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Keywords:

  • endogenous thrombin potential;
  • hypercoagulability;
  • laboratory screening;
  • thrombophilia

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Acknowledgement
  8. Disclosure of Conflict of Interests
  9. References

Summary. Background: The assessment of the risk of recurrent venous thromboembolism (VTE) is important to determine the optimal duration of secondary prophylaxis. The risk can be estimated by measuring individual parameters reflecting hypercoagulability. Because of the large numbers of such putative parameters, the assessment in individual patients is complex. Application of global assays reflecting the pro-/anti-coagulant balance in vivo would be desirable. Objectives: To investigate the relationship between recurrent VTE and thrombin generation (TG). Patients-methods: Two hundred and fifty-four patients were followed-up after a first episode of unprovoked, objectively documented VTE for a period of 2.7 years after discontinuation of treatment with vitamin K antagonists. TG was measured 1 month after discontinuation of treatment as endogenous thrombin potential (ETP), peak thrombin and lag-time in the presence or absence of thrombomodulin. The study outcome was objectively documented symptomatic recurrent VTE. Results: Patients with ETP or peak (measured in the presence of thrombomodulin) of >960 nm*min or >193 nm had hazard ratios (HR) (95% CI) for recurrent VTE of 3.41 (1.34–8.68) or 4.57 (1.70–12.2) as compared with those with an ETP <563 nm*min or peak <115 nm. Patients with lag-time <14.5 min had HR of 3.19 (1.29–7.89) as compared with those with lag-time >20.8 min. HR for ETP, peak or lag-time measured in the absence of thrombomodulin were smaller than those measured in the presence of thrombomodulin. Conclusions: The measurement of TG helps to identify patients at higher risk of VTE recurrence. The increased risk may be better appreciated if the test is performed in the presence of thrombomodulin.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Acknowledgement
  8. Disclosure of Conflict of Interests
  9. References

Venous thromboembolism (VTE) is a frequent disease with an age-standardized incidence of first event of nearly 2 per 1000 inhabitants per year [1]. The 30-day case fatality rate associated with deep vein thrombosis or pulmonary embolism has been estimated to be as high as 5% or 44%, respectively [2]. Among the patients who survive, the cumulative incidence of recurrent VTE or the occurrence of the post-thrombotic syndrome after 8 years of follow-up is 30% or 29%, respectively [3]. The risk of VTE recurrence increases with age and with a number of genetic or circumstantial risk factors, such as the presence of some thrombophilic mutations, cancer and obesity. Recent data suggest that it is possible to predict the risk of recurrence by measuring such individual plasmatic biomarkers as D-dimer [4,5], thrombin activatable fibrinolysis inhibitor [6], homocysteine [7,8], vitamin B6 [8,9], factor (F) VIII [10] or FIX [11]. Tests designed to explore the protein C pathway [12] or those designed to explore global coagulation such as the activated partial thromboplastin time (APTT) [13–15] or thrombin generation [16,17] have also received some attention, but results for the latter test are contrasting. In this study we elected to measure thrombin generation for plasma samples collected from a prospective cohort of patients enrolled in the Prolong clinical trial [18].

Patients and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Acknowledgement
  8. Disclosure of Conflict of Interests
  9. References

Patients

Some of the patients enrolled in the multicenter, prospective Prolong clinical trial [18] were included in the present study upon provision of written informed consent and approval from the Institutional Review Board. The Prolong trial was designed to assess the value of D-dimer testing to determine the duration of anticoagulant therapy and involved patients who had had a first episode of symptomatic, unprovoked VTE and had completed the period of anticoagulation with vitamin K antagonists for at least 3 months. Unprovoked VTE was defined as an event not associated with one of the following conditions: pregnancy/puerperium, recent fracture or plaster leg casting, bed rest for more than 3 days, recent surgery, cancer, antithrombin congenital deficiency or the antiphospholipid antibody syndrome. Although only capillary blood for D-dimer testing was required, thrombosis centers participating in the trial were invited to collect venous blood for further investigation. Blood was collected after 1 month of discontinuation of treatment with vitamin K antagonists into vacuum tubes containing trisodium citrate (0.109 m) as anticoagulant at a proportion of 1:9 (anticoagulant:blood) and centrifuged at 2000 × g (room temperature) for 20 min. The supernatant plasma was harvested, aliquoted in capped plastic tubes, quick-frozen by immersion in liquid nitrogen and stored at −70 ° C until testing. Each patient attended regular visits at intervals of 3–6 months for a follow-up period of 2.7 years to ascertain recurrent episodes of VTE that were diagnosed with standard procedures as reported in detail elsewhere [18]. Briefly, patients were asked to contact the center whenever they recorded symptoms suggestive of VTE. Recurrent deep vein thrombosis was diagnosed if the results of compression ultrasonography showed that a previous fully compressible segment (contra- or ipsi-lateral) was no longer compressible or if during compression there was an increase of at least 4 mm in the diameter of the residual thrombus. If the diameter of the thrombus changed by less than 4 mm, or in the case of high or moderate clinical probability and normal proximal compression ultrasonography, the examination was repeated 1 week later. Recurrent pulmonary embolism was diagnosed by means of clinical probability combined with ventilation-perfusion lung scanning or helical CT and compression ultrasonography. All suspected events were evaluated by an adjudication committee unaware of the laboratory results. The Prolong trial enrolled a total of 608 patients; those who according to the protocol [see ref. n. 18 for details] were randomized to resume anticoagulation (= 103) were excluded from this study. Of the remaining 505 patients eligible for this study, plasma samples were available for 254.

Methods

Thrombin generation was evaluated according to Hemker et al. [19], as reported in detail by Chantarangkul et al. [20]. The test is based on the activation of coagulation in thawed platelet-poor plasma after addition of human relipidated recombinant tissue factor as a trigger (Recombiplastin, Instrumentation Laboratory, Orangeburg, NY, USA) in the presence of the synthetic phospholipids 1,2-dioleoyl-sn-glycero-3-phosphoserine (DOPS), 1,2-dioleoyl-sn-glycero-3-phosphoetanolamine (DOPE) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) (Avanti Polar Lipids Inc., Alabaster, AL, USA) in the proportion of 20/20/60 (M/M) as platelet substitutes. The concentrations of tissue factor and phospholipids in the test system were 1 pm and 1.0 μm, respectively. Testing was also performed in the presence of soluble thrombomodulin (ICN Biomedicals, Aurora, OH, USA) added in the reaction mixture at a final concentration of 4 nm. Continuous registration of the generated thrombin was achieved with a fluorogenic synthetic substrate (Z-Gly-Gly-Arg-AMC HCl, Bachem, Switzerland) added to the test system at a final concentration of 417 μm. The procedure was carried out with an automated fluorometer (Fluoroskan Ascent®, ThermoLabsystem, Helsinki, Finland). Readings from the fluorometer were automatically recorded and calculated by a dedicated software (ThrombinoscopeTM, Thrombinoscope BV, Maastricht, The Netherlands), which displays thrombin generation curves [nm thrombin vs. time (min)] and calculates the time (min) of lag-phase that follows the addition of the trigger, the thrombin peak (nm), the time to peak and the area under the curve defined as endogenous thrombin potential (ETP) expressed as nm thrombin times minutes (nm*min). Thrombin generation is measured as function of an internal calibrator for thrombin (Thrombin Calibrator, Thrombinoscope BV). Operators testing for thrombin generation were unaware of the patients’ status with respect to VTE recurrence.

Statistical analyses

Continuous variables are expressed as mean and standard deviations and the unpaired t-test was used to test for differences of ETP, thrombin peak or lag-time values between patients with or without VTE recurrence. Relative risk of recurrent VTE was calculated according to tertiles (second and third tertiles compared with the first for ETP and peak, and first and second tertiles compared with the third for lag-time). Hazard ratios (HR) and 95% confidence intervals (95% CI) were calculated with the Cox proportional hazards model. Adjustments included age, gender, type of index event, duration of oral anticoagulation and normal/abnormal D-dimer. A further adjustment included the presence/absence of inherited thrombophilia defined as the presence of FV Leiden or prothrombin G20210A mutations. Kaplan–Meier survival curves were plotted to estimate the cumulative incidence of VTE recurrence. The statistical significance was set at < 0.05. All analyses were performed with the SPSS version 13.0 software (Chicago, IL, USA).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Acknowledgement
  8. Disclosure of Conflict of Interests
  9. References

The baseline characteristics of patients are shown in Table 1. During the follow-up period VTE recurred in 34/254 (13.4%) patients. Patients without recurrent VTE had lower ETP levels and thrombin peak, and longer lag-times than patients with recurrence, either in the absence or presence of thrombomodulin (Table 2). As expected, ETP and thrombin peak measured in the absence of thrombomodulin were higher than those measured in the presence of thrombomodulin; lag-time was longer in the presence than in the absence of thrombomodulin (Table 2). None of the parameters were significantly correlated with age. The distribution of patients who had VTE recurrence according to tertiles of ETP, peak thrombin or lag-time is shown in Tables 3 and 4.

Table 1.   Baseline characteristics of the investigated patients
  1. VTE, venous thromboembolism; DVT, deep vein thrombosis; PE, pulmonary embolism; VKA, vitamin K antagonists.

Sex [M/F]138/116
Age, years [median (range)] 66 (20–84)
Site of the first VTE
 Proximal DVT, n (%)159 (62.6%)
 Proximal DVT + PE, n (%) 54 (21.3%)
 Isolated PE, n (%) 41 (16.1%)
Thrombophilic abnormalities
 Factor V Leiden mutation, n (%) 29 (11.4%)
 G20210A prothrombin mutation, n (%) 16 (6.3%)
 Combined or homozygous alterations, n (%) 7 (2.8%)
Abnormal D-dimer, n (%) 63 (24.8%)
Duration of VKA therapy, mo [median (range)] 8 (3–61)
 ≤6 months, n (%) 60 (23.6%)
Table 2.   Mean (SD) ETP, peak thrombin and lag-time values for patients with and without VTE recurrence
ParameterPatients with VTE recurrence (= 34)Patients without VTE recurrence (= 220)P value
In the absence of thrombomodulin
 ETP (nm*min)1502 (446)1361 (499)0.122
 Thrombin peak (nm)232 (82)187 (89)0.005
 Lag-time (min)11.9 (5.7)12.8 (4.7)0.319
In the presence of thrombomodulin
 ETP (nm*min)986 (422)763 (468)0.009
 Thrombin peak (nm)201 (75)148 (88)<0.001
 Lag-time (min)16.8 (7.1)19.1 (9.5)0.174
Table 3.   Relative risk of recurrent VTE according to tertiles of ETP, thrombin peak or lag-time measured in the absence of thrombomodulin
 No. patientsNo. (%) VTE recurrenceHR (95% CI) univariateHR (95% CI) multivariate*HR (95% CI) multivariate
  1. *Adjusted for age, gender, type of index event, duration of anticoagulation, and normal/abnormal D-dimer.

  2. Adjusted for age, gender, type of index event, duration of anticoagulation, normal/abnormal D-Dimer, and absence/presence of inherited thrombophilic abnormalities.

ETP (nm*min)
 ≤1172847 (8.3)1 (reference)1 (reference)1 (reference)
 1172–15658310 (12)1.49 (0.57–3.92)1.35 (0.50–3.61)1.28 (0.47–3.44)
 >15658717 (19.5)2.54 (1.05–6.12)2.41 (0.99–5.86)2.37 (0.97–5.79)
 Total25434 (13.4)   
Thrombin peak (nm)
 ≤150847 (8.3)1 (reference)1 (reference)1 (reference)
 150–237837 (8.4)1.08 (0.38–3.09)0.98 (0.34–2.85)0.96 (0.33–2.81)
 >2378720 (23)3.09 (1.31–7.32)2.65 (1.10–6.39)2.56 (1.06–6.18)
 Total25434 (13.4)   
Lag-time (min)
 ≤10.78515 (17.6)2.29 (0.93–5.62)3.07 (1.23–7.66)2.91 (1.16–7.27)
 10.8–13.88212 (14.6)1.86 (0.73–4.73)2.04 (0.79–5.24)1.95 (0.76–5.01)
 >13.8877 (8.0)1 (reference)1 (reference)1 (reference)
 Total25434 (13.4)   
Table 4.   Relative risk of recurrent VTE according to tertiles of ETP, thrombin peak or lag-time measured in the presence of thrombomodulin
 No. patientsNo. (%) VTE recurrenceHR (95% CI) univariateHR (95% CI) multivariate*HR (95% CI) multivariate
  1. *Adjusted for age, gender, type of index event, duration of anticoagulation, and normal/abnormal D-dimer.

  2. Adjusted for age, gender, type of index event, duration of anticoagulation, normal/abnormal D-dimer, and absence/presence of inherited thrombophilic abnormalities.

ETP (nm*min)
 ≤563856 (7.1)1 (reference)1 (reference)1 (reference)
 563–960829 (11)1.63 (0.58–4.59)1.60 (0.57–4.50)1.52 0.54–4.33)
 >9608719 (21.8)3.35 (1.34–8.39)3.41 (1.34–8.68)3.27 (1.28–8.35)
 Total25434 (13.4)   
Thrombin peak (nm)
 ≤115845 (5.9)1 (reference)1 (reference)1 (reference)
 115–193837 (8.4)1.50 (0.48–4.73)1.42 (0.45–4.51)1.37 (0.43–4.37)
 >1938722 (25.3)4.69 (1.78–12.4)4.57 (1.70–12.2)4.36 (1.62–11.8)
 Total25434 (13.4)   
Lag-time
 ≤14.58416 (19.0)2.39 (0.98–5.80)3.19 (1.29–7.89)3.11 (1.25–7.72)
 14.6–20.88311 (13.2)1.58 (0.61–4.07)1.81 (0.67–4.85)1.87 (0.70–5.03)
 >20.8877 (8.0)1 (reference)1 (reference)1 (reference)
 Total25434 (13.4)   

Endogenous thrombin potential (ETP)

When ETP was measured in the absence of thrombomodulin the univariate HR (95% CI) for VTE recurrence of those patients falling into the third tertile (>1565 nm*min) compared with the first (lowest) tertile (≤1172 nm*min) was 2.54 (1.05–6.12). Upon adjustment for age, gender, type of index event, duration of oral anticoagulation and normal/abnormal D-dimer the correspondent HR value was 2.41 (0.99–5.86) (Table 3). Upon further adjustment for the presence/absence of inherited thrombophilic abnormalities HR was 2.37 (0.97–5.79). When the ETP was measured in the presence of thrombomodulin the univariate HR (95% CI) for VTE recurrence was 3.35 (1.34–8.39). Upon adjustment for age, gender, type of index event, duration of oral anticoagulation and normal/abnormal D-dimer the correspondent value was 3.41 (1.34–8.68) (Table 4). Upon further adjustment for the presence/absence of inherited thrombophilic abnormalities HR was 3.27 (1.28–8.35). Kaplan–Meyer thrombosis-free survival curves are shown in Fig. 1. At 2.7 years of follow-up the cumulative incidence of recurrent VTE for those patients in the highest tertile was 0.23.

image

Figure 1.  Cumulative incidence and hazard ratios (HR, 95% confidence interval) of VTE recurrence according to ETP measured in the presence of thrombomodulin. Solid, dashed and dotted lines, 3rd, 2nd and 1st tertile, respectively.

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Peak thrombin

When the peak was measured in the absence of thrombomodulin the univariate HR (95% CI) for VTE recurrence of those patients falling into the third tertile (>237 nm thrombin) compared with the first (lowest) tertile (≤150 nm thrombin) was 3.09 (1.31–7.32). Upon adjustment for age, gender, type of index event, duration of oral anticoagulation and normal/abnormal D-dimer the correspondent value was 2.65 (1.10–6.39) (Table 3). Upon further adjustment for the presence/absence of inherited thrombophilic abnormalities HR was 2.56 (1.06–6.18). When the peak was measured in the presence of thrombomodulin the univariate HR (95% CI) for VTE recurrence was 4.69 (1.78–12.4). Upon adjustment for age, gender, type of index event, duration of oral anticoagulation and normal/abnormal D-dimer the correspondent value was 4.57 (1.70–12.2) (Table 4). Upon further adjustment for the presence/absence of inherited thrombophilic abnormalities HR was 4.36 (1.62–11.8). Kaplan-Meyer thrombosis-free survival curves are shown in Fig. 2. At 2.7 years of follow-up the cumulative incidence of recurrent VTE for those patients in the highest tertile was 0.28.

image

Figure 2.  Cumulative incidence and hazard ratios (HR, 95% confidence interval) of VTE recurrence according to peak thrombin measured in the presence of thrombomodulin. Solid, dashed and dotted lines, 3rd, 2nd and 1st tertile, respectively.

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Lag-time

When the lag-time was measured in the absence of thrombomodulin the univariate HR (95% CI) for VTE recurrence of those patients falling into the first tertile (<10.7 min) compared with the third (highest) tertile (>13.8 min) was 2.29 (0.93–5.62). Upon adjustment for age, gender, type of index event, duration of oral anticoagulation and normal/abnormal D-dimer the correspondent value was 3.07 (1.23–7.66) (Table 3). Upon further adjustment for the presence/absence of inherited thrombophilic abnormalities HR was 2.91 (1.16–7.27). When the lag-time was measured in the presence of thrombomodulin the univariate HR (95% CI) for VTE recurrence was 2.39 (0.98–5.80). Upon adjustment for age, gender, type of index event, duration of oral anticoagulation and normal/abnormal D-dimer the correspondent value was 3.19 (1.29–7.89) (Table 4). Upon further adjustment for the presence/absence of inherited thrombophilic abnormalities HR was 3.11(1.25–7.72). Kaplan–Meyer thrombosis-free survival curves are shown in Fig. 3. At 2.7 years of follow-up the cumulative incidence of recurrent VTE for those patients in the lowest tertile was 0.20.

image

Figure 3.  Cumulative incidence and hazard ratios (HR, 95% confidence interval) of VTE recurrence according to lag-time measured in the presence of thrombomodulin. Solid, dashed and dotted lines, 3rd, 2nd and 1st tertile, respectively.

Download figure to PowerPoint

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Acknowledgement
  8. Disclosure of Conflict of Interests
  9. References

The assessment of the risk of recurrence after a first episode of VTE is an important prerequisite to determine the optimal duration of secondary prophylaxis. The risk may be estimated to some extent by measuring in plasma individual laboratory parameters that reflect hypercoagulability [4,5,10–15]. Hypercoagulability may in turn be due to increased levels of coagulation factors, decreased activity of anticoagulant proteins or the presence of such thrombophilic polymorphisms as FV Leiden. If one assumes that thrombosis is a multifactorial disease and that each risk factor may act independently, the overall risk in individual patients should be evaluated by measuring all the aforementioned parameters. The cost for such an evaluation would be high; furthermore, the way single risk factors (genetic or circumstantial) interact with each other to determine the overall risk is unknown; therefore, the value of performing such relatively high numbers of laboratory determinations is at present uncertain. The alternative would be the application of global coagulation assays on the assumption that they ultimately reflect the action that all risk factors (pro- and anti-coagulant) have on hypercoagulability. In this respect, shortened APTT proved reliable as a predictor of first VTE event [21] and recurrence [13–15]; the thrombin generation assay proved reliable as a predictor of first event [17], but contrasting results have been obtained with respect to prediction of recurrence [16,17]. Starting from this background, we assessed the value of thrombin generation measured as ETP (i.e. the area under the curve), peak thrombin or lag-time (i.e. the time elapsing from the addition of the trigger and the onset of thrombin generation) to assess the risk of recurrence in a prospective cohort of patients who were followed-up after a first episode of objectively documented and unprovoked VTE for a period of 2.7 years after discontinuation of treatment with vitamin K antagonists. Our results are consistent with those reported by Hron et al. [16] and indicate that patients who present with a thrombin peak greater than 193 nm have a 4.57-fold increased risk of recurrence as compared with those with a peak smaller than 115 nm. The results also indicate that the risk was independent from other confounding variables such as age, gender, type of index event, duration of anticoagulation and normal/abnormal D-dimer. The risk was somewhat reduced (but not abrogated) after further adjustment for the presence/absence of the most frequent inherited thrombophilic abnormalities, thus indicating that enhanced thrombin generation is not entirely due to the presence of FV Leiden or the prothrombin G20210A mutations. This is in line with the observation that the presence of these two mutations is not consistently associated with the risk of recurrent VTE [22–31]. When results were evaluated according to the ETP, the risk of recurrence was slightly lower [HR, 3.41 (95% CI, 1.34–8.68)], but still statistically significant. In this study we have also shown for the first time that shortened lag-time as measured in the thrombin generation test is associated with an increased risk of VTE recurrence. It should be noted that the meaning of lag-times can be regarded as that of conventional coagulation times [32]. Accordingly, our finding is in line with the observation that shortened conventional coagulation times such as the APTT are risk factors for recurrent VTE [13–15]. Although not invariably confirmed [10,11], the risk of recurrent VTE is thought to increase with age. Accordingly, thrombin generation should be associated with age. The lack of significant correlation as found in this study may, therefore, represent an apparent contradiction and would require further studies.

At variance with Hron et al. [16], we measured thrombin generation in the absence or presence of thrombomodulin. As expected, the whole amount of generated thrombin (measured as either ETP or peak) was smaller when the test was performed in the presence than in the absence of thrombomodulin, but the association between either the ETP or the thrombin peak and the risk of recurrent VTE was higher (higher HRs) when the test was carried out in the presence of thrombomodulin (Table 3 and 4). This is in line with the concept that the protein C anticoagulant system needs to be activated by thrombomodulin in order to exert its full anticoagulant activity [33]. The addition of thrombomodulin makes the thrombin generation assay more suitable to reflect the small fluctuations in the activity of the proteins involved in the protein C pathway that may occur in vivo, thus providing a more robust global test of both the pro- and anticoagulant drives. However, this raises two issues: first, thrombomodulin in vivo is located on endothelial cells whereas in the thrombin generation system it is added to the fluid phase; second, as thrombomodulin is a transmembrane protein, any inference about the appropriate concentration to be used in the test system in order to mimic the in vivo conditions is difficult. The final concentration used in this study (4 nm) was derived from our previous (unpublished) experience in which this concentration gave the best discrimination of thrombin generation between healthy subjects and patients with congenital protein C deficiency taken as a natural model of hypercoagulability. However, it should be noted that van Hylckama Vlieg et al., who measured the ETP in the presence of thrombomodulin, did not confirm the association of high levels of either ETP or peak thrombin with the increased risk of recurrence [17]. Although the reasons for such discrepancy are presently unclear, they probably rest on the design of the two assays. In this study we used smaller amounts of tissue factor and phospholipids than those used by van Hylckama Vlieg et al. [17] (1 vs. 15 pm tissue factor and 1 vs. 4 μm phospholipids) to trigger coagulation. Low concentrations of tissue factor in the thrombin generation assay are more suitable than high concentrations to mimic the conditions that occur in vivo. On the other hand, low concentrations of triggers have been reported to influence thrombin generation due to the undesirable contact activation that may variably occur in blood when such activation is not prevented by the addition of corn trypsin inhibitor [34]. This may be regarded as a limitation of the present and previous study carried out by Hron et al. [16] because in both studies blood was not collected in the presence of corn trypsin inhibitor. However, it is unlikely that the effect of contact activation on thrombin generation in both studies was different in the two populations of patients with or without VTE recurrence.

The risk of recurrent VTE in this study was assessed in a subgroup of patients enrolled in the Prolong clinical trial that included patients with a first, unprovoked episode of VTE [18]. According to the Prolong protocol D-dimer was measured 1 month after stopping anticoagulant therapy and patients with normal values discontinued anticoagulation, but were followed-up for recurrent VTE and were, therefore, eligible for the present study. Patients with abnormal D-dimer were randomized to stop or to resume anticoagulation. While the former were eligible for the present study, the latter were not, because (being anticoagulated) they had a much lower risk of recurrent VTE; therefore, their inclusion would have weakened the estimate of the association between thrombin generation and the risk of recurrent VTE. Because of these inclusion criteria the present study evaluated a smaller proportion (i.e. 25%, see Table 1) of patients with abnormal D-dimer than that evaluated in the Prolong (37%) [18] or other trials [4,5]. If one considers that patients with abnormal D-dimer have an increased risk of VTE recurrence [4,5], it would appear that we enrolled in this study patients who had a relatively low risk of recurrence. It is, therefore, tempting to speculate that the risk of recurrent VTE associated with the thrombin generation test may be even higher than that estimated in this study.

In conclusion, the results of this large prospective study show that the measurement in plasma of thrombin generation induced by small amounts of tissue factor helps to identify patients at higher risk of VTE recurrence. The increased risk is better appreciated if the test is performed in the presence of thrombomodulin. The possible advantage of the thrombin generation assay over the other individual parameters or the global test for the protein C pathway, already established by prospective studies as predictors of VTE recurrence, rests on the fact that thrombin generation may be considered as a laboratory parameter reflecting the composite effect of most of the other individual risk factors.

Acknowledgement

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Acknowledgement
  8. Disclosure of Conflict of Interests
  9. References

The authors wish to thank P. Bucciarelli for helpful discussion during the preparation of the manuscript.

Disclosure of Conflict of Interests

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Acknowledgement
  8. Disclosure of Conflict of Interests
  9. References

The authors state that they have no conflict of interest.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Acknowledgement
  8. Disclosure of Conflict of Interests
  9. References
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