Thrombosis |
From the Department of Internal Medicine II-Cardiology (W.K., A.H.), University of Ulm Medical Center, and the Department of Epidemiology (D.R., H.B.), University of Ulm, Ulm; the Department of Epidemiology (D.R., H.B.), German Center for Research on Ageing, University of Heidelberg, Heidelberg; and the Department of Internal Medicine III-Hematology and Oncology (M.G.), University of Ulm Medical Center, Ulm, Germany.
Correspondence to Hermann Brenner, MD, Department of Epidemiology, German Center for Research on Ageing, Bergheimerstr 20, D-69115 Heidelberg, Germany. E-mail brenner{at}dzfa.uni-heidelberg.de
| Abstract |
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Key Words: D-dimer hemostasis inflammation coronary artery disease case-control study
| Introduction |
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Recently, fibrin D-dimer, the degradation product of cross-linked fibrin, has gained increasing interest for several reasons. First, it can be considered as a global marker of the turnover of cross-linked fibrin and of activation of the hemostatic system. Second, in contrast to several other markers of hemostasis, D-dimer assays are more stable and more practical to measure and therefore may be more suitable for routine clinical and epidemiological purposes.5
Thus, the main aim of the present study was to investigate the association between D-dimer and angiographically determined coronary artery disease (CAD) in patients with stable angina and in controls, taking into account its potential relationship with various other hemostatic and inflammatory variables that have been related to atherosclerotic disease. In control subjects, we additionally assessed the determinants of plasma D-dimers, and in cases, its association with the severity and extent of CAD as measured by 3 different angiographic scores.
| Methods |
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The control group consisted of 477 subjects who were occasional blood donors at the local Red Cross center serving the University hospitals of Ulm. All controls had no history of definite or suspected CAD and did not report infections or surgery within the previous 4 weeks. Participation rate was 78% in eligible patients and 84% in eligible controls. Frequency matching for age and sex was performed, and a case-control ratio of 1:1.5 was intended. The sample size was sufficient to detect an odds ratio (OR) of 1.5-fold or larger for an association of D-dimer (top quartile vs combined 3 lower quartiles) and CAD with 80% power at the 5% level of significance.
All subjects underwent standardized interviews conducted by trained interviewers. Participants were asked about their medical history, including specific questions related to physician-diagnosed hypertension, diabetes, and gastroduodenal disease. Furthermore, current medication, sociodemographic data, and lifestyle habits (including smoking and alcohol consumption) were recorded.
Laboratory Methods
Venous blood was drawn in the morning under standardized conditions, and a complete blood cell count was done (Coulter STKS chamber, Coulter Co). Within 30 minutes, the remaining blood was centrifuged at 3000g for 10 minutes, immediately divided into aliquots, and frozen at -70°C until analysis. For determination of D-dimer levels, the Dimertest Gold EIA (Agen Biomedical Ltd; distributed by Hemachrom Diagnostics) was used. This assay uses DD-3B6 as a monoclonal antibody that recognizes a specific epitope on the cross-linked
-polypeptide chains in the D domain of fibrin and DD-1D2 as a polyclonal antibody that also reacts with an epitope on D-dimer that is not accessible on the intact precursor fibrinogen molecule. Other parameters also determined by ELISA were as follows: interleukin-6 (IL-6; Quantikine, R&D Systems), PAI-1 activity (Immuno), and vWF (Hemochrom). In addition, C- reactive protein (CRP) determinations were done by an immunoradiometric assay (range, 0.05 to 10 mg/L) calibrated with the World Health Organization reference standard 85/506.6 Fibrinogen was measured by immunonephelometry (Dade Behring) and according to the Clauss method. Serum amyloid A (SAA) was also determined by immunonephelometry (Dade Behring), and measurement of plasma viscosity was done in a Harkness Coulter viscometer (Coulter Electronics). Finally, total homocysteine was determined by high-performance liquid chromatography. Interassay coefficients of variation were 7.2% for D-dimer, 7% for IL-6, 12% for CRP, 4.9% for SAA, 5% for fibrinogen, 11% for PAI-1, 15.8% for vWF, 7.4% for total homocysteine, and 2% for plasma viscosity. Total and HDL cholesterol concentrations were determined by routine enzymatic methods. Lipoprotein (a) [Lp(a)] and apoproteins were determined by immunoturbidimetry on a Wako R-30 automated analyzer. All laboratory analyses were done in a blinded fashion.
Angiographic Evaluation
Coronary angiography was performed among cases by the Judkins method. Three different scores were used to evaluate the angiographic severity and extent of CAD: (1) the number of stenosed (>50% of luminal diameter) or occluded vessels (1- to 3-vessel disease); (2) the quantitative extent score (1 to 15 segments) according to the guidelines of the American Heart Association; and (3) qualitative and quantitative evaluation by the Gensini score.7 Scoring of all coronary angiograms was done by a single observer who was blinded to clinical and laboratory data. The intraclass correlation coefficient for intrarater reliability was 1.0 (1- to 3-vessel disease score), 0.79 (tertiles of the extent score), and 0.85 (tertiles of the Gensini score).
Statistical Analysis
Demographic and clinical characteristics in patients and controls were compared in a descriptive way. Levels of markers of hemostasis and inflammation are reported as arithmetic means (±1SD) except for CRP, SAA, PAI-1, and IL-6, for which the geometric means and medians are given owing to their highly skewed distributions. Categorical variables are reported as percentages. Levels of D-dimer were compared by the Kruskal-Wallis test. Spearman rank correlation coefficients were calculated between D-dimer levels and a variety of markers of hemostasis, inflammation, and classic cardiovascular risk factors. Unconditional logistic regression was used to assess the independent association of elevated D-dimer levels (top quartile vs combined 3 lower quartiles) with CAD. In a basic model, only the matching variables age (years) and sex were controlled for (model 1). Other models additionally controlled for nonlipid risk factors like body mass index (BMI, kg/m2), number of pack-years smoked, history of hypertension, history of diabetes, alcohol consumption (g/d), and years of formal school education (model 2); lipid risk factors (total cholesterol (mmol/L); HDL cholesterol (mmol/L); and apo A1, A2, B, C, and E (model 3); Lp(a) (model 4); hemostatic factors (fibrinogen, PAI-1, vWF, and plasma viscosity) (model 5); markers of inflammation (CRP, SAA, IL-6, and leukocyte count) (model 6); and all of the above factors (model 7). To assess the association of the severity of CAD according to different coronary scores with D-dimer levels, we performed a test for trend after adjustment for age and sex. A 2-tailed P value <0.05 was considered statistically significant. All computations were done with SAS software.8
| Results |
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Distribution of D-Dimer Levels
Plasma levels of D-dimer were positively skewed to the right and are therefore given as median, interquartile range, and total range. They were statistically significantly higher in cases than in controls (median, 11.2 ng/mL; interquartile range, 0 to 28.9 ng/mL, range, 0 to 309.2 ng/mL vs 2.8 ng/mL; interquartile range, 0 to 15.1 ng/mL; range, 0 to 579.3 ng/mL; P<0.001). They were also higher in females compared with males (cases: 18.2 vs 10.4 ng/mL, P=0.12; controls: 7.0 vs 1.9 ng/mL; P=0.04). Other markers of hemostasis and inflammation were also consistently higher in cases than in controls.
Correlation Between D-Dimer Levels and Hemostatic, Inflammatory, and Conventional Risk Variables (Controls Only)
Plasma D-dimer levels were positively and significantly correlated with fibrinogen (Clauss method and nephelometry), plasma viscosity, and IL-6 (Table 2). Spearman rank correlation coefficients ranged between 0.12 (IL-6) and 0.25 (fibrinogen by nephelometry). There was no correlation with conventional risk factors, total homocysteine, and hemostatic or inflammatory markers.
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Association Between D-Dimer Levels and CAD (Case-Control Analysis)
In logistic regression analysis (Table 3), the age- and sex-adjusted OR for the presence of CAD was 2.6 (95% confidence interval [CI], 1.9 to 3.5) when the highest quartile of the D-dimer distribution was compared with the combined lower 3 quartiles. Because we were specifically interested in the potential independent contribution of D-dimers to the risk of CAD, we defined several pathophysiological meaningful clusters of variables, carried out adjustments for these sets of variables separately, and included all of them in the final model. The OR did not change appreciably after controlling for nonlipid risk factors (OR, 2.7; 95% CI, 1.9 to 3.9) and lipid risk variables, excluding Lp(a) (OR, 2.7; 95% CI, 1.9 to 3.8). Because Lp(a) may act in a prothrombogenic manner through several potential pathways, we separately adjusted for this variable. However, there was no appreciable effect seen on the risk estimate (OR, 2.6; 95% CI, 1.9 to 3.6). After adjustment for the other hemostatic parameters, the OR remained essentially the same (OR, 2.4; 95% CI, 1.7 to 3.3), and for the markers of inflammation it was reduced only slightly (OR, 2.1; 95% CI, 1.5 to 2.9); in both instances, the OR remained meaningful and statistically significant. In the final model, after controlling simultaneously for all the above variables, the OR was reduced only slightly compared with the age- and sex-adjusted analysis (OR, 2.4; 95% CI, 1.6 to 3.6). Neither additional adjustment for total homocysteine (data not shown) nor controlling for cardiovascular active compounds (aspirin, ß-adrenoceptor blockers, angiotensin-converting enzyme inhibitors, lipid-lowering drugs, and diuretics) altered the results appreciably (OR, 2.3; 95% CI, 1.2 to 4.5). Results were similar in those with a history of previous myocardial infarction compared with those without (data not shown).
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D-Dimer and the Severity and Extent of CAD (Cases Only)
In 305 of 312 patients with angiographically determined CAD, the severity of CAD was evaluated by 3 different coronary scores (Table 4). No association between D-dimer plasma levels and any of the 3 scores, representing severity and extent of CAD, was found.
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D-Dimer and Left Ventricular Function (Cases Only)
There was no correlation between D-dimer levels and angiographically determined left ventricular ejection fraction. However, the number of patients with an ejection fraction <50% was rather small.
| Discussion |
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Thrombosis and inflammation play an important role not only in the pathophysiology of acute ischemic syndromes14,15 but also in the process of atherogenesis, especially in the progression of disease.16 Based on these considerations, a variety of epidemiological and clinical studies have been carried out to investigate new biochemical markers for their ability to improve risk prediction for future cardiovascular events. In the vast majority of these studies, however, only 1 of these markers has been tested. Although several of them have been shown to predict risk independently of conventional risk factors, the question remains which marker should be preferred in the routine clinical setting, or which cluster of variables reflecting different pathophysiological aspects of atherothrombosis should be used. Besides analytical and technical considerations, the main reason in favor or against a given marker consists in its independence of other markers reflecting the same pathophysiological pathway. Thus, the main aim of the present study was to investigate the association between D-dimer and CAD, taking into account its potential relationship with various other hemostatic and inflammatory variables that have been related to atherosclerotic disease.
The association that we found between D-dimer levels and CAD was strong and at least on the order of that found for conventional risk factors, and it decreased only slightly after controlling for a large variety of potential confounders in multivariable analysis. It is important to note that the association was essentially independent of other prothrombotic variables like fibrinogen, PAI-1 activity, vWF, total homocysteine, and Lp(a) and was only slightly reduced after controlling for several markers of inflammation. Furthermore, major cardiovascular compounds used in this population did not affect the association between D-dimer levels and CAD.
Elevated D-dimer levels have been found to predict the risk of future coronary events independently of conventional risk factors in initially healthy, middle-aged male and female subjects,1719 in elderly men and women,20 as well as in patients with known peripheral arterial occlusive disease,21 and after myocardial infarction.22 In the Edinburgh Artery Study, a positive association with stroke was also found.23 Only in the Physicians Health Study24 was the association between D-dimer and risk of myocardial infarction no longer significant after controlling for either total and HDL cholesterol or markers of the fibrinolytic system. In a formal meta-analysis of prospective studies,25 an OR of 1.7 (95% CI, 1.3 to 2.2) was discovered when individuals with baseline D-dimer values in the top third versus those in the bottom third were compared, with no relevant differences between population-based cohorts and patients with preexisting vascular disease.
Similar to our results obtained in a case-control design, Lowe et al17,18 reported no significant confounding of the association between D-dimer and the future risk of myocardial infarction by either traditional nonlipid or lipid risk factors. Only the introduction of markers of inflammation decreased the association.18 The strongest decrease in the OR in our analyses, though still moderate in magnitude, was also seen after controlling for markers of inflammation. However, the association between D-dimers and CAD remained significant. D-dimer may also be involved in pathophysiological pathways mediating inflammation, because it is known to represent a measure of extracellular fibrin turnover, for example, in various inflammatory states.5 Alternatively, the reduction in the OR seen in this study may be a random event, because in the fully adjusted model, it was essentially unchanged compared with age-adjusted analysis.
Finally, no association was observed between the severity or extent of atherosclerosis of the coronary tree and D-dimer levels in patients. In 1 study,26 a weak correlation was seen, which seemed to be due to the inclusion of controls without significant CAD. In another study,27 the severity of peripheral atherosclerosis was significantly related to D-dimer levels. Such a finding is conceivable because peripheral arterial disease usually involves much larger atheromatous beds and results in higher D-dimer levels and greater variability compared with CAD.
Our study has several limitations that need to be addressed. First, its case-control design did not allow causal inferences to be made but only hypotheses to be generated. Second, asymptomatic CAD in control subjects cannot be ruled out because no electrocardiogram or angiogram was available; however, the prevalence of CAD in an asymptomatic, middle-aged population appears to be low.28 Third, blood donors tend to be healthier than population-based controls. However, we tried to minimize this potential bias by performing multivariable adjustments. The present study has also several strengths. We investigated a homogeneous group of patients with exclusively chronic stable CAD. Furthermore, we measured a variety of biomarkers reflecting the hemostatic system and inflammation, which enabled us to carefully analyze the relationship between D-dimers and these other emerging markers of CAD risk.
Conclusions
D-dimers can be regarded as a global marker of the turnover of cross-linked fibrin and of activation of the hemostatic system. D-dimer levels seem to be essentially independent of other cardiovascular risk factors, which suggests that they might add relevant information in addition to lipid variables and other classic risk factors. In contrast to several other markers of hemostasis, D-dimer assays are more stable and more practical to measure and therefore, may be more suitable from a technical point of view for epidemiological purposes.29 However, among several other reasons, a lack of standardization still represents 1 major problem that must be solved before its introduction into the clinical routine can be recommended to help improve risk prediction in atherothrombotic diseases.
| Acknowledgments |
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| Footnotes |
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Received April 22, 2001;
revision received July 20, 2001;
| References |
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