacity was increased to pre-surgery levels in the absence of antithrombin, TG peak height increases to 454 ± 72 nM (figure A). A slight increase of prothrombin conversion (up to 83% of the pre-surgery level) brings TG peak height into the normal range (386±61 nM), but the ETP remains elevated (3278 ± 1141 nM). The optimal way to restore thrombin generation is the simultaneous infusion of PCC and antithrombin. Our in silico experiments indicate that this restores both the peak height (362 ± 69 nM) and the ETP (1601 ± 369 nM∙min) to the pre-surgery level (figure B). Conclusions Cardiac surgery with CPB causes the hemostatic balance to shift toward bleeding due to a reduction of prothrombin conversion. This is only partially compensated by a reduction of thrombin inactivation, resulting in reduced TG after surgery. Post-surgery bleeding can be (and often is) remedied by the transfusion of PCC. However, full restoration of the prothrombin conversion capacity results in a TG profile that predicts a prothrombotic state, as an ETP value in the highest quartile is associated with a 5 times elevated thrombosis risk according to Winckers et al. Equilibrated restoration of the hemostatic capacity asks for the simultaneous supplementation of both pro- and anticoagulant factors, i.e. PCC containing AT or fresh frozen plasma. Figure 1. The effect of prothrombin complex concentrate (± antithrombin) transfusion on in silico thrombin generation. ■ Pre-surgery TG, ● Post-surgery TG, □ in silico PCC transfusion (left panel), and ○ in silico PCC and AT transfusion (right panel). Figure 1. The effect of prothrombin complex concentrate (± antithrombin) transfusion on in silico thrombin generation. ■ Pre-surgery TG, ● Post-surgery TG, □ in silico PCC transfusion (left panel), and ○ in silico PCC and AT transfusion (right panel). Disclosures Kremers: Synapse bv: Employment. Bloemen:Synapse bv: Employment. De Laat:Synapse bv: Employment. Wagenvoord:Synapse bv: Employment. Al Dieri:Synapse bv: Employment. Hemker:Synapse bv: Employment.

Introduction: During the last 50 years vitamin K antagonists (VKAs) have been widely used for the primary prevention of thromboembolism in patients with atrial fibrillation, or for the secondary prevention in patients with a history of venous thromboembolism. The most prevalent adverse effect of anticoagulant therapy is an increased risk of bleeding. Annually approximately 1-4% of patients treated with VKAs suffer from major bleeding episodes. In the past several attempts have been made to predict the bleeding risk for these patients. Among the methods used at this moment are clinical decision scores, of which the HAS-BLED score is the most common one. Up to this point there are no laboratory methods available to predict patients at risk for bleeding. Thrombin generation (TG) tests have been shown to provide an intermediate phenotype for thrombosis. Additionally, TG has the capacity to detect the anticoagulant effect of many, if not all, anticoagulants, including VKAs. This method was only applicable in plasma, but recently this TG test has been introduced in whole blood (WB). With this study we aimed to investigate whether TG, in plasma or WB, could detect bleeding in patients taking VKAs. Materials & methods: Blood samples were collected in citrate tubes from 129 patients taking VKAs for over 3 months, who were older than 18 years of age and followed over time for bleeding episodes. The patients signed informed consent forms and the study was approved by the local medical ethical committee. TG was determined in WB, platelet rich plasma (PRP) and platelet poor plasma (PPP) by means of calibrated automated thrombinography (CAT). Tissue factor was used as a trigger at a final concentration of 1 pM in WB and PRP and at both 1 pM and 5 pM with 4µM phospholipids in PPP. Hematocrit and the hemoglobin concentration were determined in WB. The PPP of the patients was used to define the International normalized ratio (INR). Results: In our study the average INR value of all patients was 2.95 (± 0.92 (SD)). Twenty six patients (20.2 %) suffered from 44 clinically relevant bleeding episodes during a mean follow-up of 15.5 months. Applying TG in PPP and PRP we found no differences in either endogenous thrombin potential (ETP), an indication for the total amount of thrombin that can be converted, nor peak height (the highest amount of active thrombin present during coagulation). Interestingly, when we applied TG in whole blood, so including the blood cells, we found a significantly lower ETP (p < 0.01) and peak (p < 0.05) in the patients that suffered from bleeding (median: 182.5 nM.min and 23.9 nM, respectively) compared to patients that did not bleed (median: 256.2 nM.min and 39.1 nM). When examining the INR, hematocrit and hemoglobin levels, no significant differences were detected. A receiver operating curve (ROC) was constructed for the ETP and peak (figure 1). By determining the area under the curve (AUC) of the ROC we found that the ETP and peak are significantly (p < 0.05) associated with the tendency to bleed (ETP, AUC: 0.700; peak, AUC: 0.642). This compares favorably with the AUC reported for the HAS-BLED score related to clinically relevant bleeding in patients on VKAs (0.60). Conclusions: The TG test in whole blood was only recently developed and this is the first study implementing it to test patients using VKAs. In our study TG measured in WB proved to be the first laboratory test that is able to detect patients at risk of bleeding when treated with VKAs. The INR and plasma TG (CAT-based method), on the other hand, did not discriminate between bleeding and non-bleeding patients. For WB TG, based on the AUC of the ROC, the ETP and peak in WB have a predictive value for bleeding in patients taking VKAs superior over the currently used HAS-BLED score. Figure 1: Analysis of patients with and without bleeding symptoms. Receiver operating curves (ROC) of the ETP and peak in WB TG. Figure 1:. Analysis of patients with and without bleeding symptoms. Receiver operating curves (ROC) of the ETP and peak in WB TG. Disclosures No relevant conflicts of interest to declare.

Introduction We tested whether the recently introduced measurement of thrombin generation (TG) in whole blood can be used to evaluate the clotting status of patients on vitamin K antagonist (VKA) prophylaxis. The prothrombin time, and hence the International Normalized Ratio (INR), only evaluates the vitamin K dependent factors II, VII and X but not the anticoagulant factors, protein C and S as well as factor IX. In TG all factors play their role and when thrombomodulin (TM) is added the function of proteins C and S is stressed. The thrombotic tendency in congenital protein C resistance proves the importance of this protein C pathway. Aim To compare the INR in samples from patients under VKA prophylaxis to TG in whole blood and in platelet rich and platelet poor plasma (PRP, PPP) both in the presence and in the absence of TM. Materials & Methods Blood samples were collected from 123 consenting patients on VKA. In two thirds (67%) the indication for prophylaxis was atrial fibrillation. Other indications included prosthetic valves, lung embolisms or thrombosis. The INR was determined in the PPP of the samples and the patients were stratified into 5 groups: INR of 1.0 to 1.5, 1.5 to 2.5, 2.5 to 3.5, 3.5 to 4.5 and higher than 4.5. Thrombin generation (TG) was measured via Calibrated Automated Thrombinography (CAT) in whole blood and in PRP and PPP, with and without 20 nM added TM. From the TG curve lag time and time to peak were obtained as well as the maximal thrombin concentration (peak) and the area under the curve (endogenous thrombin potential: ETP). Also red and white blood cells and platelets were counted. Results With increasing INR values, the ETP and peak height decrease and lag time and time to peak prolong. All TG parameters measured in whole blood were significantly correlated (p-values< 0.01) with the values determined in both PRP and PPP. INR was linearly correlated with lag time and time to peak (p-value< 0.01), whereas for the concentration dependent parameters (ETP and peak height) the correlation with the INR was hyperbolical (p-value< 0.01). In plasma, 20 nM TM causes a diminution of ETP and peak of 50-60 % in normals and in patients in the INR 1 – 1.5 group. At higher INR values inhibition is between 25 and 40%, independent of the INR value. In whole blood, on the contrary, the same concentration of TM causes around 30 % of inhibition in normals and in all patients alike. Conclusions Whole blood TG data correlate well with INR and reflect more of the coagulation mechanism than the INR does. Like the INR it does not reflect the function of the VKAs on the natural anticoagulant factors, however. In PPP and PRP addition of TM shows that VKA treatment induces TM resistance in patients with an INR value higher than 1.5. Disclosures: No relevant conflicts of interest to declare.

Introduction Age specific differences in the function and concentration of haemostatic proteins are well established and are thought to contribute to the lower incidence of thromboembolism in infants and children compared to adults. We hypothesized that there are age-specific differences in the fibrin clot nanostructure and that these differences impact on fibrinolytic and anticoagulant treatment. Aim To determine the differences in fibrin clot nanostructure and response to tissue Plasminogen Activator (tPA) and Unfractionated Heparin (UFH) in infants compared to adults. Materials and Methods Four plasma samples from infants (0.86±0.16 years) and four samples from adults (32.04±15.80 years) were investigated. Thrombin generation (TG) was performed on paper discs in a microplate with 300mmol/L rhodamine substrate (P2Rho), 4mmol/L phospholipids, 5x10-9 mmol/L tissue factor and 16.7 mmol/L CaCl2; with and without additional 50ng/ml tPA, and 0.01IU/ml UFH. Fluorescence signal was recorded with a Fluoroskan Ascent fluorometer, with samples tested in triplicate. The resulting fibrin clots were fixed with 2.5% glutaraldehyde; washed with PBS and fixed with osmium tetroxide OsO4. The clots were dehydrated in ethanol and hexamethyldisilazane and were placed on carbon adhesive tabs. Each clot was coated with gold and studied using Scanning Electron Microscopy (SEM) on a Phenom Pro desktop SEM. The age-specific differences in fibrin fibre thickness and pore size were quantified using the Fibermetric software, with 36 representative images for infants and the same number for adults at each baseline, tPA and UFH, at magnification 10.000, counting 500 fibres per image. Results We observed significant differences in fibrin clot nanostructure between infants and adults (Fig 1). Results are expressed as median and range. Fibrin fibers were thicker in infants compared to adults (343.3nm, 61.2nm to 997.1nm vs 352.3nm, 26.1nm to 997.5nm, p In the presence of tPA in the adult sample, the fibrin fibers were thicker (352.3nm, 26.1nm to 997.5nm vs 355.8nm, 70.0nm to 999.6nm, p0.05) and pore size (0.057pm2, 0.0041pm2 to 19.97pm2 vs 0.050pm2, 0.0041pm2 to 19.34pm2, p>0.05). In the presence of UFH in adults fibrin fibers were thicker (352.3nm, 26.1nm to 997.5nm vs 339.0, 66.9nm to 996.4nm, p Discussion and Conclusion Our study suggests that the fibrin clots in infants are weaker compared to those of adults and is consistent with the lower TG in infants. This is further supported by published evidence that clots produced with low thrombin concentrations are composed of thick fibrin fibres, whereas those generated with high thrombin concentrations are composed of a tight network of thin fibres. The effect of both tPA and UFH on fibrin clots nanostructure in adults is consistent with published literature and is investigated in children for the first time. The fact that tPA did not have an effect on the fibrin clots of infants is not surprising considering that the concentration of plasminogen is 39%, tPA is 40% and TAFI function is 60% of the adult value; while PAI function is 50% higher compared to adults. UFH had an effect of the fibrin clot nanostructure in both infants and adults, with the effect on pore size being more pronounced in adults. While developmental haemostasis has been understood for many years, this is the first study to provide definitive proof that the end product of coagulation is a different clot structure in children. This further enhances our understanding of why thrombosis are less common in children, and has significant implications for treatment. Disclosures: No relevant conflicts of interest to declare.

Introduction Developmental Haemostasis, the concept describing maturation of the haemostatic system is based on established age specific differences in the quantity and function of haemostatic proteins. Interestingly, infants and children have a lower incidence of thromboembolic events than adults. Understanding the precise mechanism of thromboprotection in infants and children may therefore provide novel prevention and treatment strategies for adults. We hypothesized that a significant component of the thromboprotective mechanism is contributed to by the age-specific differences in thrombin generation. Aim This study was designed to investigate differences in thrombin generation in different age groups from infants to adults, to obtain more insight into age related differences in thromboembolic rates. Materials and Methods Plasma samples from 86 infants, children and adults were tested individually and the results were grouped according to age: 1 month to 1 year (N=13), 1-5 years (N=20), 6-10 years (N=19), 11-16 years (N=16) and adults (N=18). Thrombin generation (TG) was measured using Calibrated Automated Thrombinography (CAT) triggered with tissue factor (TF) at 5pM and 1pM final concentration. Parameters measured from the TG curve included: lag time, maximal thrombin concentration (peak), time to peak, as well as the area under the curve (endogenous thrombin potential, ETP). The raw data from TG was also used to calculate the overall, as well as alpha-2-macroglobulin (A2M) dependent decay constants. A2M function was measured with a thrombin inhibition assay developed in house. Results Both ETP and peak thrombin in infants and children was consistently significantly lower than in adults at both 1pM and 5pM tissue factor concentrations. The difference in ETP ranged from 27% to 35% (p Discussion and Conclusions This study confirms that TG in children (including adolescents) is lower than in adults and shows for the first time that this is predominantly caused by higher thrombin inhibition, largely due to an increase of A2M activity. Considering that the levels of A2M are increased by 50%, Antithrombin is decreased by approximately 37%, whilst the levels of Prothrombin, Fibrinogen and Heparin Cofactor II remain the same in infants and children compared to adults; therefore A2M is an important inhibitor of thrombin in early life and during childhood. Age-specific differences in thrombin generation as demonstrated by our study possibly play an important role in the thromboprotective mechanism functioning in infants and children. Disclosures: No relevant conflicts of interest to declare.