COVID-19: Not a thrombotic disease but a thromboinflammatory disease.

While Coronavirus Disease in 2019 (COVID-19) may no longer be classified as a global public health emergency, it still poses a significant risk at least due to its association with thrombotic events. This study aims to reaffirm our previous hypothesis that COVID-19 is fundamentally a thrombotic disease. To accomplish this, we have undertaken an extensive literature review focused on assessing the comprehensive impact of COVID-19 on the entire hemostatic system. Our analysis revealed that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection significantly enhances the initiation of thrombin generation. However, it is noteworthy that the thrombin generation may be modulated by specific anticoagulants present in patients' plasma. Consequently, higher levels of fibrinogen appear to play a more pivotal role in promoting coagulation in COVID-19, as opposed to thrombin generation. Furthermore, the viral infection can stimulate platelet activation either through widespread dissemination from the lungs to other organs or localized effects on platelets themselves. An imbalance between Von Willebrand Factor (VWF) and ADAMTS-13 also contributes to an exaggerated platelet response in this disease, in addition to elevated D-dimer levels, coupled with a significant increase in fibrin viscoelasticity. This paradoxical phenotype has been identified as 'fibrinolysis shutdown'. To clarify the pathogenesis underlying these hemostatic disorders in COVID-19, we also examined published data, tracing the reaction process of relevant proteins and cells, from ACE2-dependent viral invasion, through induced tissue inflammation, endothelial injury, and innate immune responses, to occurrence of thrombotic events. We therefrom understand that COVID-19 should no longer be viewed as a thrombotic disease solely based on abnormalities in fibrin clot formation and proteolysis. Instead, it should be regarded as a thromboinflammatory disorder, incorporating both classical elements of cellular inflammation and their intricate interactions with the specific coagulopathy.

Fibrin Network Porosity and Endo-/Exogenous Thrombin Cross-talk.

The earliest assessment of fibrin network porosity used a liquid permeation system and confocal 3D microscopy, which was later replaced by scanning electron microscopy. Although the methods have extensively been applied in studies of health or disease, there remains debate on the choice of a proper clotting trigger. In this review, we assess published data and convey our opinions with regard to several issues. First, when the coagulation process is initiated by recombinant tissue factor (rTF) and phospholipids, the fibrin network porosity is regulated by the endogenous thrombin based on enzymatic activations of multiple coagulants. If purified thrombin (1.0 IU/mL) is employed as the clotting trigger, fibrin network porosity may be affected by exogenous thrombin, which directly polymerizes fibrinogen in plasma, and additionally by endogenous thrombin stemming from a "positive feedback loop" action of the added thrombin. Second, with use of either endogenous or exogenous thrombin, the concentration and clotting property of available fibrinogen both influence the fibrin network porosity. Third, in the assay systems in vitro, exogenous thrombin but not rTF-induced endogenous thrombin seems to be functional enough to activate factor XIII, which then contributes to a decrease in the fibrin network porosity. Fourth, fibrin network porosity determines the transport of fibrinolytic components into/through the clots and therefore serves as an indicator of the fibrinolysis potential in plasma.

The Clotting Trigger Is an Important Determinant for the Coagulation Pathway In Vivo or In Vitro-Inference from Data Review.

Blood coagulation comprises a series of enzymatic reactions leading to thrombin generation and fibrin formation. This process is commonly illustrated in a waterfall-like manner, referred to as the coagulation cascade. In vivo, this "cascade" is initiated through the tissue factor (TF) pathway, once subendothelial TF is exposed and bound to coagulation factor VII (FVII) in blood. In vitro, a diminutive concentration of recombinant TF (rTF) is used as a clotting trigger in various global hemostasis assays such as the calibrated automated thrombogram, methods that assess fibrin turbidity and fibrin viscoelasticity tests such as rotational thromboelastometry. These assays aim to mimic in vivo global coagulation, and are useful in assessing hyper-/hypocoagulable disorders or monitoring therapies with hemostatic agents. An excess of rTF, a sufficient amount of negatively charged surfaces, various concentrations of exogenous thrombin, recombinant activated FVII, or recombinant activated FIXa are also used to initiate activation of specific sub-processes of the coagulation cascade in vitro. These approaches offer important information on certain specific coagulation pathways, while alterations in pro-/anticoagulants not participating in these pathways remain undetectable by these methods. Reviewing available data, we sought to enhance our knowledge of how choice of clotting trigger affects the outcome of hemostasis assays, and address the call for further investigations on this topic.

A ROTEM method using APTT reagent and tissue factor as the clotting activators may better define bleeding heterogeneity in moderate or severe haemophilia A (part I: Study in plasma samples).

Bleeding heterogeneity observed in haemophilia A (HA) may attribute to that the available monitoring methods cannot appropriately reflect the coagulation profile. The present study aimed to develop a global approach by changing the clotting initiation way in rotational thromboelastometry (ROTEM) assay. ROTEM was run in Factor VIII (FVIII)-immune-depleted plasma to which different concentrations of recombinant VIII (rFVIII) had been added, and also in 31 patients with HA. The clotting activators were APTT reagent (1.2 × 10-3 of the dose used in the original APTT method) and recombinant tissue factor (0.02 pmol/L). In FVIII-immune-depleted plasma spiked with rFVIII, maximum velocity of coagulation reliably mirrored the rFVIII levels. This dose-response disappeared after the samples were pre-incubated with an antibody against TFPI, protein S, activated prothrombin complex concentrate or rFVIIa known to favour the extrinsic activation. In the HA patients with FVIII 0-0.21 IU/mL, APTT and ROTEM outcomes varied in significant correlations to FVIII activity; however, this correlation became non-significant when only samples with FVIII 0-0.05 IU/mL were included. Conclusions: The decreased coagulation in HA mostly result from deficiency/absence of FVIII; other pro-/anti-thrombotic proteins are also influential. The multiple effects may cause a mismatch between bleeding phenotype and FVIII concentrations. The ROTEM assay with the clotting activators i.e., tiny doses of APTT reagent and TF are more effective than the original APTT method as regards the assay sensitivity to influence by VIII activity and also to that by other pro-/anti-thrombotic proteins, showing the whole coagulation picture behind the phenotypic heterogeneity in HA.

Monitoring of coagulation factor therapy in patients with von Willebrand disease type 3 using a microchip flow chamber system.

Patients with type 3 von Willebrand disease (VWD-3) have no measurable levels of VW factor (VWF) and usually require treatment with VWF-FVIII concentrate to prevent and/or stop bleeding. Even though the patients are treated prophylactically, they may experience bleeding symptoms. The aim of this study was to evaluate the effect of VWF-FVIII concentrate treatment in VWD-3 patients with the Total Thrombus Analysis System (T-TAS®), which measures thrombus formation under flow conditions. Coagulation profiles of 10 VWD-3 patients were analysed using T-TAS before and 30 minutes after VWF-FVIII concentrate (Haemate®) injection. Results were compared to VWF- and FVIII activity in plasma, and results with thromboelastometry and ristocetin-activated platelet impedance aggregometry (Multiplate®) in whole blood. For comparison, 10 healthy controls were also analysed with T-TAS. A median dose of 27 (range 15-35) IU/kg of VWF-FVIII concentrate increased VWF- and FVIII activity as expected. T-TAS thrombus formation was enhanced when a tissue factor/collagen-coated flow chamber was used at low shear, but treatment effects at high shear using a collagen-coated flow chamber were minimal. Whole blood coagulation assessed by thromboelastometry was normal and did not change (p > 0.05) but ristocetin-induced platelet aggregation improved (p < 0.001). In conclusion, T-TAS detects effects of VWF-FVIII concentrate treatment on coagulation-dependent thrombus formation at low shear, but minor effects are observed on platelet-dependent thrombus formation at high shear. The poor prediction of bleeding by conventional laboratory monitoring in VWD-3 patients might be related to insufficient restoration of platelet-dependent thrombus formation.

Increased fibrin formation and impaired fibrinolytic capacity in severe chronic kidney disease.

Chronic kidney disease (CKD) is associated with a concurrent increased risk of thrombosis and bleeding. We aimed to investigate whether CKD is associated with increased fibrin formation, impaired fibrin degradation, or both. Twenty-one patients with CKD stage 4 (CKD 4), 15 haemodialysis patients, and 13 controls (C) without kidney disease were studied. We used a global assay to determine fibrin formation and degradation in plasma. Fibrin turbidity was measured over time to obtain a value of the coagulation activation profile (Cp) and the fibrinolysis activation profile (Fp), and the amount of fibrin formed, termed fibrin optical density sum (fibrin OD-sum). We used scanning electron microscopy (SEM) to visualize the fibrin network. Plasminogen activator inhibitor type-1 antigen, thrombin-activatable fibrinolysis inhibitor activity, fibrinogen, von Willebrand factor, antithrombin, albumin, and C-reactive protein were measured in plasma. Fibrin OD-sum was significantly elevated in haemodialysis patients [312 a.u.; 278-435 (median; interquartile range); P < 0.0013] and in CKD 4 (293 a.u.; 169-434; P = 0.0119) compared with controls (115 a.u.; 82-234). SEM showed a tight fibrin network in haemodialysis and CKD 4 patients. Fp was lower in the haemodialysis group than in controls (P = 0.030). Plasminogen activator inhibitor type-1 was lower in haemodialysis patients (P = 0.034). Thrombin-activatable fibrinolysis inhibitor activity, Cp, antithrombin, and C-reactive protein did not differ between groups. Fibrinogen was significantly elevated and albumin decreased in both haemodialysis and CKD 4 patients compared with controls. Von Willebrand factor was elevated in haemodialysis patients compared with controls (P = 0.010). The prothrombotic state in severe CKD is characterized by impaired fibrinolysis in association with increased fibrin formation despite normal levels of endogenous fibrinolysis inhibitors.

Tranexamic acid--an old drug still going strong and making a revival.

Experience with tranexamic acid, an indirect fibrinolytic inhibitor, started as soon as it was released from Shosuke Okamoto's lab in the early 1960s. It was first prescribed to females with heavy menstrual blood loss and to patients with hereditary bleeding disorders. Soon the indications were widened to elective surgery because of its blood saving effects. Contraindications are few, most important is ongoing venous or arterial thrombosis and allergy to tranexamic acid, and the doses has to be reduced in renal insufficiency. In randomized controlled trials, however, patients with other risk factors are excluded as well (patients with history of cardiovascular disease, thromboembolism, bleeding diathesis, renal failure with creatinine >250µmol/L, pregnancy, and patients on treatment with anticoagulants). Recent meta-analyses of several randomized controlled trials in orthopedic arthroplasty have shown that tranexamic acid reduces peri- and postoperative blood loss, blood transfusion requirements and reoperations caused by bleedings. In general, the preoperative dose was 10-15mg/kg i.v. (or 1g), followed or not, by one or two doses, some as continuous infusion i.v. To validate relationship between dose and effect more data are needed. No evidence was found of increased thromboembolic accidents or other adverse events in the patients on tranexamic acid compared to the control groups. In major cardiac surgery tranexamic acid has been used in a large number of controlled trials with various dosing schemes in which the highest dosages seem to be associated with neurotoxicity; therefore a maximum total dose of 100mg/kg especially in patients over 50years of age is recommended by ISMICS (International Society for Minimally Invasive Cardiothoracic Surgery). Other indications for tranexamic acid are reviewed here as well. In recent years the extensive trial in severe trauma with massive bleedings using tranexamic acid was presented, CRASH-2 (Clinical Randomisation of an Antifibrinolytic in Significant Haemorrhage) comprising more than 20,000 patients. It showed that the survival was increased when tranexamic acid was given early after the accident compared to placebo; further studies are taking place is this field to get more information. Of utmost importance is the ongoing WOMAN (World Maternal Antifibrinolytic) a randomized, double-blind, placebo controlled trial among 15,000 with clinical diagnosis of postpartum haemorrhage bearing in mind that each year a large number of women in low and middle income countries, die from causes related to childbirth. In summary, we consider tranexamic acid is a drug of great value to reduce almost any kind of bleeding, it is cheap and convenient to use and has principally few contraindications. It may be added, that tranexamic acid is included in the WHOs list of essential medicines.

Assays of fibrin network properties altered by VKAs in atrial fibrillation - importance of using an appropriate coagulation trigger.

Atrial fibrillation (AF) is a prothrombotic condition, involving increased thrombin generation and fibrinogen concentrations. Vitamin K antagonists (VKAs) prevent arterial thromboembolism if optimal anticoagulation is achieved by individualised drug doses, assessed by determining the Prothrombin time-related International Normalized Ratio (Pt-INR). There is evidence that formation of tight-laced fibrin networks is pathogenic in prothrombotic diseases. This study was performed among AF patients, to test whether long-term treatment with VKAs affects the structure of fibrin networks, and whether the effect is altered by employing different coagulation triggers: exogenous thrombin (1 IU/ml), 10 pM tissue factor (TF) or a commercial Pt-INR reagent (containing 400-fold more TF). In the thrombin-based method, fibrin network porosity (scanning electron microscopy) and liquid permeability (flow measurements) correlated inversely to fibrinogen concentrations, while positive correlations to the degree of anticoagulation were shown with the Pt-INR reagent. In the method with 10 pM TF, the two above relationships were detected, though the influence of Pt-INR was more profound than that of fibrinogen concentrations. Moreover, greater shortening of clot lysis time (CLT) arose from more permeable clots. As a coagulation trigger, 10 pM TF vs exogenous thrombin or the Pt-INR reagent is more informative in reflecting the in vivo process from thrombin generation to fibrin formation. Since fibrin network permeability rose in parallel to elevations of INR and shortening of CLT in AF patients, antithrombotic effects on prevention of thrombotic complications may be achieved from impairment of thrombin generation, resulting in formation of permeable clots susceptible to fibrinolysis.

A fibrinogen concentrate Haemocomplettan (Riastap) or a Factor XIII concentrate Fibrogammin combined with a mini dose of tranexamic acid can reverse the fibrin instability to fibrinolysis induced by thrombin- or FXa-inhibitor.

To assess whether Haemocomplettan(®) (fibrinogen concentrate) or Fibrogammin(®) (Factor XIII concentrate) can be used to manage bleeding complications of antithrombotic treatment, we examined a normal plasma pool spiked with AR-H067637 (thrombin inhibitor) or rivaroxaban (activated factor X-inhibitor), to which one of the concentrates was added. Fibrin network permeability (Ks), images of Scanning Electron Microscopy (SEM) and Clot Lysis Time (CLT) were examined. Both inhibitors increased the Ks levels, which could be fully or partly reversed by Haemocomplettan(®) or Fibrogammin(®) respectively. However, these modified clots with tightened network remained non-resistant to fibrinolysis, shown as unaffected CLT. Tranexamic acid at a very low concentration (0·4 mg/ml) aided the two concentrates to stabilize the clots, where the prolongation of CLT was more pronounced for a lower dose than a higher dose of Haemocomplettan(®) while Fibrogammin(®) brought the greatest delay to CLT out of all additions. These observations were partly supported by SEM images, displaying alterations of fibrin fibre arrangement known to influence fibirinolysis. The in vitro data suggest that Haemocomplettan(®) or Fibrogammin(®) given in combination with a mini dose of tranexamic acid may slow down the natural clearance of fibrin clot by plasmin and thus prevent patients from haemorrhagic complications during antithrombotic therapy.

In vitro studies using a global hemostasis assay to examine the anticoagulation effects in plasma by the direct thrombin inhibitors: dabigatran and argatroban.

This study aimed to assess whether a global hemostatic assay we developed can measure the anticoagulant effects of the direct thrombin inhibitors (DTIs)--dabigatran and argatroban. A normal plasma pool (NPP) spiked with one of the DTIs and five plasma samples from patients with coronary heart disease spiked with dabigatran were examined. Fibrin formation and fibrin degradation were initiated by adding recombinant tissue factor (together with washed-frozen-thawed platelets and CaCl(2)) and recombinant tissue plasminogen activator. Fibrin optical density (OD) was recorded, based on which coagulation activation profile (Cp) and fibrinolysis activation profile (Fp) were determined. Moreover, the sum of OD values registered over time (fibrin OD-sum) was calculated to reflect the capacity of fibrin formation under the general effect by Cp and Fp. The endogenous thrombin potential (ETP) and the standard clotting markers i.e., activated partial thromboplastin time (APTT) and prothrombin time expressed as International Normalized Ratio (INR) were also analyzed. Results demonstrated that APTT, INR and ETP could detect the effects of the DTIs except for INR in NPP containing dabigatran. In our global assay, the DTIs depressed the fibrin formation (shown as decreased fibrin OD-sum value) by leading to decrease of Cp and increase of Fp. Thus, our global assay which examines both fibrin formation and degradation seems more advantageous than the other methods mentioned above, as regards the possibility of being a laboratory tool to monitor the antithrombotic therapy with DTIs.

In vivo roles of factor XII.

Coagulation factor XII (FXII, Hageman factor, EC = 3.4.21.38) is the zymogen of the serine protease, factor XIIa (FXIIa). FXII is converted to FXIIa through autoactivation induced by "contact" to charged surfaces. FXIIa is of crucial importance for fibrin formation in vitro, but deficiency in the protease is not associated with excessive bleeding. For decades, FXII was considered to have no function for coagulation in vivo. Our laboratory developed the first murine knockout model of FXII. Consistent with their human counterparts, FXII(-/-) mice have a normal hemostatic capacity. However, thrombus formation in FXII(-/-) mice is largely defective, and the animals are protected from experimental cerebral ischemia and pulmonary embolism. This murine model has created new interest in FXII because it raises the possibility for safe anticoagulation, which targets thrombosis without influence on hemostasis. We recently have identified platelet polyphosphate (an inorganic polymer) and mast cell heparin as in vivo FXII activators with implications on the initiation of thrombosis and edema during hypersensitivity reactions. Independent of its protease activity, FXII exerts mitogenic activity with implications for angiogenesis. The goal of this review is to summarize the in vivo functions of FXII, with special focus to its functions in thrombosis and vascular biology.

Acetylation and glycation of fibrinogen in vitro occur at specific lysine residues in a concentration dependent manner: a mass spectrometric and isotope labeling study.

Aspirin may exert part of its antithrombotic effects through platelet-independent mechanisms. Diabetes is a condition in which the beneficial effects of aspirin are less prominent or absent - a phenomenon called "aspirin resistance". We investigated whether acetylation and glycation occur at specific sites in fibrinogen and if competition between glucose and aspirin in binding to fibrinogen occurs. Our hypothesis was that such competition might be one explanation to "aspirin resistance" in diabetes. After incubation of fibrinogen in vitro with aspirin (0.8 mM, 24 h) or glucose (100 mM, 5-10 days), we found 12 modified sites with mass spectrometric techniques. Acetylations in the α-chain: αK191, αK208, αK224, αK429, αK457, αK539, αK562, in the ß-chain: ßK233, and in the γ-chain: γK170 and γK273. Glycations were found at ßK133 and γK75, alternatively γK85. Notably, the lysine 539 is a site involved in FXIII-mediated cross-linking of fibrin. With isotope labeling in vitro, using [(14)C-acetyl]salicylic acid and [(14)C]glucose, a labeling of 0.013-0.084 and 0.12-0.5 mol of acetylated and glycated adduct/mol fibrinogen, respectively, was found for clinically (12.9-100 µM aspirin) and physiologically (2-8 mM glucose) relevant plasma concentrations. No competition between acetylation and glycation could be demonstrated. Thus, fibrinogen is acetylated at several lysine residues, some of which are involved in the cross-linking of fibrinogen. This may mechanistically explain why aspirin facilitates fibrin degradation. We find no support for the idea that glycation of fibrin(ogen) interferes with acetylation of fibrinogen.

Impaired fibrinolytic capacity and increased fibrin formation associate with myocardial infarction.

We assessed whether abnormality of haemostasis measured by a newly developed global method is associated with risk of a first myocardial infarction (MI). The global markers coagulation activation profile (Cp), fibrinolysis activation profile (Fp) and sum of fibrin optical density over time (Fibrin OD-sum) were determined in plasma from 800 MI cases and 1,123 controls included in the Stockholm Heart Epidemiology Program. Clot lysis time (CLT) was also determined based on raw data of fibrin OD from the global assay. Odds ratios (OR) of MI with 95% confidence intervals (CI) were calculated using logistic regression. A Fp value <10th percentile value in controls was significantly associated with increased MI risk; OR after multivariate adjustments for conventional cardiovascular risk factors 1.66 (95% CI 1.22-2.27). For an abnormally long CLT (>90th percentile value in controls) the adjusted OR of MI was 2.62 (95% CI 1.87-3.66) and for a high Fibrin OD-sum value (>90th percentile in controls) it was 1.86 (95% CI 1.37-2.53). A high Cp value was not significantly associated with MI. In conclusion, we found that abnormal haemostasis in platelet-poor plasma, reflected either as an attenuated fibrinolytic capacity or the resulting increase of fibrin formation, was associated with increased MI risk.

Is fibrin formation and thrombin generation increased during and after an acute coronary syndrome?

INTRODUCTION AND METHODS: In order to study coagulation and fibrinolysis in acute coronary syndrome (ACS) we used a recently developed assay, called OH-index, which provides simultaneous measurements of fibrin formation and fibrinolysis (fibrin degradation) in the patients' plasma. We also investigated thrombin generation using the calibrated automated thrombogram (CAT), and assessed thrombin generation in vivo by measuring F1+2 plasma concentrations. In addition, to better characterize the patients we also assessed markers of inflammation and endothelial function. Eighty-seven ACS patients were sampled at admission, within 24 hours during treatment with low molecular weight heparin (LMH), and 6 months later; 65 healthy controls were also sampled. RESULTS: As assessed by OH-index fibrin formation was slightly depressed at admission, profoundly depressed during LMH treatment and comparable to controls at 6 months, whereas fibrin degradation was elevated, particularly during LMH treatment. F1+2 levels decreased during LMH treatment but did not deviate significantly from controls at admission or in convalescence. CAT data showed that peak thrombin was higher at admission and after 6 months compared to controls, whereas the endogenous thrombin potential only tended to be elevated. Both variables were strongly reduced during LMH treatment. Patients had elevated levels of markers of inflammation and endothelial function as expected. CONCLUSION: ACS-patients have an increased capacity to generate thrombin and an enhanced capacity to degrade fibrin in the acute phase. Increased thrombin generation persists also 6 months after the event.

Effects on fibrin network porosity of anticoagulants with different modes of action and reversal by activated coagulation factor concentrate.

Orally available direct thrombin inhibitors (DTI) and direct activated factor X inhibitors (DFXaI) may replace vitamin K antagonists in patients needing long-term anticoagulant treatment. We investigated the influence on the fibrin network of anticoagulants with different modes of action: AR-H067637 (DTI), the active metabolite of AZD0837, apixaban (DFXaI), fondaparinux (indirect FXaI) and warfarin. Counteraction of the anticoagulant effect by FEIBA(®) (Factor Eight Inhibitor Bypass Activity) was also investigated. Tissue factor, phospholipids and calcium were used to initiate coagulation in human platelet poor plasma. The permeability constant (Ks), reflecting the amount of buffer passing through the coagulum, was calculated and the fibrin network was visualized by 3D confocal microscopy. Warfarin (International Normalized Ratio 2-3) increased Ks in plasma by 28-50% compared with control. 'Therapeutic' plasma concentrations of AR-H067637 (0·3-0·6 µmol/l), apixaban (0·2-0·4 µmol/l) and fondaparinux (0·1-0·3 µmol/l) increased Ks by 72-91%, 58-76% and 36-53% respectively. Addition of FEIBA(®) totally reversed the warfarin effect but only partially reversed effects of the other anticoagulants at concentrations that increased Ks by 50% or more. Fibrin network observed with 3D confocal microscopy agreed well with the permeability results. In conclusion, all examined anticoagulants rendered the fibrin network more porous. FEIBA(®) reversed the increased permeability in warfarin plasma but had only partial effects on the other anticoagulants.

Changes in von Willebrand factor and ADAMTS13 during IVF.

During IVF, circulating estradiol concentrations are strongly increased, and this may have direct effects on hemostasis. Elevated von Willebrand factor levels represent an important risk factor for arterial and venous thrombosis. ADAMTS13, also known as von Willebrand factor-cleaving protease, has an important regulatory function of von Willebrand factor but has not been studied during IVF. Blood was sampled from 31 women at maximal downregulation of estradiol synthesis using gonadotropin-releasing hormone analogues and during high-level stimulation of estradiol synthesis using follicle-stimulating hormone during the first phase of IVF. Von Willebrand factor antigen, von Willebrand factor ristocetin cofactor activity, factor VIII and ADAMTS13 antigen and activity levels in plasma were determined at the time of downregulation and at high-level stimulation. Estradiol increased from a mean of 154 pg/ml at downregulation to 5889 pg/ml at high-level stimulation (range 1620-19 500 pg/ml). Factor VIII increased from 0.96 ± 0.34 to 1.26 ± 0.41 kIU/l (P < 0.001). Von Willebrand factor antigen and activity increased from 0.75 ± 0.22 to 1.06 ± 0.40 kIU/l (P < 0.001) and from 0.83 ± 0.26 to 1.24 ± 0.48 kIU/l (P < 0.001), respectively. ADAMTS13 antigen decreased from 72.2 ± 13.5 to 67.9 ± 9.9% (P < 0.05, P = 0.01) and ADAMTS13 activity from 88.6 ± 18.3 to 80.8 ± 15.7% (P < 0.01). The increments in estradiol and factor VIII during IVF were paralleled by an increase in von Willebrand factor antigen and activity, and a decrease in circulating ADAMTS13 antigen and activity, respectively. This could in part explain why these patients have an increased risk of thrombotic events.

Molecular aspects in clinical hemostasis research at Karolinska Institutet.

The development of hemostasis research at Karolinska Institutet is described, focusing first on the initial findings of the fibrinogen structure and the hereditary bleeding disorders, hemophilia A and von Willebrand's disease. Basic research has focused on new biomarkers for cardiovascular/thromboembolic disorders, such as myocardial infarction and stroke, including preeclampsia and diabetes, with studies on the importance of decreased fibrinolysis in these disorders. Since long, the structure of the fibrin network has been evaluated, and recently the influence of aspirin and new thrombin and factor Xa inhibitors has been investigated. Research on the contact pathway of coagulation has also started at the Unit.