Wide variation in thrombin generation in patients with atrial fibrillation and therapeutic International Normalized Ratio is not due to inflammation

A correlation of thrombin generation assay and clot waveform analysis in patients on warfarin

OBJECTIVES

Thrombin generation assays and activated partial thromboplastin time (aPTT)-based clot waveform analysis (CWA), are some examples of global coagulation assays. Each modality evaluates different aspects of the clot forming process to globally define haemostasis with exclusive measurement parameters. Data on CWA are emerging, but its performance against other haemostatic assays is yet to be ascertained. This study evaluates the correlation between aPTT-based CWA and CAT parameters across a range of INR in warfarin-treated patients.

PATIENTS/METHODS

A prospective study consisting of patients on warfarin anticoagulation with varying INR levels. CWA and CAT were performed for the study subjects.

RESULTS

54 samples were included covering an INR range from 1.33-6.89, with a mean of 4.31 +/- 1.13. For CAT parameters, endogenous thrombin potential (ETP) and peak thrombin were assessed. Both unadjusted and adjusted (adjusted for final plateau transmittance) aPTT-based CWA were evaluated for parameters min1 (maximum velocity), min2 (maximum acceleration) and max2 (maximum deceleration). Peak thrombin showed significant correlation with all CWA parameters (min1: r = 0.435, P<0.001; min2: r = 0.485, P<0.001; max2: r = 0.578, P<0.001; adjusted min1: r = 0.734, P<0.001, adjusted min2: r = 0.693, P<0.001; adjusted max2: r = 0.751, P<0.001). ETP correlated significantly with all CWA parameters except unadjusted min1 (min1: r = 0.235, P = 0.087; min2: r = 0.326, P = 0.016; max2: r = 0.437, P<0.001; adjusted min1: r = 0.610, P<0.001, adjusted min2: r = 0.563, P<0.001; adjusted max2: r = 0.642, P<0.001).

CONCLUSION

We demonstrated a modest correlation between CAT and CWA parameters. Adjusted CWA improved this correlation. These findings provide additional understanding of CWA and it's role in the evaluation of global haemostatic function.

Gender-specific differences in haemostatic parameters and their influence on blood loss in bimaxillary surgery

Objective

The objectives of this prospective cohort study were to establish gender-related differences in blood loss and haemostatic profiles associated with bimaxillary surgery. In addition, we aimed to identify if any gender differences could be established which might help predict blood loss volume.

Materials and methods

Fifty-four patients (22 males; 32 females) undergoing bimaxillary surgery for skeletal dentofacial deformities were eligible for inclusion. Blood samples were taken 1 day preoperatively and 48 h postoperatively for detailed gender-specific coagulation analysis incorporating global coagulation assays (endogenous thrombin potential) and specific coagulation parameters. Blood loss was measured at two different time points: (1) the end of surgery, visible intraoperative blood loss (IOB) using ‘subtraction method’; and (2) 48 h postoperatively perioperative bleeding volume (CBL-48 h) using ‘haemoglobin-balance method’ and Nadler’s formula. Correlation and regression analyses were performed to identify relevant parameters affecting the amount of blood loss.

Results

Significant differences in IOB and CBL-48 h were observed (p < 0.001). Men had higher IOB versus women, lacking statistical significance (p = 0.056). In contrast, men had significantly higher CLB-48 h (p = 0.019). Reduced CBL-48 h was shown to be most closely associated with the level of Antithrombin-III being decreased in females.

Conclusions

Male gender is associated with higher IOB and CBL-48 compared with females. Gender does not affect IOB regarding haemostatic profile but does correlate strongly with procedure length. Conversely, CBL-48 is closely associated with gender-specific imbalances in the anticoagulant system.

Clinical relevance

Knowledge of gender-related differences will help clinicians establish predictive factors regarding excessive blood loss in orthognathic surgery and identify at-risk patients.

Nanomechanics of Blood Clot and Thrombus Formation

Mechanical properties have been extensively studied in pure elastic or viscous materials; however, most biomaterials possess both physical properties in a viscoelastic component. How the biomechanics of a fibrin clot is related to its composition and the microenvironment where it is formed is not yet fully understood. This review gives an outline of the building mechanisms for blood clot mechanical properties and how they relate to clot function. The formation of a blood clot in health conditions or the formation of a dangerous thrombus go beyond the mere polymerization of fibrinogen into a fibrin network. The complex composition and localization of in vivo fibrin clots demonstrate the interplay between fibrin and/or fibrinogen and blood cells. Studying these protein–cell interactions and clot mechanical properties may represent new methods for the evaluation of cardiovascular diseases (the leading cause of death worldwide), creating new possibilities for clinical diagnosis, prognosis, and therapy.

Expected final online publication date for the Annual Review of Biophysics, Volume 51 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Global coagulation assays in healthy controls: are there compensatory mechanisms within the coagulation system?

Global coagulation assays (GCAs) may provide a more comprehensive individual hemostatic profiling. We aim to evaluate GCAs (thromboelastography, thrombin generation) in healthy controls, and correlate results with age, gender, lipid status, tissue factor pathway inhibitor (TFPI) and P-selectin. Blood samples were collected from healthy controls (> 18 years of age) not taking anticoagulation or antiplatelet agents and without known cardiovascular disease. Thromboelastography (TEG) was performed on citrated whole blood while calibrated automated thrombogram (CAT), P-selectin (endothelial marker) and TFPI (principle inhibitor of tissue factor-initiated coagulation) were performed on platelet-poor plasma. 153 healthy controls (mean age 42 years, 98 females (64%)) were recruited. Female controls demonstrated more hypercoagulable TEG and CAT parameters while those over 50 years of age demonstrated more hypercoagulable TEG parameters despite comparable thrombin generation. Paradoxically, individuals with "flattened" thrombin curves (lower velocity index (rate of thrombin generation) despite preserved endogenous thrombin potential (amount of thrombin)) were more likely to be male (49% vs 20%, p = 0.003) with increased low-density lipoprotein cholesterol (3.3 vs 2.6 mmol/L, p = 0.003), P-selectin (54.2 vs 47.3 ng/mL, p = 0.038) and TFPI (18.7 vs 8.6 ng/ml, p = 0.001). In addition to reduced velocity index and thrombin peak, controls in the highest TFPI tertile also demonstrated a poorer lipid profile. GCAs can detect subtle changes of the hemostatic profile. Interestingly, reduced thrombin generation was paradoxically associated with increased cardiovascular risk factors, possibly attributable to increased TFPI. This finding may suggest compensation by the coagulation system in response to endothelial activation and represent a biomarker for early cardiovascular disease. A larger prospective study evaluating these assays in the cardiovascular disease population is ongoing.

Thrombin generation in cardiovascular disease and mortality - results from the Gutenberg Health Study

Thrombin generation may be a potential tool to improve risk stratification for cardiovascular diseases. This study aims to explore the relation between thrombin generation and cardiovascular risk factors, cardiovascular diseases, and total mortality. For this study, N=5000 subjects from the population-based Gutenberg Health Study were analysed in a highly standardized setting. Thrombin generation was assessed by the Calibrated Automated Thrombogram method at 1 and 5 pM tissue factors trigger in platelet poor plasma. Lag time, endogenous thrombin potential, and peak height were derived from the thrombin generation curve. Sex-specific multivariable linear regression analysis adjusted for age, cardiovascular risk factors, cardiovascular diseases and therapy, was used to assess clinical determinants of thrombin generation. Cox regression models adjusted for age, sex, cardiovascular risk factors and vitamin K antagonists investigated the association between thrombin generation parameters and total mortality. Lag time was positively associated with obesity and dyslipidaemia for both sexes (p<0.0001). Obesity was also positively associated with endogenous thrombin potential in both sexes (p<0.0001) and peak height in males (1 pM tissue factor, p=0.0048) and females (p<0.0001). Cox regression models showed an increased mortality in individuals with lag time (1 pM tissue factor, hazard ratio=1.46, [95% CI: 1.07; 2.00], p=0.018) and endogenous thrombin potential (5 pM tissue factor, hazard ratio = 1.50, [1.06; 2.13], p=0.023) above the 95th percentile of the reference group, independent of the cardiovascular risk profile. This large-scale study demonstrates traditional cardiovascular risk factors, particularly obesity, as relevant determinants of thrombin generation. Lag time and endogenous thrombin potential were found as potentially relevant predictors of increased total mortality, which deserves further investigation.

Calibrated Automated Thrombinography (CAT), a Tool to Identify Patients at Risk of Bleeding during Anticoagulant Therapy: A Systematic Review

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  < 0.001). Another study found no difference in ETP or peak levels between bleeding and nonbleeding patients in PPP or PRP. When measured in whole blood, both levels were significantly lower in patients with bleeding compared with nonbleeding patients (median [interquartile range, IQR] ETP: 182.5 [157.2-2,847 nM/min] vs. median [IQR] ETP: 256.2 [194.9-344.2 nM/min]; p  < 0.001) and median [IQR] peak: 23.9 [19.6-41.8 nM] vs. median [IQR] peak: 39.1 [24.9-53.2 nM]; p  < 0.05). From the remaining studies, four suggested that CAT-TG is more sensitive in detecting hemostatic abnormalities than INR and one article found ETP and INR to be equally useful. However, insufficient data were provided to validate these conclusions. 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.

The Influence of Race on Plasma Thrombin Generation In Healthy Subjects In Singapore

Race is touted as an independent risk factor for venous thromboembolism (VTE), although the basis for this is varied and contentious. Comparison of plasma thrombin generation (TG) using calibrated automated thrombogram (CAT) across races offers a modality that objectively measures global hemostatic function to evaluate this influence. Direct comparative data across races are currently not available. Aim is to establish the influence of race on plasma TG. Sixty normal participants, matched for age and gender, equally representing 4 races—Caucasian, Chinese, Indian, and Malay—were recruited. Thrombin generation parameters (lag time, time to peak, peak, and endogenous thrombin potential [ETP]) in platelet-poor plasma were measured using CAT. The mean ETP (standard deviation) for the different races were Caucasians: 1338.18 (194.19) nM·min; Chinese, 1318.91 (108.90) nM·min; Indians, 1389.81 (182.61) nM·min; and Malays, 1436.21 (184.24) nM·min. Caucasians had the longest mean lag time of 2.59 ± 0.37 seconds; Indians had the highest mean peak of 284.22 ± 30.74 nM, and Malays had the longest mean time to peak of 5.47 ± 0.59 seconds. Analysis based on race did not demonstrate any significant difference for all TG parameters. The greatest mean difference of ETP between any 2 races (Malays and Chinese) was 117.30 nM·min (95% confidence interval: −45.86 to 280.46 nM·min) which was within the predefined limit of equivalence. In a cohort of healthy participants, TG mediated by plasma factors is not influenced by race and does not explain the reported racial differences in VTE incidence. For the 4 racial groups studied, the use of separate normal ranges for plasma TG might not be essential.

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.

Comparing thrombin generation in patients with hemophilia A and patients on vitamin K antagonists

Essentials It is unknown if hemophilia patients with atrial fibrillation need anticoagulation. Endogenous thrombin potentials (ETP) in hemophilia patients and patients on coumarins were compared. Severe hemophilia patients had comparable ETP to therapeutic international normalized ratio (INR). In non-severe hemophilia, 33% had higher ETP than therapeutic INR and may need anticoagulation. Click to hear Dr Negrier's perspective on global assays for assessing coagulation SUMMARY: Background It is unknown whether patients with hemophilia A with atrial fibrillation require treatment with vitamin K antagonists (VKAs) to the same extent as the normal population. Objective To compare hemostatic potential in hemophilia patients and patients on VKAs using thrombin generation (TG). Methods In this cross-sectional study, TG, initiated with 1pM tissue factor, was measured in 133 patients with severe (FVIII < 1%, n = 15) and non-severe (FVIII 1-50%, n = 118) hemophilia A, 97 patients on a VKA with an international normalized ratio (INR) ≥ 1.5 and healthy controls. Endogenous thrombin potential (ETP) (nm*min) was compared according to FVIII level (< 1%, 1-19% and 20-50%) with healthy controls and patients with sub-therapeutic INR (1.5-1.9) and therapeutic INR (≥ 2.0). Medians and interquartile ranges (IQRs) were calculated. Results Compared with healthy controls (898 [IQR 803-1004]), both hemophilia patients and patients on VKAs had lower median ETPs at 304 (196-449) and 176 (100-250), respectively. ETP was quite similar in severe hemophilia patients (185 [116-307]) and patients with a therapeutic INR (156 [90-225]). Compared with patients with therapeutic INR, ETP in patients with FVIII 1-19% and patients with FVIII 20-50% was higher at 296 (203-430) and 397 (219-632), respectively. All patients with therapeutic INR had an ETP < 400. Considering this threshold, 93% of severe hemophilia patients, 70% of patients with FVIII 1-19% and 52% of patients with FVIII 20-50% had an ETP < 400. Conclusion In severe hemophilia patients, TG was comparable to that in patients with a therapeutic INR. In one-third of non-severe hemophilia patients, TG was higher. These results suggest that anticoagulation therapy should be considered in a substantial proportion of non-severe hemophilia patients.

During warfarin induction, the Fiix-prothrombin time reflects the anticoagulation level better than the standard prothrombin time

Essentials Fiix-prothrombin time (PT) monitoring of warfarin measuring factor (F) II and X, is effective. Plasma obtained during warfarin induction and stable phase in Fiix-trial was assayed. Fiix-PT stabilized anticoagulation earlier than monitoring with traditional PT-INR. FVII had little effect on thrombin generation that was mainly determined by FII and FX.

SUMMARY

Background The prothrombin time (PT) is equally prolonged by reduction of each of the vitamin K-dependent (VKD) factors (F) II, VII and X. The Fiix-PT is only affected by FII and FX, the main contributors to thrombin generation (TG). Objective To test the hypothesis that variability in warfarin anticoagulation is reduced early during monitoring with the normalized PT-ratio calculated from Fiix-PT (Fiix-International Normalized Ratio [INR]) compared with traditional PT-INR monitoring. Also, that because of its insensitivity to FVII, Fiix-PT more accurately reflects TG when Fiix-INR and PT-INR are discrepant. Methods Samples from Fiix-trial participants monitored with either Fiix-PT or PT were used. VKD coagulation factors and TG were measured in samples from 40 patients during stable anticoagulation and in serial samples obtained from 26 patients during warfarin induction. TG was assessed in relation to selective reduction in single VKD factors. Results During Fiix-warfarin induction full anticoagulation measured as FII or FX activity was achieved at a similar rate to that with PT-warfarin but subsequently stabilized better. Fiix-INR but not PT-INR mirrored total TG during initiation. During induction, FII (R2 = 0.66) and FX (R2 = 0.52) correlated better with TG and with a steeper slope than did FIX (R2 = 0.37) and in particular FVII (R2 = 0.21). In vitro, FII and FX were the main determinants of TG at concentrations observed during VKA anticoagulation, whereas FVII and FIX had little influence. Conclusions Fiix-PT monitoring reduces anticoagulation variability, suggesting that monitoring FVII has a limited role during VKA management. TG is better reflected by Fiix-PT.

Thrombin generation and international normalized ratio in inherited thrombophilia patients receiving thromboprophylactic therapy

BACKGROUND

Thrombin generation assay (TGA) is useful as a global functional test for assessing bleeding or thrombotic risk and its modification with therapy. We investigated TGA to assess anticoagulation status compared with the international normalized ratio (INR) system in patients with primary thrombophilia receiving and not undergoing thromboprophylaxis.

MATERIALS AND METHODS

We studied 50 patients with at least one thrombotic event and a confirmed diagnosis of inherited thrombophilia. Thrombin generation was measured in platelet-poor plasma by calibrated automated thrombography (CAT).

RESULTS

Patients in optimal anticoagulation (INR: 2.0-3.0) showed an endogenous thrombin potential (ETP) of 14-56% of normal and a peak of 18-55% of normal. A significant inverse relationship between INR and thrombin generation parameters (ETP, peak and velocity index) and a linear correlation for lag time was found in patients treated with vitamin-K antagonists (VKA). Receiver-operating characteristics (ROC) analysis showed that the optimal cutoff for ETP was 1600.2 nM · min (111.6% of normal, with a sensitivity of 96.6% and a specificity of 92.9%) and for the peak was 298.3 nM (112.1% of normal, with a sensitivity of 96.4% and a specificity of 100%). According to this analysis, ETP was able to identify patients with increased thrombotic and hemorrhagic risk, correlating with severe clinical complications.

CONCLUSION

TGA showed excellent sensitivity and specificity for assessing anticoagulation status in patients with primary thrombophilia receiving VKA, with significant advantages with regard to INR. Clinical data strongly support ETP as a valuable indicator of thrombotic or hemorrhagic risk in patients receiving or not receiving thromboprophylaxis.

Rivaroxaban in antiphospholipid syndrome (RAPS) protocol: a prospective, randomized controlled phase II/III clinical trial of rivaroxaban versus warfarin in patients with thrombotic antiphospholipid syndrome, with or without SLE

INTRODUCTION

The current mainstay of the treatment of thrombotic antiphospholipid syndrome (APS) is long-term anticoagulation with vitamin K antagonists (VKAs) such as warfarin. Non-VKA oral anticoagulants (NOACs), which include rivaroxaban, have been shown to be effective and safe compared with warfarin for the treatment of venous thromboembolism (VTE) in major phase III prospective, randomized controlled trials (RCTs), but the results may not be directly generalizable to patients with APS.

AIMS

The primary aim is to demonstrate, in patients with APS and previous VTE, with or without systemic lupus erythematosus (SLE), that the intensity of anticoagulation achieved with rivaroxaban is not inferior to that of warfarin. Secondary aims are to compare rates of recurrent thrombosis, bleeding and the quality of life in patients on rivaroxaban with those on warfarin.

METHODS

Rivaroxaban in antiphospholipid syndrome (RAPS) is a phase II/III prospective non-inferiority RCT in which eligible patients with APS, with or without SLE, who are on warfarin, target international normalized ratio (INR) 2.5 for previous VTE, will be randomized either to continue warfarin (standard of care) or to switch to rivaroxaban. Intensity of anticoagulation will be assessed using thrombin generation (TG) testing, with the primary outcome the percentage change in endogenous thrombin potential (ETP) from randomization to day 42. Other TG parameters, markers of in vivo coagulation activation, prothrombin fragment 1.2, thrombin antithrombin complex and D-dimer, will also be assessed.

DISCUSSION

If RAPS demonstrates i) that the anticoagulant effect of rivaroxaban is not inferior to that of warfarin and ii) the absence of any adverse effects that cause concern with regard to the use of rivaroxaban, this would provide sufficient supporting evidence to make rivaroxaban a standard of care for the treatment of APS patients with previous VTE, requiring a target INR of 2.5.

Global assays and the management of oral anticoagulation

Coagulation tests range from global or overall tests to assays specific to individual clotting factors and their inhibitors. Whether a particular test is influenced by an oral anticoagulant depends on the principle of the test and the type of oral anticoagulant. Knowledge on coagulation tests applicable in monitoring status and reversal of oral anticoagulation is a prerequisite when studying potential reversal agents or when managing anticoagulation in a clinical setting. Specialty tests based on the measurement of residual activated factor X (Xa) or thrombin activity, e.g., are highly effective for determining the concentration of the new generation direct factor Xa- and thrombin inhibitors, but these tests are unsuitable for the assessment of anticoagulation reversal by non-specific prohemostatic agents like prothrombin complex concentrate (PCC) and recombinant factor VIIa (FVIIa). Global coagulation assays, in this respect, seem more appropriate. This review evaluates the current status on the applicability of the global coagulation assays PT, APTT, thrombin generation and thromboelastography in the management of oral anticoagulation by vitamin K antagonists and the direct factor Xa and thrombin inhibitors. Although all global tests are influenced by both types of anticoagulants, not all tests are useful for monitoring anticoagulation and reversal thereof. Many (pre)analytical conditions are of influence on the assay readout, including the oral anticoagulant itself, the concentration of assay reagents and the presence of other elements like platelets and blood cells. Assay standardization, therefore, remains an issue of importance.

The influence of coagulation factors on the in-treatment biological variation of international normalized ratio for patients on warfarin

BACKGROUND

Biological variation is usually estimated in healthy individuals during steady-state conditions. The aim of this study was to estimate the in-treatment biological variation of the International normalised ratio (INR) and to investigate to what extent the different levels of coagulation factors could explain this variation.

METHODS

Blood samples were collected from randomly included patients on warfarin treatment. INR was determined on a laboratory instrument (STA Compact(®)) and on three point-of-care instruments (Simple Simon(®)PT, CoaguChek(®)XS and INRatio(™)). The level of fibrinogen, and the activity of coagulation factors II, V, VII and X were determined.

RESULTS

The in-treatment within- and between-subject coefficients of variation of INR were dependent on the method and varied between 18 and 24% and 13 and 19%, respectively, and were reduced to 3.9-5.1% and 2.3-5.8%, after correction for coagulation factors which could explain 91-95% of the variance of INR.

CONCLUSIONS

The in-treatment biological variation of INR was higher than reported for healthy individuals as well as patients in a steady-state condition, but by correcting for appropriate coagulation factors it was reduced. The association between INR and coagulation factors was different for the different PT methods mainly due to different sensitivity towards FII and FVII.

Contributions of procoagulants and anticoagulants to the international normalized ratio and thrombin generation assay in patients treated with warfarin: potential role of protein Z as a powerful determinant of coagulation assays

BACKGROUND

The effects of warfarin are measured with the international normalized ratio (INR). However, the thrombin generation assay (TGA) may offer more information about global coagulation. We analyzed the monitoring performance of the TGA and INR and investigated the impact of procoagulants (fibrinogen, factor (F)II, FVII, FIX, and FX) and anticoagulants (proteins C, S, and Z) on them.

METHODS

The TGA was performed on a calibrated automated thrombogram, producing lag time, endogenous thrombin potential (ETP), and peak thrombin in 239 patients treated with warfarin. Pro- and anticoagulant levels were also measured.

RESULTS

The INR was significantly and inversely correlated with ETP. The therapeutic range of ETP comparable to an INR range of 2.0-3.0 was 290.1-494.6. ETP showed comparable performance to the INR as a warfarin-monitoring parameter with respect to clinical complication rate. The median levels of FII, FVII, FIX, and FX and proteins C and Z tended to decrease gradually with increasing anticoagulation intensity according to the INR or ETP. Of note, protein Z levels decreased dramatically with increasing anticoagulation status. INRs were significantly determined by FII, FVII, and protein Z. ETP was significantly dependent on FVII, and proteins C and Z concentration. Protein Z significantly reduced the total amount of thrombin generation and prolonged PT value in vitro.

CONCLUSIONS

The INR and ETP exhibit similar efficacy for warfarin monitoring according to the clinical complication rate. Protein Z is considered to be a significant determinant of INR and ETP in patients on warfarin therapy.

Bleeding risk in warfarinized patients with a therapeutic international normalized ratio: the effect of low factor IX levels

OBJECTIVE

Bleeding is the main complication of warfarin therapy, even patients with an international normalized ratio (INR) in the target range can suffer bleeding, suggesting that INR does not perfectly reflect the therapeutic effect of warfarin. We hypothesized the INR might underestimate the level of anticoagulation in a subject with a lower factor (F) IX level than average.

METHODS AND RESULTS

We modeled warfarin anticoagulation in our in vitro thrombin generation (TG) model by adjusting the levels of vitamin K-dependent factors to those of patients with an INR of 2-3. Variation in FIX had a marked effect on TG but had no effect on the prothrombin time (PT)-INR. A prospective observational, cross-sectional clinical study including 341 consecutive patients admitted to the emergency department with an INR between 2 and 3, showed a statistically lower FIX activity in bleeders (P = 0.004) compared with others. No correlation was found between TG capacity and PT-INR results (P = 0.36). However, in patients, presenting with a warfarin-related hemorrhage, TG was significantly lower (P < 0.001) than others. A correlation on the boundary of significance was observed between TG capacity and FIX levels (P = 0.09).

CONCLUSION

These data demonstrates that patients who bleed when their PT-INR is in the target range 2-3 might have defective TG related to a lower level of FIX than expected.

Thrombin generation assay: a new tool to predict and optimize clinical outcome in cardiovascular patients?

Antithrombotic therapy (including antiplatelet and anticoagulant drugs) is the cornerstone of the current medical treatment of patients with acute coronary syndromes (ACS). This therapy and particularly the new antiplatelet and anticoagulant drugs have significantly reduced the ischemic risk, but have increased bleeding complications. Recently, several studies have emphasized the negative prognostic impact on long-term mortality of these bleeding adverse events. Thus, new assays to estimate the bleeding risk and the efficacy of these antithrombotic drugs are clearly in demand. Regarding the anticoagulant drugs, new promising data have emerged about the thrombin generation assay (TGA). TGA measures the ability of plasma to generate thrombin. TGA may be used to check coagulation function, to value risk of thrombosis and to compare the efficacy of different anticoagulants employed in clinical management of patients with ACS. The TGA result is a curve which describes the variation of thrombin's amount during the activation of the coagulation cascade. All available anticoagulant drugs influence the principal parameters generated by TGA and so it is possible to evaluate the effects of the medical treatment. In this review we provide a brief description of the assay and we summarize the principals of previous studies by analyzing the relationship between anticoagulant drugs and TGA. Moreover, a brief summary of its ability to predict ischemic and bleeding risks has been provided.

The clinical significance of differences between point-of-care and laboratory INR methods in over-anticoagulated patients

INTRODUCTION

Patients receiving warfarin are at increased risk of bleeding when their International Normalised Ratio (INR) >4.5. Although not standardised above 4.5 the INR is measured in over-anticoagulated patients, consequently we have examined the reliability of INR results ≥4.5. We assessed: the relationship between different prothrombin time systems for INRs >4.5; the relationships between the INR and levels of vitamin K-dependent coagulation factors (VKD-CF) and thrombin generation test (TGT) parameters; and the impact that variation in results would have on warfarin dosing.

METHODS

INRs were performed using a CoaguChek XS Plus point-of-care (POC) device (measuring range 0.6-8.0). For POC INRs ≥4.5, laboratory INRs were also measured using a recombinant tissue factor (rTF) and a rabbit brain (RBT) thromboplastin.

RESULTS

There was good correlation between POC (INR ≥4.5, <8.0) and Lab INRs (rTF n=154, rs=0.87, p<0.0001; RBT n=102, rs=0.76, p<0.0001); and significant correlations between each of the VKD-CF and the INR, the strongest being with FVII (POC INR rs=-0.53 p<0.0001; Lab rTF-INR rs=-0.70 p<0.0001). TGT peak thrombin and ETP also showed good correlations with INR values (R(2)>0.71). Using POC and Lab rTF-INR, 109/154 (71%), or POC and Lab RBT-INR 75/102 (74%) results exhibited dosage concordance and/or were within 0.5 INR units. In the remaining patients variation in warfarin dosing was generally slight.

CONCLUSIONS

Our data suggest that CoaguChek XS Plus INRs >4.5 and <8.0 are comparable to laboratory INRs (both methods) and it is probably unnecessary to perform laboratory INRs for clinical management of patients with INRs >4.5 including those >8.0.

Optimizing warfarin reversal--an ex vivo study

BACKGROUND

Warfarin reversal is a common clinical situation. This is commonly performed using vitamin K and, depending on the urgency, fresh frozen plasma (FFP), prothrombin complex concentrates (PCCs), or activated factor VII. Even though PCCs are widely used, the ideal dosing regimen is far from established.

OBJECTIVES

To verify differences in warfarin reversal patterns using FFP, recombinant FVIIa (rFVIIa), and PCC; and to test the hypothesis that supratherapeutic International Normalized Ratios (INRs) might not correlate with thrombin generation (TG) and identify the ideal concentrations of PCC required to reverse various INR thresholds.

METHODS

We studied the effects of FFP, rFVIIa and Beriplex P/N on the INR and TG, using the calibrated automated thrombography assay in ex vivo warfarinized plasma. Plasmas with different INRs were spiked with different concentrations of Beriplex P/N.

RESULTS

Beriplex P/N was the only agent that completely normalized TG and the INR. The endogenous thrombin potential (ETP) and the peak thrombin showed a significant negative correlation with all INRs. The ETP and velocity of TG reached a plateau at an INR of approximately 4.0. A concentration equivalent to a dose of 30 IU kg(-1) Beriplex P/N normalized the ETP, the INR, FII, FVII, FIX and FX of samples with INRs > or = 4.0. Higher doses resulted in hypercoagulable TG patterns. A concentration equivalent to a dose of 20 IU kg(-1) was sufficient to reverse warfarin at an INR range of 2.0-3.9, as judged by the same tests.

CONCLUSIONS

Warfarin reversal algorithms could be simplified with the adoption of this strategy utilizing two doses of PCC, depending on the INR of the patient. This would also lead to cost reductions and, possibly, a reduction in thrombotic risk.