Platelet activation is not involved in acceleration of the coagulation system in acute cardioembolic stroke with nonvalvular atrial fibrillation

Atrial fibrillation and stroke

INTRODUCTION

Stroke is one of the leading causes of mortality and morbidity globally. Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia. It is set to reach epidemic proportions. AF is associated with a five-fold increase in risk of stroke. Strokes caused by AF more often are fatal or result in severe disability. Even though the incidence of stroke has been significantly reduced by oral anticoagulation, AF is thought to account for a significant proportion of cryptogenic strokes where no etiology is identified.

AREAS COVERED

This article reviews the literature related to AF and stroke, pathophysiological insights, diagnosis of AF in stroke patients, and its management (Graphical Abstract).

EXPERT OPINION

The pathophysiology of thrombogenesis that links AF and stroke is not well understood and is an area of active research to identify new therapeutic targets to prevent AF and stroke. As the nature of AF and stroke is multifaceted, an integrated care approach to managing AF and stroke is increasingly essential.

Prospective pilot study of tirofiban in progressive stroke after intravenous thrombolysis

Background

Intravenous thrombolysis (IVT) is a standard procedure for the treatment of patients with acute ischemic stroke (AIS). Improving the therapeutic efficacy of IVT is an important task for neurologists. The aim of this study was to evaluate the efficacy and safety of early low-dose tirofiban treatment in AIS patients with early neurological deterioration (END) after IVT.

Methods

In this prospective and randomized pilot study, 73 AIS patients with END were recruited from a local hospital in China. Of these, 14 patients were treated with regular antiplatelet agents (aspirin plus clopidogrel) and 59 patients were treated with tirofiban within 24 h of IVT, followed by regular antiplatelet therapy. Neurological deficits and functional recovery were assessed with NIHSS and modified Rankin Scale (mRS) at 7 and 90 days. During the 90-day follow-up period, both hemorrhagic (e.g., intracerebral hemorrhage) and non-hemorrhagic (e.g., pneumonia) events were recorded.

Results

Treatment with tirofiban compared with regular antiplatelet therapy: (1) improved functional recovery of AIS patients to mRS (≤2) at both 7 and 90 days (odds ratios [ORs], 1.37 and 1.64; 95% confidence interval [CI], 1.16–1.61 and 1.26–2.12; P = 0.008 and < 0.001, respectively), and (2) reduced NIHSS scores from 11.14 ± 2.38 to 5.95 ± 3.48 at day 7 (P < 0.001) and from 8.14 ± 2.74 to 4.08 ± 3.50 at day 90 (P < 0.001). Tirofiban treatment did not increase the risk of hemorrhagic complications. Multivariate regression analysis showed that tirofiban treatment independently predicted a favorable functional outcome (P ≤ 0.001).

Conclusion

Early treatment with low-dose tirofiban in AIS patients with neurologic deterioration after IVT potentially improved functional recovery and attenuated neurologic deficits as early as 7 days and did not increase the risk of various hemorrhagic complications. However, the therapeutic efficacy of tirofiban treatment in END patients needs to be determined by future randomized clinical trials with a large study population.

Clinical trial registration

http://www.chictr.org.cn/, Identifier ChiCTR2200058513.

Atrial fibrillation and the prothrombotic state: revisiting Virchow’s triad in 2020

Atrial fibrillation (AF) is characterised by an increased risk of pathological thrombus formation due to a disruption of physiological haemostatic mechanisms that are better understood by reference to Virchow’s triad of ‘abnormal blood constituents’, ‘vessel wall abnormalities’ and ‘abnormal blood flow’. First, there is increased activation of the coagulation cascade, platelet reactivity and impaired fibrinolysis as a result of AF per se, and these processes are amplified with pre-existing comorbidities. Several prothrombotic biomarkers including platelet factor 4, von Willebrand factor, fibrinogen, β-thromboglobulin and D-dimer have been implicated in this process. Second, structural changes such as atrial fibrosis and endothelial dysfunction are linked to the development of AF which promote further atrial remodelling, thereby providing a suitable platform for clot formation and subsequent embolisation. Third, these factors are compounded by the presence of reduced blood flow secondary to dilatation of cardiac chambers and loss of atrial systole which have been confirmed using various imaging techniques. Overall, an improved understanding of the various factors involved in thrombus formation will allow better clinical risk stratification and targeted therapies in AF.

Predictive Role of Coagulation, Fibrinolytic, and Endothelial Markers in Patients with Atrial Fibrillation, Stroke, and Thromboembolism: A Meta-Analysis, Meta-Regression, and Systematic Review

Background

The pathophysiological mechanism associated with the higher prothrombotic tendency in atrial fibrillation (AF) is complex and multifactorial. However, the role of prothrombotic markers in AF remains inconclusive.

Material/Methods

We conducted a meta-analysis of observational studies evaluating the association of coagulation activation, fibrinolytic, and endothelial function with occurrence of AF and clinical adverse events. A comprehensive subgroup analysis and meta-regression was performed to explore potential sources of heterogeneity.

Results

A literature search of major databases retrieved 1703 studies. After screening, a total of 71 studies were identified. Pooled analysis showed the association of coagulation markers (D-dimer (weighted mean difference (WMD)=197.67 and p<0.001), fibrinogen (WMD=0.43 and p<0.001), prothrombin fragment 1–2 (WMD=0.53 and p<0.001), antithrombin III (WMD=23.90 and p=0.004), thrombin-antithrombin (WMD=5.47 and p=0.004)); fibrinolytic markers (tissue-type plasminogen activator (t-PA) (WMD=2.13 and p<0.001), plasminogen activator inhibitor (WMD=11.44 and p<0.001), fibrinopeptide-A (WMD=4.13 and p=0.01)); and endothelial markers (von Willebrand factor (WMD=27.01 and p<0.001) and soluble thrombomodulin (WMD=3.92 and p<0.001)) with AF.

Conclusions

The levels of coagulation, fibrinolytic, and endothelial markers have been reported to be significantly higher in AF patients than in SR patients.

Platelets Cellular and Functional Characteristics in Patients with Atrial Fibrillation: A Comprehensive Meta-Analysis and Systematic Review

BACKGROUND This systematic review with meta-analysis aimed to determine the strength of evidence for evaluating the association of platelet cellular and functional characteristics including platelet count (PC), MPV, platelet distribution width (PDW), platelet factor 4, beta thromboglobulin (BTG), and p-selectin with the occurrence of atrial fibrillation (AF) and consequent stroke. MATERIAL AND METHODS We conducted a meta-analysis of observational studies evaluating platelet characteristics in patients with paroxysmal, persistent and permanent atrial fibrillations. A comprehensive subgroup analysis was performed to explore potential sources of heterogeneity. RESULTS Literature search of all major databases retrieved 1,676 studies. After screening, a total of 73 studies were identified. Pooled analysis showed significant differences in PC (weighted mean difference (WMD)=-26.93 and p<0.001), MPV (WMD=0.61 and p<0.001), PDW (WMD=-0.22 and p=0.002), BTG (WMD=24.69 and p<0.001), PF4 (WMD=4.59 and p<0.001), and p-selectin (WMD=4.90 and p<0.001). CONCLUSIONS Platelets play a critical and precipitating role in the occurrence of AF. Whereas distribution width of platelets as well as factors of platelet activity was significantly greater in AF patients compared to SR patients, platelet count was significantly lower in AF patients.

Contraction of Blood Clots Is Impaired in Acute Ischemic Stroke

OBJECTIVE

Obstructive thrombi or thrombotic emboli are the pathogenic basis of ischemic stroke. In vitro blood clots and in vivo thrombi can undergo platelet-driven contraction (retraction), resulting in volume shrinkage. Clot contraction can potentially reduce vessel occlusion and improve blood flow past emboli or thrombi. The aim of this work was to examine a potential pathogenic role of clot contraction in ischemic stroke.

APPROACH AND RESULTS

We used a novel automated method that enabled us to quantify time of initiation and extent and rate of clot contraction in vitro. The main finding is that clot contraction from the blood of stroke patients was reduced compared with healthy subjects. Reduced clot contraction correlated with a lower platelet count and their dysfunction, higher levels of fibrinogen and hematocrit, leukocytosis, and other changes in blood composition that may affect platelet function and properties of blood clots. Platelets from stroke patents were spontaneously activated and displayed reduced responsiveness to additional stimulation. Clinical correlations with respect to severity and stroke pathogenesis suggest that the impaired clot contraction has the potential to be a pathogenic factor in ischemic stroke.

CONCLUSIONS

The changeable ability of clots and thrombi to shrink in volume may be a novel unappreciated mechanism that aggravates or alleviates the course and outcomes of ischemic stroke. The clinical importance of clot or thrombus transformations in vivo and the diagnostic and prognostic value of this blood test for clot contraction need further exploration.

Association of hemostatic markers with atrial fibrillation: a meta-analysis and meta-regression

BACKGROUND

There is growing evidence that indicates the presence of a prothrombotic state in atrial fibrillation (AF). However, the role of hemostatic markers in AF remains inconclusive.

METHODS

We conducted a meta-analysis of observational studies to evaluate the association between hemostatic markers and AF. A meta-regression was performed to explore potential sources of heterogeneity.

RESULTS

A total of 59 studies met our inclusion criteria for the meta-analysis. For platelet activation, increased circulating platelet factor-4, β-thromboglobulin (BTG) and P-selectin were significantly higher in AF cases compared with controls (standardized mean difference [SMD][95% confidence interval (CI)]: 1.72[0.96-2.49], 1.61[1.03-2.19] and 0.50[0.23-0.77], respectively). For coagulation activation, increased levels of plasma D-dimer, fibrinogen, thrombin-antithrombin, prothrombin fragment 1+2, and antithrombin-III were significantly associated with AF (SMD[95% CI]: 1.82[1.38-2.26], 0.72[0.55-0.89], 0.42[0.13-0.72], 1.00 [0.00-1.99] and 1.38[0.16-2.60], respectively). For fibrinolytic function, tissue-type plasminogen activator and plasminogen activator inhibitor-1 were significantly increased in AF cases compared with controls (SMD[95% CI]: 0.86[0.04-1.67] and 0.87[0.28-1.47], respectively) but the associations became nonsignificant after performing subgroup analysis by anticoagulants treatment status. For endothelial function, increased von Willebrand factor was significantly associated with AF (SMD, 0.79; 95% CI, 0.60-0.99); however, no association was observed for soluble thrombomodulin (SMD, 0.60; 95% CI, -0.13-1.33).

CONCLUSIONS

Increased circulating hemostatic factors (PF-4, BTG, P-selectin, D-dimer, fibrinogen, TAT, F1+2, AT- III, and vWf) are significantly associated with AF. Future research is necessary to elucidate the precise mechanism of the prothrombotic state and how hemostatic markers promote thromboembolism in AF.

Evaluation of platelet activation, coagulation, and fibrinolytic activation in patients with symptomatic lacunar stroke

BACKGROUND

It is unclear whether hemostasis plays a role in the pathogenesis of ischemic stroke subtypes.

OBJECTIVE

We aimed to investigate the possible relationship between different hemostatic markers and lacunar stroke.

RESULTS

The study consisted of 30 patients with symptomatic lacunar stroke and 30 healthy age-matched healthy individuals. We analyzed the values of "Mean Platelet Volume," D-dimer, "soluble p-selectin," "Plasminogen Activator Inhibitor Type-1" (PAI-1), "Thrombin-Activatable Fibrinolysis Inhibitor" (TAFI), and "Platelet Factor 4" (PF4) in patients with lacunar infarct and compared these values to those of control individuals. There were significant differences for D-dimer, mean platelet volume, thrombin-activatable fibrinolysis inhibitor, and platelet factor 4 values in symptomatic lacunar stroke group compared with the control group (P < 0.01).

CONCLUSIONS

Different hemostatic factors may play a role in the pathogenesis of lacunar stroke. Evaluating the role of hemostatic factors on different types of strokes may help us identify new therapeutic strategies and different prognostic stratifications for ischemic stroke.

Is D-dimer helpful in evaluating stroke patients? A systematic review

D-dimer (DD) is a fibrin degradation product present in negligible amounts in healthy individuals, but in thrombotic/fibrinolytic conditions substantially increases in plasma. Over the last two decades numerous studies have explored whether DD measurements would help stroke clinicians. An easy, reliable, and inexpensive test for stroke diagnosis, determination of stroke subtype, severity, prognosis, and recurrence risk is being sought. We searched the database, of studies indexed in English on MEDLINE, using the keywords 'cerebral venous thrombosis, D-dimer, deep vein thrombosis, intracerebral hemorrhage, ischemic stroke, outcome, prognosis, and subarachnoid hemorrhage' for relevant studies. Here, we systematically review current evidence on plasma DD levels in patients with ischemic and hemorrhagic strokes, transient ischemic attacks, and cerebral venous thrombosis. Numerous studies showed that patients with various strokes and stroke-related diseases had acutely increased plasma DD levels. Plasma DD levels, however, are neither sensitive nor specific enough to be utilized in stroke diagnostics and cannot replace either clinical or radiological evaluation. Regarding prediction of patient outcome, good clinical evaluation is clearly superior to DD testing.

Left atrial appendage exclusion and the risk of thromboembolic events following mitral valve surgery

OBJECTIVES

We aimed to evaluate left atrial appendage (LAA) exclusion in patients undergoing mitral valve surgery with respect to thromboembolic events.

BACKGROUND

LAA is the predominant source of emboli in patients with atrial fibrillation. Prophylactic LAA exclusion at the time of heart surgery has been recommended to reduce the risk of future thromboembolism.

METHODS

An observational cohort of 136 patients undergoing LAA exclusion during mitral valve surgery was identified between May 1993 and November 1998 at our institution.

RESULTS

During a mean follow-up of 3.6 +/- 1.3 years, there were 14 (12.3%) thromboembolic events. Compared with patients who received warfarin upon hospital discharge, there were more thromboembolic events in patients not prescribed warfarin upon hospital discharge (n = 7/67, 10% vs n = 6/40, 15%, respectively). The warfarin status was not known for one patient. The majority of thromboembolic events (n = 10/14, 71%) occurred in those who underwent mitral valve repair.

CONCLUSION

In this observational study, patients who undergo LAA exclusion during mitral valve surgery to reduce the risk of thromboembolism have a significant incidence of thromboembolic events, especially when warfarin therapy is not prescribed upon hospital discharge.

Emerging role of direct thrombin inhibitors in nonvalvular atrial fibrillation: potential and peril

Ximelagatran, an emerging new direct, oral, fixed-dose thrombin inhibitor may offer an attractive alternate to warfarin for patients with nonvalvular atrial fibrillation. The lack of drug-to-drug interactions and need for coagulation monitoring parameters such as international normalized ratio and a short half-life make therapeutic control and withdrawal in the event of bleeding easier. The side-effect profile, including elevation of liver enzymes, may temper the clinical utility of long-term use ximelagatran.

Serial changes in fibrinolysis and coagulation activation markers in acute and convalescent phase of ischemic stroke

BACKGROUND

The changes in the activity of a number of plasma markers of coagulation and fibrinolysis have previously been studied in patients with ischemic stroke, with conflicting results. We aimed to find out the changes in the activities of a wide array of markers of the coagulation and the fibrinolytic system of mildly or moderately affected first-ever ischemic stroke patients.

METHODS

In a prospective, longitudinal, case-control study, we studied plasma plasminogen activator inhibitor type-1 (PAI-1) activity, tissue-type plasminogen activator antigen (t-PA:Ag), d-dimer, prothrombin fragment 1+2 (F 1+2), and thrombin-antithrombin III complex (TAT) levels in 55 consecutive patients on admission, 1 week, 1 month, and 3 months after an ischemic stroke. Sex- and age-matched controls were studied once. All patients underwent blood sampling at each study time point; comprehensive stroke risk factors were recorded, and the etiology of the ischemic stroke was determined. All patients were contacted 3 years later for possible recurrent ischemic events.

RESULTS

PAI-1 activity was increased in the acute phase and at 3 months, D-dimer levels were significantly higher at 1 week and 1 month after stroke, whereas t-PA:Ag, TAT and F 1+2 levels remained stable during the whole study period.

CONCLUSIONS

The changes of the fibrinolytic and coagulation system activity in the patients with mild or moderate ischemic stroke appeared minor compared with the results of previous studies, which included more severely ill patients.

Role of transesophageal echocardiography in the evaluation of patients with stroke

This review article summarizes recent advances in the care of patients presenting with neurologic events, in which transesophageal echocardiography plays an important role in diagnosis, prognosis, and treatment. New research on the use of transesophageal echocardiography in patients with stroke and atrial fibrillation is discussed, including left atrial clot formation, maintenance of sinus rhythm after cardioversion, and techniques of left atrial appendage occlusion. A discussion of developments in the diagnosis and management of thoracic aortic plaque follows. The association of patent foramen ovale and atrial septal aneurysm with stroke is outlined, and possible reasons for this association are discussed. Recent literature on the use of percutaneous closure devices for patent foramen ovale is reviewed.

Role of homocysteine for thromboembolic complication in patients with non-valvular atrial fibrilation

Thromboembolism is the most important complication in patients with atrial fibrilation (AF). Homocysteine is a toxic amino acid that has been recently accepted as a risk factor for atherosclerosis and stroke. The aim of the present study is to show whether there is a relation between hyperhomocysteinemia and thromboembolic complications in patients with non-valvular AF. We admitted 38 patients with non-valvular AF. The patients were divided into two groups: group A (n = 20; mean age, 75.7 +/- 10.4 years; three males/17 females), and group B (n = 18; mean age, 68.0 +/- 10.6 years; 11 males/seven females). While group A consisted of the patients with AF and stroke, group B was composed of the patients with AF but without stroke. The patients having sinus rhythm (15 subjects) were used as the reference group to obtain the cut-off value. Homocysteine was measured by the immunoassay method. The means of the homocysteine levels were 12.4 +/- 3.3 micromol/l in group A, 8.3 +/- 2.3 micromol/l in group B and 9.3 +/- 1.8 micromol/l in the reference group. The cut-off value was 10.6 micromol/l. Group A had a statistically higher homocysteine level than not only group B, but also the reference group (P < 0.05). While 60% of group A (n = 12) had the elevated homocysteine level, the rate was only 22% for group B (n = 4). In conclusion, hyperhomocysteinemia may be one of the explanations for the increased rate of thromboembolic complications in older patients with AF.

Urinary 11-dehydro-thromboxane B(2) and coagulation activation markers measured within 24 h of human acute ischemic stroke

The aim of this study was to determine the extent of change in platelet and coagulation markers in the acute phase of ischemic stroke and to assess the utility of marker measurement in stroke subtype classification. Urinary 11-dehydro-thromboxane B(2) (11-dTXB2), a marker of in vivo platelet activation, and markers of coagulation activation, including prothrombin fragment 1+2 (F1+2), thrombin-antithrombin complex (TAT), and fibrinogen, were measured in 25 patients with ischemic stroke within 24 h of onset of symptoms. Marker levels in patients with ischemic stroke were compared with those in 19 age-matched controls who had not taken aspirin for at least 2 weeks before sampling and 25 healthy controls. Median marker levels were significantly increased in stroke over those in age-matched controls for fibrinogen (344 vs. 289 mg/dl; P=0.030), F1+2 (1.40 vs. 0.80 nmol/l; P=0.003), and TAT (6.65 vs. 2.20 microg/l; P<0.0001). Median marker levels for seven patients with cardioembolic stroke and 18 with non-cardioembolic stroke were not significantly different for any marker test. Eight patients taking aspirin at the time of the stroke had significantly lower 11-dTXB2 values than patients not taking aspirin (964 vs. 4,314 pg/mg of creatinine; P=0.007). Stroke patients not taking aspirin had significantly higher 11-dTXB2 concentration than age-matched controls (4,314 vs. 1,788 pg/mg of creatinine; P=0.006). Coagulation and platelet activation markers are increased in the acute phase of stroke regardless of the clinical mechanism. This finding suggests that the markers may not be useful for predicting clinical subtype of ischemic stroke in the acute phase.

Coagulation indicators in patients with paroxysmal atrial fibrillation: effects of electric and pharmacologic cardioversion

The aim of this study was to determine whether paroxysmal atrial fibrillation (PAF) and/or restoration to sinus rhythm with electric or pharmacologic cardioversion induce modifications to the coagulation system. Thirty-five patients with PAF undergoing either electric (n = 11) or pharmacologic (n = 24) cardioversion were studied. Fibrinopeptide A and D-dimer blood samples were taken immediately before and after cardioversion at different intervals. When compared with the control group (n = 70), the precardioversion fibrinopeptide A plasma values were significantly elevated (11.8 vs 2.5 ng/mL). Fibrinopeptide A plasma values were significantly reduced 5 minutes after cardioversion (11.8 vs 5.3 ng/mL) and remained stable throughout the follow-up sequential measurements. D-dimer plasma values were significantly increased (measured at 12 hours and at day 7) in patients who underwent electrical cardioversions only. A positive correlation (R(2) = 0.76) was found between the energy delivered for cardioversion to sinus rhythm and D-dimer plasma values on day 7. In patients with PAF, levels of fibrinopeptide A, an indicator of coagulation activation, are elevated and soon reduced by the restoration of sinus rhythm. Electric, but not pharmacologic, cardioversion induces an early activation of the fibrinolytic system.

A cross-sectional and diurnal study of thrombogenesis among patients with chronic atrial fibrillation

OBJECTIVES

First, we sought to determine whether there is diurnal variation in hemostatic factors related to thrombogenesis and hypercoagulability among patients with chronic atrial fibrillation (AF). Second, we sought to determine whether levels of soluble thrombomodulin (sTM), a marker of endothelial function, or soluble P-selectin (sP-sel), an index of platelet activation, are altered in patients with AF as compared with subjects in sinus rhythm.

BACKGROUND

Atrial fibrillation is associated with an increased risk of stroke and thromboembolism and is known to confer a hypercoagulable state, with abnormalities of thrombosis, platelet activation and endothelial cell function. Many cardiovascular events, such as acute myocardial infarction, have thrombosis as an underlying process, and they undergo diurnal variation.

METHODS

Fifty-two patients (45 men, mean [+/- SD] age 66 +/- 6 years) with chronic AF, none of whom received antithrombotic therapy, were studied. Baseline levels of fibrinogen, sP-sel, sTM and von Willebrand factor (vWF) were compared to those levels in matched healthy control subjects in sinus rhythm. In a subgroup of 20 patients, five venous blood samples were collected through an indwelling cannula at 6-h intervals from 12 PM to 12 PM the following day and were analyzed for the same markers.

RESULTS

Patients with chronic AF had higher plasma sP-sel, sTM, vWF and fibrinogen levels as compared with control subjects in sinus rhythm. Significant correlations were found between fibrinogen and sP-sel in patients with AF (r = 0.567 [Spearman], p < 0.001) and in control subjects (r = 0.334, p = 0.016). There was no significant diurnal variation in plasma levels of sP-sel, sTM, vWF or fibrinogen over the 24-h study period (repeated measures analysis of variance, p = NS).

CONCLUSIONS

There is no circadian or diurnal variation in the hypercoagulable state seen in AF, as assessed by plasma fibrinogen and markers of platelet (sP-sel) and endothelial function (vWF and sTM). The persistent hypercoagulable state, together with the loss of diurnal variation in various hemostatic markers, in chronic AF may contribute to the high risk of stroke and thromboembolic complications in these patients.

[Aspirin and cerebral ischemic accidents]

At the acute phase of cerebral infarction, two recent large studies found that the use of aspirin reduces both mortality and the risk of the recurrence of stroke. In primary prevention, aspirin nearly halves the risk of myocardial infarction but does not reduce that of stroke. Concerning the secondary prevention of atherothrombotic brain infarcts, aspirin has been the most extensively studied drug, and is efficient between 50 mg and 1.3 g. In spite of the efficacy of other antiplatelets in this indication--ticlopidine (500 mg), clopidogrel (75 mg) and dipyridamole (400 mg)--aspirin remains the most cost-effective, doses between 100 and 300 mg being the most widely used. Cardiac diseases with a high embolic risk require the use of oral anticoagulation. In nonvalvular atrial fibrillation, the choice of antithrombotic drugs depends on risk stratification: oral anticoagulants are indicated in high-risk subjects, whereas aspirin is recommended in low-risk subjects and when oral anticoagulants are contraindicated. Studies with associations of aspirin and other antiplatelets are required to increase the yield of this medication in high-risk subjects, in parallel with efforts to detect and to treat the vascular risk factors.

Pharmacodynamic models to evaluate antithrombotics in clinical pharmacology

The need to prevent thromboembolic events effectively and safely has stimulated an intense search for novel antithrombotics. Parameters derived from in vitro tests with patients' blood are essential for therapeutic monitoring of anticoagulants. Clinical pharmacologic evaluation of novel antithrombotic therapies based on such parameters can easily fail, however, by neglecting pivotal pathophysiologic determinants of thrombus formation. When vascular injury occurs, blood cells, plasma proteins, and the vessel wall intimately cooperate for an adequate local repair. Much remains to be learned about the local and transient interaction of these components. In most tests of platelet function and coagulation proteins, blood samples from treated individuals are stimulated in vitro to assess inhibitory effects. Limitations of such a test strategy for dose-finding studies with antithrombotics may be overcome by measuring activation markers specifically generated on the surface of blood cells or in plasma at the site of thrombosis in patients.