(Circulation. 1997;95:1777-1782.)
© 1997 American Heart Association, Inc.
Articles |
Interrelation of Hyperhomocyst(e)inemia, Factor V Leiden, and Risk of Future Venous Thromboembolism
From the Division of Preventive Medicine (P.M.R., C.H.H.), the Cardiovascular Division (P.M.R.), and the Channing Laboratory (M.J.S.), Department of Medicine, Brigham and Women's Hospital, and the Department of Ambulatory Care and Prevention (C.H.H.), Harvard Medical School, Boston, Mass; the Departments of Nutrition (M.J.S.) and Epidemiology (M.J.S., C.H.H.), Harvard School of Public Health, Boston, Mass; the Laboratory Medicine Division (J.P.M.), Washington University School of Medicine, St Louis, Mo; the Oregon Regional Primate Research Center (M.R.M.), Beaverton; and the Jean Mayer Human Nutrition Research Center at Tufts University (J.S.), Boston, Mass.
Correspondence to Paul M. Ridker, Department of Medicine, Brigham and Women's Hospital, 900 Commonwealth Ave E, Boston, MA 02115. E-mail PMRIDKER{at}BICS.BWH.HARVARD.EDU
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Abstract |
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Background Because patients with rare familial homocystinuria who also carry factor V Leiden have an increased incidence of venous thromboembolism (VTE), we hypothesized an interrelation of moderate hyperhomocyst(e)inemia, factor V Leiden, and risk of VTE in the general population.
Methods and Results In a large prospective cohort, we determined total homocysteine level and factor V Leiden mutation in baseline blood samples from 145 initially healthy men who subsequently developed VTE and among 646 men who remained free of vascular disease during a 10-year follow-up period. Hyperhomocyst(e)inemia was defined as a total homocysteine level above the 95th percentile (17.25 µmol/L). Compared with men with normal total homocysteine levels, those with hyperhomocyst(e)inemia had no increase in risk of any VTE but were at increased risk of idiopathic VTE (relative risk [RR]=3.4, P=.002). Compared with men without Leiden mutation, those with mutation were at increased risk of developing any VTE (RR=2.3, P=.005) as well as idiopathic VTE (RR=3.6, P=.0002). Compared with men with neither abnormality, those affected by both disorders had a 10-fold increase in risk of any VTE (RR=9.65, P=.009) and a 20-fold increase in risk of idiopathic VTE (RR=21.8, P=.0004).
Conclusions Apparently healthy men with coexistent hyperhomocyst(e)inemia and Leiden mutation are at substantially increased risk of developing future VTEs, particularly those events considered idiopathic. In these data, the risk of VTE among doubly affected individuals was far greater than the sum of the individual risks associated with either abnormality alone.
Key Words: factor V Leiden • thrombosis, venous • homocysteine • embolism, pulmonary
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Introduction |
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Although marked elevations of tHcy are found in patients with rare inherited defects of methionine metabolism,1 2 3 many cases of moderate hyperhomocyst(e)inemia result from less severe genetic defects and from inadequate intake of folate and vitamins B6 and B12.4 Elevated levels of tHcy are associated with increased risk of arterial occlusions and correlate with extent of atherosclerotic disease.5 6 7 8 Recent data suggest that hyperhomocyst(e)inemia may also be a risk factor for VTE. In two recent case-control studies,9 10 patients with tHcy levels above the 95th percentile were found to have a risk of VTE twice that of patients with lower levels, and a third study11 reported a high prevalence of hyperhomocyst(e)inemia in young adults with deep venous thrombosis or pulmonary embolism. However, two other studies12 13 report no association between tHcy level and risk of VTE.
One possible explanation for these apparently conflicting results is that tHcy level may be a relatively weak risk factor for VTE unless a second defect of endogenous anticoagulation coexists. In this regard, the most common inherited factor thus far recognized that predisposes patients to venous thrombosis is activated protein C resistance,14 15 16 a defect usually caused by a single point mutation in the gene coding for coagulation factor V.17 18 19 This mutation, commonly referred to as factor V Leiden, is present in 3% to 7% of the white population and is associated with increased risks of both first and recurrent VTEs, particularly those of idiopathic origin.20
Almost no data are available assessing risks of venous thrombosis among individuals who carry the factor V Leiden mutation and who also have moderate hyperhomocyst(e)inemia. However, in a recent report of seven families with clinically severe inherited homocystinuria, the coexistence of factor V Leiden was associated with an increased incidence of thromboembolism.21 We therefore hypothesized that apparently healthy men with elevations of tHcy who also carry the factor V Leiden mutation might be at increased risk of developing future deep venous thromboses and pulmonary emboli.
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Methods |
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We evaluated the roles of hyperhomocyst(e)inemia and factor V Leiden as markers of risk for VTE (deep venous thrombosis or pulmonary embolism) among 14 916 apparently healthy men participating in the PHS who provided baseline blood and plasma samples and were prospectively followed up for the future occurrence of vascular diseases and cancer. Detailed descriptions of the PHS, a randomized, double-blind, placebo-controlled trial of aspirin and ß-carotene in the primary prevention of cardiovascular disease and cancer, have been presented elsewhere.22 23
All participants in the PHS completed annual questionnaires concerning risk factors and disease outcomes. For any self-report of either pulmonary embolism or deep venous thrombosis, clinic and hospital records, death certificates, and autopsy reports were requested and reviewed by an end-points committee of physicians who used standardized criteria to confirm or reject the diagnosis of each reported case. The diagnosis of pulmonary embolism was confirmed only when a positive angiogram or ventilation-perfusion scan showed at least two segmental defects without ventilation defects. The diagnosis of deep vein thrombosis was confirmed only if there was documentation of either a positive venographic study or ultrasound study; reported cases of deep vein thrombosis documented by impedance plethysmography or Doppler examination but not by ultrasound were not considered to be confirmed. Deep vein thromboses and pulmonary emboli not associated with cancer, recent surgery, or trauma were considered idiopathic.
During the 
12 years between randomization in 1982 and August 1994,
158 VTEs were confirmed among the 14 916 study subjects with banked
baseline blood samples; of these, 145 (92%) underwent successful
analysis for both tHcy level and factor V Leiden status. Plasma levels
of tHcy were assessed as the sum of homocysteine and homocysteinyl
moieties of the disulfides homocystine and cysteine-homocysteine using
high-performance liquid chromatography and electrochemical
detection.8 24 25 The mean within-pair coefficient of
variation in paired samples run during these analyses was
<5%.6 26 Genotyping for the presence or absence of
factor V Leiden was performed by use of a polymerase chain reaction
technique as previously described.16
With the use of a nested case-control design, each participant who provided adequate whole blood and plasma samples at baseline and subsequently suffered a confirmed VTE was matched to 1 control subject who was also a study participant, provided adequate baseline blood samples for analysis, and reported no cardiovascular disease during follow-up through the time of matching. To increase the statistical power of the study, we further included as control subjects a group of study participants who remained free of vascular disease during follow-up, provided baseline blood samples for analysis, and had previously served as control subjects for analyses of myocardial infarction and stroke.6 16 26 Thus, in addition to the 145 case subjects, a total of 646 study participants who were free of vascular disease at the time of control assignment also had plasma and DNA assayed for both tHcy level and factor V Leiden.
Statistical Analysis
Means and proportions of baseline vascular risk factors were
computed for the case patients and the control subjects. The
significance of any difference in means was tested by Student's
t test, and the difference of any proportions was tested by
the 
2 statistic. On the basis of prior
reports,9 10 hyperhomocyst(e)inemia (tHcy+) was defined as
tHcy levels exceeding the 95th percentile of the study
distribution.
To evaluate for evidence of association between tHcy level and risk of future VTE, logistic regression analyses were performed comparing incidence rates for individuals with tHcy levels above and below the 50th, 75th, 80th, 85th, 90th, and 95th percentiles of the study distribution. To evaluate the role of factor V Leiden (Leiden+), similar analyses were performed in which the referent group was those individuals who did not carry the mutation (Leiden-). To evaluate the combined role of hyperhomocyst(e)inemia and factor V Leiden, logistic regression analyses were performed in which the referent group was those individuals with normal tHcy levels who did not carry factor V Leiden (tHcy-,Leiden-). In this latter analysis, RRs of developing VTE were computed for individuals with hyperhomocyst(e)inemia free of factor V Leiden (tHcy+,Leiden-), individuals with normal tHcy levels affected by factor V Leiden (tHcy-,Leiden+), and individuals with both hyperhomocyst(e)inemia and factor V Leiden (tHcy+,Leiden+). Separate analyses were performed for those VTEs considered by the end-points committee to be idiopathic (ie, not associated with cancer, surgery, or trauma). All analyses were two-tailed, and all CIs were computed at the 95% level.
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Results |
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Table 1
shows baseline characteristics for the 145
study participants who developed confirmed VTE during follow-up (case
subjects) and for the 646 study participants who remained free of
vascular disease (control subjects). No significant differences were
observed between study groups for blood pressure, exercise frequency,
alcohol use, or prevalence of diabetes. Of the 145 VTEs, 73 (50.3%)
were idiopathic.
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Mean plasma tHcy levels were similar in the case (11.5±7.6 µmol/L) and control groups (10.9±4.1 µmol/L; P=.4), and the 95th percentile for tHcy in the study population was 17.25 µmol/L.
Table 2 shows the RR of developing VTE for study
subjects with tHcy levels above and below the 50th, 75th, 80th, 85th,
90th, and 95th percentiles of the study distribution. No statistically
significant association was observed between tHcy and VTE of any cause.
However, compared with individuals with lower tHcy levels (tHcy-),
those with tHcy levels above the 95th percentile (>17.25
µmol/L) (tHcy+) were at significantly increased risk of developing
idiopathic VTE (RR=3.4, P=.002). As previously
described,16 individuals with factor V Leiden were at
significantly increased risk of developing any VTE (RR=2.3,
P=.005) as well as idiopathic VTE (RR=3.6,
P=.0002). These data are summarized in Fig 1.
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Table 3 shows the distribution of case and control
subjects defined by the presence or absence of hyperhomocyst(e)inemia
and by factor V Leiden status. Fig 2A illustrates the RR
of developing any future VTE for each of the four study groups.
Compared with individuals without hyperhomocyst(e)inemia or factor V
Leiden (tHcy-,Leiden-), the RR of developing any future venous
thrombosis among those with both disorders (tHcy+,Leiden+) was 9.65
(P=.009). Lower RRs were found for individuals with only one
of these defects (RR=1.07, P=.9 for the tHcy+,Leiden- group
and RR=1.90, P=.045 for the tHcy-,Leiden+ group). As shown
in Table 3, adjustment for age, body mass index, and smoking status had
no important effects on these relationships.
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All of the VTEs that occurred among doubly affected study participants
were idiopathic. Thus, as shown in Table 4 and
illustrated in Fig 2B
, the RRs of VTE not related to surgery, trauma,
or cancer for the tHcy-,Leiden-, tHcy+,Leiden-, tHcy-,Leiden+, and
tHcy+,Leiden+ groups were 1.00 (referent), 2.43 (P=.06),
2.87 (P=.006), and 21.8 (P=.0004),
respectively.
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Discussion |
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The initiation and propagation of venous thrombosis is a dynamic and multifactorial process that appears to increase in frequency when two or more concomitant defects of hemostasis are present. For example, among patients with deficiencies of protein C or protein S who have suffered venous thromboses, the prevalence of factor V Leiden is increased.27 28 29 Similarly, a recent report30 described elevated risks of thrombosis among patients with inherited antithrombin deficiencies who carry the Leiden mutation. However, functional deficiencies of protein C, protein S, and antithrombin are rare. In contrast, moderate hyperhomocyst(e)inemia is a common disorder. Because some9 10 11 but not all12 13 studies of VTE and hyperhomocyst(e)inemia have reported positive associations, it has been controversial whether elevation of tHcy is an independent risk factor for these events.
The current data indicate that individuals with moderate hyperhomocyst(e)inemia who also carry factor V Leiden are at significantly increased risk of developing future VTE compared with men with neither or only one of these abnormalities. In our prospective cohort of initially healthy men, all VTEs that developed among doubly affected individuals occurred in the absence of cancer, surgery, or trauma. Thus, the risk of developing such idiopathic VTE was >20 times higher among hyperhomocyst(e)inemic patients with factor V Leiden than among individuals without either defect. Our finding that hyperhomocyst(e)inemia achieved statistical significance for the subgroup of idiopathic but not any venous thrombosis also may explain in part why prior studies evaluating the role of tHcy in VTE have been inconsistent.9 10 11 12 13 This observation could also arise due to differing frequencies of factor V Leiden, a hypothesis supported in our data showing an overall increase in risk of any VTE among those with factor V Leiden and among those with factor V Leiden and hyperhomocyst(e)inemia but not among those with hyperhomocyst(e)inemia alone.
Our observation that the coexistence of hyperhomocyst(e)inemia and factor V Leiden results in markedly increased thrombotic risk is consistent with a recent report of individuals affected by hereditary homocystinuria. Specifically, among seven families with inherited defects of methionine metabolism, only those patients with both factor V Leiden and clinical homocystinuria suffered venous thromboembolism.21 Thus, taken together, these two reports suggest that the marked variability in thrombotic risk among hyperhomocyst(e)inemic patients may be due in part to factor V Leiden status. The biological mechanism of this adverse synergy is uncertain, although elevations of tHcy have been reported to inhibit protein C activation as well as to augment factor V function.31 32
Moderate to intermediate hyperhomocyst(e)inemia can result from poor dietary intake of folate and vitamins B6 and B12, although recent data indicate that individuals homozygous for a common thermolabile polymorphism in the gene coding for MTHFR also have elevated tHcy levels and may be at increased risk for arterial thrombosis.33 34 35 As described elsewhere,36 participants in the PHS who are homozygous for this MTHFR polymorphism have significantly higher tHcy levels than do heterozygous carriers or homozygous normal individuals. However, in the current analysis, 57% of study subjects who were defined as hyperhomocyst(e)inemic (tHcy >17.25 µmol/L) were not homozygous for the abnormal MTHFR polymorphism. Thus, it is probable that absolute tHcy level is of greater physiological importance than the presence or absence of a particular MTHFR variant. Because methionine loading was not performed in the PHS and some baseline plasma samples were nonfasting, some individuals with hyperhomocyst(e)inemia may not have been detected in the present study. However, this potential limitation cannot account for our findings because any such misclassification would lead to an underestimation of true risks, if anything.
The current data, combined with those of other investigators, raise
several issues and extend prior observations from this
cohort.16 First, because hyperhomocyst(e)inemia can often
be corrected by vitamin supplementation, these data raise the
possibility that simple dietary interventions might be adequate to
reduce long-term risks for some patients.37 Second, these
data provide support for the hypothesis that individuals genetically
predisposed to thrombosis due to carriage of factor V Leiden may
require additional screening for other concomitant abnormalities of
hemostasis and thrombosis, such as tHcy level.21 Finally,
because the prevalence of factor V Leiden is 5% and
hyperhomocyst(e)inemia has been defined as levels in excess of the 95th
percentile, 1 in every 400 healthy individuals is likely to be affected
by both of these abnormalities. Given the high RRs of first and
recurrent VTEs associated with factor V Leiden14 15 16 20 and
the demonstration in these and other data that concomitant defects can
further increase risks, clinicians may consider such patients for
long-term anticoagulation. However, because the absolute risks of
venous thrombosis associated with factor V Leiden and
hyperhomocyst(e)inemia remain uncertain and long-term prophylaxis with
full-dose warfarin is associated with higher risks of bleeding,
including cerebral hemorrhage, randomized trials of adequate sample
size are required to evaluate the benefit-to-risk ratio of different
anticoagulation regimens for these patients.
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Selected Abbreviations and Acronyms |
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Received August 8, 1996; revision received November 13, 1996; accepted November 25, 1996.
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 P. Simioni, D. Tormene, P. Prandoni, P. Zerbinati, S. Gavasso, P. Cefalo, and A. Girolami Incidence of venous thromboembolism in asymptomatic family members who are carriers of factor V Leiden: a prospective cohort study Blood, March 15, 2002; 99(6): 1938 - 1942. [Abstract] [Full Text] [PDF]
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 P. Lee and K. Prasad Hyperhomocysteinemia and Venous Thrombosis International Journal of Lower Extremity Wounds, March 1, 2002; 1(1): 4 - 12. [Abstract] [PDF]
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 M. Cattaneo, R. Lombardi, A. Lecchi, P. Bucciarelli, and P. M. Mannucci Low Plasma Levels of Vitamin B6 Are Independently Associated With a Heightened Risk of Deep-Vein Thrombosis Circulation, November 13, 2001; 104(20): 2442 - 2446. [Abstract] [Full Text] [PDF]
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 A. Tripodi and P. M. Mannucci Laboratory Investigation of Thrombophilia Clin. Chem., September 1, 2001; 47(9): 1597 - 1606. [Abstract] [Full Text] [PDF]
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 U. Seligsohn and A. Lubetsky Genetic Susceptibility to Venous Thrombosis N. Engl. J. Med., April 19, 2001; 344(16): 1222 - 1231. [Full Text] [PDF]
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 J. S. Miles, J. P. Miletich, S. Z. Goldhaber, C. H. Hennekens, and P. M. Ridker G20210A mutation in the prothrombin gene and the risk of recurrent venous thromboembolism J. Am. Coll. Cardiol., January 1, 2001; 37(1): 215 - 218. [Abstract] [Full Text] [PDF]
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 T C F Sykes, C Fegan, and D Mosquera Thrombophilia, polymorphisms, and vascular disease Mol. Pathol., December 1, 2000; 53(6): 300 - 306. [Abstract] [Full Text]
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P. J. Stubbs, M. K. Al-Obaidi, R. M. Conroy, MusB, P. O. Collinson, MRCPath, I. M. Graham, FRCPI, and M. I. M. Noble Effect of Plasma Homocysteine Concentration on Early and Late Events in Patients With Acute Coronary Syndromes Circulation, August 8, 2000; 102(6): 605 - 610. [Abstract] [Full Text] [PDF]
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P. M Ueland, H. Refsum, S. A. Beresford, and S. E. Vollset The controversy over homocysteine and cardiovascular risk Am. J. Clinical Nutrition, August 1, 2000; 72(2): 324 - 332. [Abstract] [Full Text] [PDF]
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G. G. Klee Cobalamin and Folate Evaluation: Measurement of Methylmalonic Acid and Homocysteine vs Vitamin B12 and Folate Clin. Chem., August 1, 2000; 46(8): 1277 - 1283. [Abstract] [Full Text] [PDF]
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A. Khajuria and D. S. Houston Induction of monocyte tissue factor expression by homocysteine: a possible mechanism for thrombosis Blood, August 1, 2000; 96(3): 966 - 972. [Abstract] [Full Text] [PDF]
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P. M. Ridker, N. Rifai, M. J. Stampfer, and C. H. Hennekens Plasma Concentration of Interleukin-6 and the Risk of Future Myocardial Infarction Among Apparently Healthy Men Circulation, April 18, 2000; 101(15): 1767 - 1772. [Abstract] [Full Text] [PDF]
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 L. J. Langman, J. G. Ray, J. Evrovski, E. Yeo, and D. E.C. Cole Hyperhomocyst(e)inemia and the Increased Risk of Venous Thromboembolism: More Evidence From a Case-Control Study Arch Intern Med, April 10, 2000; 160(7): 961 - 964. [Abstract] [Full Text] [PDF]
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 J. M. Foody, J. A. Milberg, K. Robinson, G. L. Pearce, D. W. Jacobsen, and D. L. Sprecher Homocysteine and Lipoprotein(a) Interact to Increase CAD Risk in Young Men and Women Arterioscler Thromb Vasc Biol, February 1, 2000; 20(2): 493 - 499. [Abstract] [Full Text] [PDF]
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 S. Lanthier, L. Carmant, M. David, A. Larbrisseau, and G. de Veber Stroke in children: The coexistence of multiple risk factors predicts poor outcome Neurology, January 25, 2000; 54(2): 371 - 371. [Abstract] [Full Text] [PDF]
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H.H. van Boven, J.P. Vandenbroucke, E. Briet, and F.R. Rosendaal Gene-Gene and Gene-Environment Interactions Determine Risk of Thrombosis in Families With Inherited Antithrombin Deficiency Blood, October 15, 1999; 94(8): 2590 - 2594. [Abstract] [Full Text] [PDF]
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V. De Stefano, I. Martinelli, P. M. Mannucci, K. Paciaroni, P. Chiusolo, I. Casorelli, E. Rossi, and G. Leone The Risk of Recurrent Deep Venous Thrombosis among Heterozygous Carriers of Both Factor V Leiden and the G20210A Prothrombin Mutation N. Engl. J. Med., September 9, 1999; 341(11): 801 - 806. [Abstract] [Full Text] [PDF]
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L. E. P. Rohde, C. H. Hennekens, and P. M. Ridker Cross-Sectional Study of Soluble Intercellular Adhesion Molecule-1 and Cardiovascular Risk Factors in Apparently Healthy Men Arterioscler Thromb Vasc Biol, July 1, 1999; 19(7): 1595 - 1599. [Abstract] [Full Text] [PDF]
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L. E. P. Rohde, L. H. Arroyo, N. Rifai, M. A. Creager, P. Libby, P. M. Ridker, and R. T. Lee Plasma Concentrations of Interleukin-6 and Abdominal Aortic Diameter Among Subjects Without Aortic Dilatation Arterioscler Thromb Vasc Biol, July 1, 1999; 19(7): 1695 - 1699. [Abstract] [Full Text] [PDF]
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D. Gemmati, M. Previati, M. L. Serino, S. Moratelli, S. Guerra, S. Capitani, E. Forini, G. Ballerini, and G. L. Scapoli Low Folate Levels and Thermolabile Methylenetetrahydrofolate Reductase as Primary Determinant of Mild Hyperhomocystinemia in Normal and Thromboembolic Subjects Arterioscler Thromb Vasc Biol, July 1, 1999; 19(7): 1761 - 1767. [Abstract] [Full Text] [PDF]
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S. J. Jay and S. R. Kahn Cigarette Smoking: Risk Factor for Venous Thromboembolic Disease? Arch Intern Med, May 24, 1999; 159(10): 1144 - 1144. [Full Text] [PDF]
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 D. SHEMIN, K. L. LAPANE, L. BAUSSERMAN, E. KANAAN, S. KAHN, L. DWORKIN, and A. G. BOSTOM Plasma Total Homocysteine and Hemodialysis Access Thrombosis: AProspective Study J. Am. Soc. Nephrol., May 1, 1999; 10(5): 1095 - 1099. [Abstract] [Full Text]
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O. Salomon, D. M. Steinberg, A. Zivelin, S. Gitel, R. Dardik, N. Rosenberg, S. Berliner, A. Inbal, A. Many, A. Lubetsky, et al. Single and Combined Prothrombotic Factors in Patients With Idiopathic Venous Thromboembolism : Prevalence and Risk Assessment Arterioscler Thromb Vasc Biol, March 1, 1999; 19(3): 511 - 518. [Abstract] [Full Text] [PDF]
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M. Pahor, M. B. Elam, R. J. Garrison, S. B. Kritchevsky, and W. B. Applegate Emerging Noninvasive Biochemical Measures to Predict Cardiovascular Risk Arch Intern Med, February 8, 1999; 159(3): 237 - 245. [Abstract] [Full Text] [PDF]
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S. R. Kahn The Clinical Diagnosis of Deep Venous Thrombosis: Integrating Incidence, Risk Factors, and Symptoms and Signs Arch Intern Med, November 23, 1998; 158(21): 2315 - 2323. [Abstract] [Full Text] [PDF]
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 S. MURIN, G. P. MARELICH, A. C. ARROLIGA, and R. A. MATTHAY Hereditary Thrombophilia and Venous Thromboembolism Am. J. Respir. Crit. Care Med., November 1, 1998; 158(5): 1369 - 1373. [Abstract] [Full Text] [PDF]
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L. E. Rohde, R. T. Lee, J. Rivero, M. Jamacochian, L. H. Arroyo, W. Briggs, N. Rifai, P. Libby, M. A. Creager, and P. M. Ridker Circulating Cell Adhesion Molecules Are Correlated With Ultrasound-Based Assessment of Carotid Atherosclerosis Arterioscler Thromb Vasc Biol, November 1, 1998; 18(11): 1765 - 1770. [Abstract] [Full Text] [PDF]
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J. G. Ray Meta-analysis of Hyperhomocysteinemia as a Risk Factor for Venous Thromboembolic Disease Arch Intern Med, October 26, 1998; 158(19): 2101 - 2106. [Abstract] [Full Text] [PDF]
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 M. Laffan Genetics and pulmonary medicine bullet 4: Pulmonary embolism Thorax, August 1, 1998; 53(8): 698 - 702. [Full Text]
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S. Z. Goldhaber Pulmonary Embolism N. Engl. J. Med., July 9, 1998; 339(2): 93 - 104. [Full Text] [PDF]
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M. Walter, H. Reinecke, G. Breithardt, G. Assmann, J. Heinrich, and P. M. Ridker Factor V Leiden and Thromboembolism • Response Circulation, April 14, 1998; 97(14): 1426 - 1427. [Full Text]
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G. N. Welch and J. Loscalzo Homocysteine and Atherothrombosis N. Engl. J. Med., April 9, 1998; 338(15): 1042 - 1050. [Full Text] [PDF]
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L. A.J. Kluijtmans, G. H.J. Boers, B. Verbruggen, F. J.M. Trijbels, I. R.O. Novakova, and H. J. Blom Homozygous Cystathionine beta -Synthase Deficiency, Combined With Factor V Leiden or Thermolabile Methylenetetrahydrofolate Reductase in the Risk of Venous Thrombosis Blood, March 15, 1998; 91(6): 2015 - 2018. [Abstract] [Full Text] [PDF]
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S. Z Goldhaber Clinical overview of venous thromboembolism Vascular Medicine, February 1, 1998; 3(1): 35 - 40. [Abstract] [PDF]
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P. M Ridker and PREVENT Investigators Long-term, low-dose warfarin among venous thrombosis patients with and without factor V Leiden mutation: rationale and design for the Prevention of Recurrent Venous Thromboembolism (PREVENT) trial Vascular Medicine, February 1, 1998; 3(1): 67 - 73. [Abstract] [PDF]
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A. Ulvik, J. Ren, H. Refsum, and P. M. Ueland Simultaneous determination of methylenetetrahydrofolate reductase C677T and factor V G1691A genotypes by mutagenically separated PCR and multiple-injection capillary electrophoresis Clin. Chem., February 1, 1998; 44(2): 264 - 269. [Abstract] [Full Text] [PDF]
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M. Cattaneo, M. L. Monzani, I. Martinelli, C. R. Falcon, P. M. Mannucci, and P. M. Ridker Interrelation of Hyperhomocyst(e)inemia, Factor V Leiden, and Risk of Future Venous Thromboembolism • Response Circulation, January 27, 1998; 97(3): 295 - 296. [Full Text]
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 P. Robbins, M. Forrest, and D. Royston Hypercoagulable States Seminars in Cardiothoracic and Vascular Anesthesia, November 1, 1997; 1(4): 295 - 318. [Abstract] [PDF]
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 Factor V Leiden Risk Not Limited to Young Patients Journal Watch Cardiology, April 28, 1997; 1997(428): 13 - 13. [Full Text]
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 P. M. Ridker, J. P. Miletich, C. H. Hennekens, and J. E. Buring Ethnic Distribution of Factor V Leiden in 4047 Men and Women: Implications for Venous Thromboembolism Screening JAMA, April 23, 1997; 277(16): 1305 - 1307. [Abstract] [PDF]
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M. D. Phillips Interrelated Risk Factors for Venous Thromboembolism Circulation, April 1, 1997; 95(7): 1749 - 1751. [Full Text]
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 A. Undas, E. B. Williams, S. Butenas, T. Orfeo, and K. G. Mann Homocysteine Inhibits Inactivation of Factor Va by Activated Protein C J. Biol. Chem., February 2, 2001; 276(6): 4389 - 4397. [Abstract] [Full Text] [PDF]
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