Funded by the NIH • Developed at the University of Washington, Seattle
Funded by the NIH • Developed at the University of Washington, Seattle
[Factor V Leiden Mutation, Hereditary Resistance to Activated Protein C. Includes: Hereditary Resistance to Activated Protein C, Factor V Leiden Mutation]
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Author:
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Jody L Kujovich, MD
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Initial Posting:
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Last Update:
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Disease characteristics. Factor V Leiden thrombophilia is characterized by a poor anticoagulant response to activated protein C (APC) and an increased risk of venous thromboembolism (VTE). Deep venous thrombosis (DVT) is the most common VTE, with the legs being the most common site. Thrombosis in unusual locations is less common. Evidence suggests that a heterozygous factor V Leiden mutation has at most a modest effect on the risk of recurrence after initial treatment of a first VTE. Heterozygosity for factor V Leiden is associated with a two- to threefold increase in relative risk of pregnancy loss, and possibly other pregnancy complications such as preeclampsia, fetal growth retardation, and placental abruption. The clinical expression of factor V Leiden thrombophilia is influenced by: (1) the number of factor V Leiden alleles (heterozygotes have a slightly increased risk for venous thrombosis; homozygotes have a much greater thrombotic risk); (2) coexisting genetic thrombophilic disorders, which have a supra-additive effect on overall thrombotic risk; (3) acquired thrombophilic disorders: hyperhomocysteinemia, high factor VIII levels, malignancy; (4) circumstantial risk factors: travel, central venous catheters, pregnancy, oral contraceptive use, hormone replacement therapy (HRT), selective estrogen receptor modulators (SERMs), organ transplantation, advancing age, and surgery.
Diagnosis/testing. Factor V Leiden thrombophilia is suspected in individuals with a history of venous thromboembolism (VTE) manifest as deep vein thrombosis (DVT) or pulmonary embolism, especially in women with a history of VTE during pregnancy or in association with oral contraceptive use, and in individuals with a personal or family history of recurrent thrombosis.The diagnosis of factor V Leiden thrombophilia is made either using a coagulation screening test or by DNA analysis of the F5 gene, which encodes the factor V protein. The term "factor V Leiden" refers to the specific G-to-A substitution at nucleotide 1691 in the gene for factor V that predicts a single amino acid replacement (R506Q) at one of three APC cleavage sites in the factor Va molecule.
Management. Treatment of manifestations: The first acute thrombosis is treated according to standard guidelines [course of intravenous unfractionated heparin or low molecular-weight heparin and concurrent oral administration of warfarin (except during pregnancy)]. The duration of oral anticoagulation therapy is debated. Long-term oral anticoagulation is considered in those with recurrent VTE, multiple thrombophilic disorders, or coexistent circumstantial risk factors and in factor V Leiden homozygotes. Prevention of primary manifestations: In the absence of a history of thrombosis, long-term prophylactic anticoagulation is not routinely recommended for asymptomatic factor V Leiden heterozygotes. A short course of prophylactic anticoagulation when circumstantial risk factors are present may prevent initial thrombosis in factor V Leiden heterozygotes. Prevention of secondary complications: Enoxaparin prophylaxis in women heterozygous for factor V Leiden who have a history of recurrent pregnancy loss seems to increase the likelihood of a favorable pregnancy outcome. Surveillance: periodic reevaluation of individuals on long-term anticoagulation to assess risks (bleeding) vs. benefits. Agents/circumstances to avoid: oral contraceptives and HRT (homozygous women with or without prior VTE; heterozygous women and a history of VTE); asymptomatic heterozygous women using oral contraceptives should avoid third-generation formulations. Testing of relatives at risk: Molecular genetic testing can establish the genetic status of asymptomatic at-risk family members; however, the indications for family testing are unresolved. Clarification of factor V Leiden allele status may be useful in at-risk relatives considering hormonal contraception or pregnancy or in families with a strong history of recurrent venous thrombosis at a young age. Asymptomatic factor V Leiden heterozygotes and homozygotes should be aware of the signs and symptoms of VTE that require immediate medical attention and the potential need for prophylactic anticoagulation in high-risk circumstances.
Genetic counseling. Heterozygosity for the factor V Leiden allele and the associated risk for venous thrombosis are inherited in an autosomal dominant manner. Homozygosity for the factor V Leiden allele and a much greater risk for venous thrombosis are inherited in an autosomal recessive manner. Because of the high prevalence of the factor V Leiden allele in the general population, the genetic status of both parents and/or the reproductive partner of an affected individual needs to be evaluated before information regarding potential risks to sibs or offspring can be provided. While technically possible, prenatal testing does not seem relevant for this complex disorder, in which the genetic change is common in the general population and is predisposing to, but not predictive of, thrombosis.
No clinical features are specific for factor V Leiden thrombophilia. The diagnosis of factor V Leiden thrombophilia requires either the APC resistance assay as a coagulation screening test or DNA analysis of F5, the gene encoding factor V, to identify the Leiden mutation, a specific G-to-A substitution at nucleotide 1691 that predicts a single amino acid replacement (R506Q).
Factor V Leiden thrombophilia is suspected in individuals with a history of venous thromboembolism (VTE) manifest as deep vein thrombosis (DVT) or pulmonary embolism, especially in women with a history of VTE during pregnancy or in association with oral contraceptive use, and in individuals with a personal or family history of recurrent thrombosis.
The growing consensus is that factor V Leiden testing should be performed in the following circumstances [ACMG Consensus Statement 2001 , CAP Consensus Conference Statement 2002 , Manco-Johnson et al 2002 , Bates et al 2004]:
Factor V Leiden testing may be considered in the following individuals:
Factor V Leiden testing is not recommended for the following:
Factor V Leiden is inactivated at a rate approximately ten times slower than normal factor V and persists longer in the circulation, resulting in increased thrombin generation and a mild hypercoagulable state, reflected by elevated levels of prothrombin fragment F1+2 and other activated coagulation markers [Martinelli et al 1996 , Zoller et al 1996].
The APC resistance assay is a coagulation screening test based on the aPTT; two versions are available:
The "original" APC resistance assay involves performing an aPTT on the individual's plasma in the presence and absence of a standardized amount of exogenous APC; the two results are expressed as a ratio (aPTT + APC / aPTT – APC). This assay is based on the principle that when added to normal plasma, APC inactivates factors Va and VIIIa, which slows coagulation and prolongs the aPTT. The APC-resistant phenotype is characterized by a minimal prolongation of the aPTT in response to APC and a corresponding low ratio. The original assay has a sensitivity and specificity of 85%-90% for factor V Leiden. It is unreliable in individuals with a baseline prolonged aPTT resulting from warfarin or heparin anticoagulation, other coagulation defects, or a lupus inhibitor, and it may be altered by the hemostatic changes that occur during pregnancy or acute thrombosis.
The modified ("second-generation") APC resistance assay overcomes these limitations, is now more widely available, and has a sensitivity and specificity for factor V Leiden approaching 100% [Kapiotis et al 1996]. In this assay, the individual's plasma is first diluted (1:4) in factor V-deficient plasma that contains polybrene, a heparin neutralizer. The addition of the factor V-deficient plasma corrects for deficiencies of all other coagulation proteins, neutralizes therapeutic concentrations of heparin, and also eliminates the effect of some lupus inhibitors. The assay can be used for individuals receiving warfarin or heparin anticoagulation and for many individuals with lupus inhibitors, as well as in the setting of acute thrombosis, pregnancy, or inflammation [Svensson et al 1997].
GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information. —ED.
Gene. F5, the gene encoding factor V, is the only gene associated with factor V Leiden thrombophilia.
Clinical uses
Note: It is the policy of GeneReviews to include clinical uses of testing available from laboratories listed in the GeneTests Laboratory Directory; inclusion does not necessarily reflect the endorsement of such uses by the author(s), editor(s), or reviewer(s).
Clinical testing
Targeted mutation analysis. Targeted mutation analysis for factor V Leiden is performed by a variety of comparable methods [ACMG Consensus Statement 2001].
Table 1
summarizes molecular genetic testing for this disorder.
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Test Method
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Genetic Mechanism
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Mutation Detection Rate
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Test Availability
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G-to-A substitution at nucleotide 1691 in the
F5 gene
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100%
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Clinical
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Interpretation of test results. Molecular genetic tests are reliable in individuals on warfarin or heparin anticoagulation, and independent of thrombotic episodes.
Test results on DNA extracted from peripheral blood leukocytes should be interpreted with caution in the setting of liver transplantation or hematopoietic stem cell transplantation [Camire et al 1998 , Loew et al 2005]. Diagnosis of factor V Leiden in hematopoietic stem cell transplant recipients requires molecular analysis of non-hematopoietic tissue [Crookston et al 1998].
Note: Hematopoietic stem cell transplantation from a donor with factor V Leiden thrombophilia should not increase the thrombotic risk in the recipient.
Resistance to APC resulting from factor V Leiden may be acquired or corrected by liver transplantation [Leroy-Matheron et al 2003 , Willems et al 2003 , Loew et al 2005]. "Acquired factor V Leiden" is suggested in a liver transplant recipient who has the combination of an abnormal APC resistance screening assay and a normal factor V genotype in DNA extracted from peripheral blood leukocytes. Diagnosis of factor V Leiden in liver transplant recipients requires molecular genetic testing of donor tissue.
Two different mutations (designated as Factor V Cambridge, Factor V Hong Kong) at the arginine 306 activated protein C cleavage site in F5 have been reported rarely in persons with thrombosis (see Pathologic allelic variants).
The clinical expression of factor V Leiden thrombophilia is variable. Many individuals with the factor V Leiden allele never develop thrombosis [Heit et al 2005]. Although most individuals with factor V thrombophilia do not experience their first thrombotic event until adulthood, some have recurrent thromboembolism before age 30 years.
Two studies found that heterozygosity for the factor V Leiden allele was not associated with an increase in mortality or reduction in normal life expectancy [Hille et al 1997 , Heijmans et al 1998].
The primary clinical manifestation of factor V Leiden thrombophilia is venous thromboembolism (VTE) (see Clinical expression of factor V Leiden thrombophilia).
Deep venous thrombosis (DVT) is the most common VTE. The most common site for DVT is the legs, but upper-extremity thrombosis also occurs.
Superficial venous thrombosis may also occur. Factor V Leiden is associated with a sixfold increased risk of superficial vein thrombosis [Martinelli, Cattaneo et al 1999]. A significant fraction of individuals with venous leg ulcerations have APC resistance and the factor V Leiden allele [Larsson et al 1996 , Munkvad & Jorgensen 1996 , Zuber et al 1996]. Superficial vein thrombosis was the most common thrombotic complication reported in factor V Leiden homozygotes [Ehrenforth et al 2004].
Thrombosis in unusual locations may also occur, but less commonly.
Risk for VTE in adults. Multiple studies report that pulmonary embolism is less common than DVT in individuals with the factor V Leiden allele [Manten et al 1996 , Vandenbroucke et al 1998]. Analysis of pooled data from these studies suggests that the prevalence of the factor V Leiden allele in individuals with isolated pulmonary embolism is approximately one half that in individuals with DVT [de Moerloose et al 2000]. Another study found that factor V Leiden heterozygotes had a nearly eightfold lower incidence of DVT involving the iliofemoral veins and significantly fewer extensive thromboses compared to individuals without the mutation [Karemaker et al 2000 , de Moerloose et al 2000]. This observation could account for the lower risk of pulmonary embolism, as the iliofemoral veins are the most common source of pulmonary emboli [Bjorgell et al 2000]. Isolated DVT was also the most common major thrombotic event in a large cohort of factor V Leiden homozygotes [Ehrenforth et al 2004]. However, in a population-based cohort study, a factor V Leiden allele was not associated with a higher risk for DVT than for pulmonary embolism [Juul et al 2004].
A factor V Leiden allele was reported in 9%-12% of individuals with upper-extremity DVT, suggesting that the mutation confers a two- to sixfold increased risk of thrombosis in this location [Martinelli et al 2004 , Blom et al 2005b].
Risk for VTE in children. Although venous thrombosis is far less common in children than in adults, the prevalence of thrombophilic disorders in children with thrombosis is higher than in a corresponding adult population. A combination of risk factors appears to be required to provoke thrombosis in children [Rosendaal 1997 , Nowak-Gottl et al 2001 , Revel-Vilk & Kenet 2006]. An increased prevalence of a factor V Leiden allele was found in neonates and children with venous thromboembolism in most, but not all studies. The variation in the reported prevalences of factor V Leiden likely reflects differences in study design and clinical characteristics of children studied [Revel-Vilk & Kenet 2006].
The majority of the individuals reported had other coexisting inherited and circumstantial risk factors in addition to the factor V Leiden mutation. For example, in one study, 50% of factor V Leiden heterozygotes had a coexisting thrombophilic disorder, and circumstantial risk factors were present in all children with venous thromboembolism.
In a prospective study, asymptomatic heterozygous and homozygous children who were family members of symptomatic probands with the factor V Leiden mutation had no thrombotic complications during an average follow-up period of five years [Tormene et al 2002]. Thus, the available data suggest that asymptomatic children with a factor V Leiden allele are at low risk for thrombosis except in the setting of strong circumstantial risk factors.
Risk for recurrent thrombosis in adults heterozygous for factor V Leiden alone. Recent evidence suggests that a heterozygous factor V Leiden mutation has at most a modest effect on the risk of recurrence after initial treatment of a first VTE.
Several earlier studies suggested that individuals heterozygous for factor V Leiden had a two- to fourfold increased risk of recurrent thrombosis [Simioni et al 1997 , Simioni et al 2000], althouth other studies found no significant increase in risk [Eichinger et al 1997 , De Stefano et al 1999 , Lindmarker et al 1999]. A meta-analysis including 3104 individuals with a first VTE concluded that a heterozygous factor V Leiden mutation is associated with a significantly increased risk of recurrent VTE after a first event (odds ratio 1.4) [Ho et al 2006].
In contrast, two recent prospective cohort studies that evaluated the risk of recurrent thrombosis in unselected individuals with a first VTE followed for a mean of two years [Baglin et al 2003] and seven years [Christiansen et al 2005] concluded that heterozygotes for factor V Leiden did not have a greater risk of recurrent VTE than those without the mutation. In addition, a prospective study of families with a strong history of thrombosis found that persons with factor V Leiden had the lowest rate of recurrent VTE (3.5%/year) [Vossen, Walker et al 2005].
Risk for recurrent thrombosis in factor V Leiden homozygotes and heterozygotes with other risk factors. The risk of recurrent VTE in factor V Leiden homozygotes is not well defined, but presumed to be higher than in heterozygotes. In a retrospective cohort study, 34% of factor V Leiden homozygotes had a history of recurrent VTE [Ehrenforth et al 2004]. A prospective follow-up of the Leiden Thrombophilia study reported a five year cumulative recurrence rate of 12.5% in a small group of factor V Leiden homozygotes not receiving long-term anticoagulation [Christiansen et al 2005]. Other studies included few or no factor V Leiden homozygotes, and those included were often on long-term anticoagulation [Vossen, Walker et al 2005].
Individuals who are heterozygous for both factor V Leiden and the prothrombin gene mutation or homozygous for factor V Leiden have a three- to ninefold higher risk of recurrence [De Stefano et al 1999 , Lindmarker et al 1999 , Meinardi et al 2002].
In one study, the annual incidence of recurrent VTE was 12%/year in persons with homozygous factor V Leiden or combined factor V Leiden and the prothrombin gene mutation, compared to 3%/year in those who were heterozygous for factor V Leiden alone [Gonzalez-Porras et al 2006].
The risk of recurrent VTE is four- to fivefold higher in factor V Leiden heterozygotes with hyperhomocysteinemia than in individuals with a factor V Leiden allele alone [Meinardi et al 2002].
Risk for recurrent thrombosis in children. The risk of recurrent VTE is likely higher in children with an initial spontaneous event, a strong family history of thrombosis, and multiple thrombophilic defects [Revel-Vilk & Kenet 2006].
Heterozygous and homozygous factor V Leiden mutations were found in 29% and 2.3% of children with a first spontaneous venous thrombosis, respectively.
Children with a factor V Leiden mutation had a four- to sixfold higher risk of recurrence, which occurred in 28% of homozygotes and 19% of heterozygotes, compared to 5% of those with a normal genotype [Nowak-Gottl et al 2001].
Risk for recurrent thrombosis in pregnant women. Women with a prior history of venous thrombosis probably have a higher risk of recurrence during pregnancy, although recurrence rates range from 0% to 15% among published studies. The risk is likely higher in women with a prior spontaneous event, and/or coexisting genetic or acquired risk factors. One prospective study evaluated the safety of withholding anticoagulation during pregnancy in 125 women with a history of venous thromboembolism. In subgroup analysis, women with a previous spontaneous thromboembolic event and thrombophilia (especially factor V Leiden), had the highest recurrence rate during pregnancy (20%, odds ratio 10). Women with either thrombophilia or a prior unprovoked VTE (but not both) had a recurrence rate of 13% and 7.7%, respectively [Brill-Edwards et al 2000].
Factor V Leiden thrombophilia may increase the risk of pregnancy loss and other obstetric complications. The available data indicate that heterozygosity for factor V Leiden is associated with a two- to threefold increase in relative risk of pregnancy loss, and possibly other complications such as preeclampsia, fetal growth retardation, and placental abruption; however, the precise risk is unknown pending prospective longitudinal studies. Overall, the probability of a successful pregnancy outcome is high.
Pregnancy loss.
In addition to the increased risk of venous thromboembolism during pregnancy, a large number of case-control studies consistently found a high prevalence of factor V Leiden heterozygosity in women with unexplained recurrent pregnancy loss (
30%), compared to 1%-10% of controls (odds ratio range: 2-5) [Ridker et al 1998
, Brenner et al 1999
, Gris et al 1999
, Kupferminc et al 1999
, Martinelli et al 2000].
A small prospective study reported miscarriage in 11% of factor V Leiden heterozygotes compared to 4.2% of women without a factor V Leiden allele [Murphy et al 2000]. In another prospective study, factor V Leiden heterozygotes with a history of recurrent early miscarriage had a significantly lower live birth rate than women with a similar history of unsuccessful pregnancies but without the mutation. The live birth rate was 38% in factor V Leiden heterozygotes compared to 69% in women with a normal factor V genotype, suggesting that the mutation confers a three- to fourfold higher risk of an adverse pregnancy outcome [Rai et al 2002].
In a meta-analysis including 3000 women, a factor V Leiden allele significantly increased the risk of early first-trimester recurrent loss (odds ratio 2.1) and late recurrent and non-recurrent loss (odds ratios 7.8 and 3.2, respectively) [Rey et al 2003]. Two other meta-analyses also found a strong association with fetal loss [Dudding & Attia 2004 , Kovalevsky et al 2004].
In contrast, a prospective follow-up study of thrombophilic women with no prior history of pregnancy loss found that a factor V Leiden allele conferred only a slight increase in risk of fetal loss (relative risk 1.4) [Vossen et al 2004].
Evidence of increased second- and third-trimester losses. Some evidence suggests that thrombophilic women have a higher risk of loss in the second and third trimester. A large case-control study identified factor V Leiden as an independent risk factor for a first unexplained fetal loss after ten weeks' gestation (odds ratio 3.5) [Lissalde-Lavigne et al 2005]. Mulitple other studies and three meta-analyses suggest that factor V Leiden heterozygotes have a higher risk of late pregnancy loss than early first-trimester loss [Preston et al 1996 , Rai et al 1996 , Tormene et al 1999 , Rey et al 2003 , Dudding & Attia 2004 , Kovalevsky et al 2004]. One possible explanation is that late-pregnancy losses reflect thrombosis of the placental vessels, in contrast to first-trimester losses, which are more commonly attributable to other causes. In several studies, the majority of placentas from women heterozygous for factor V Leiden and late fetal loss had evidence of thrombotic vasculopathy or infarction, supporting this hypothesis [Gris et al 1999 , Martinelli et al 2000].
Evidence of increased first-trimester losses. A factor V Leiden allele also increases the risk of early first-trimester loss [Rey et al 2003].
Thirty-five per cent of all fetal losses in factor V Leiden heterozygotes were "pre-clinical" (prior to ultrasound confirmation of fetal heart activity), compared to 12% of those in women without the mutation [Tal et al 1999].
Preeclampsia, fetal growth retardation, and placental abruption. Although preeclampsia, fetal growth retardation, and placental abruption may also involve impaired placental perfusion, their association with thrombophilia remains controversial. The conflicting results reported in different studies may reflect the varying diagnostic and selection criteria, different ethnic groups, and small number of cases included.
Preeclampsia. Multiple case-control studies found a significantly higher prevalence of factor V Leiden in women with preeclampsia (8%-26%) compared to women with normal pregnancies (2%-10%) with odds ratios ranging from two to six [Grandone et al 1997 , Grandone et al 1999 , Kupferminc et al 1999 , Agorastos et al 2002 , Mello et al 2005].
Several large meta-analyses found an overall two to threefold increased risk of preeclampsa [Kosmas et al 2003 , Dudding & Attia 2004 , Lin & August 2005]. However, these risk estimates were based on pooled data from contradictory studies. The conflicting results reported may be due at least in part to differences in the severity of preeclampsia [Morrison et al 2002 , Mello et al 2005]. Factor V Leiden has a stronger association with severe and early-onset preeclampsia than with mild forms of the disease [Mello et al 2005 , Nurk et al 2006].
Women with thrombophilia including factor V Leiden and severe preeclampsia may have a higher risk of serious maternal complications and adverse perinatal outcomes than those without thrombophilia [Kupferminc et al 2000 , Mello et al 2005].
In contrast, other studies found no association of the mutation with preeclampsia [Alfirevic et al 2001 , Livingston et al 2001 , Morrison et al 2002 , De Maat et al 2004]. A factor V Leiden allele did not increase the risk of preeclampsia in three prospective studies of unselected women screened during the first trimester [Lindqvist et al 1999 , Murphy et al 2000 , Dizon-Townson et al 2005].
Fetal growth retardation. The data on the risk of fetal growth retardation are more limited and conflicting. A factor V Leiden allele was found in 8%-35% of women with pregnancies complicated by fetal growth retardation compared to 2%-4% of controls (odds ratio range: 7-13) [Kupferminc et al 1999 , Martinelli et al 2001 , Kupferminc et al 2002]. Another study suggested that factor V Leiden heterozygotes have a twofold higher risk of delivering a neonate with fetal growth retardation [Grandone et al 2002]. Two recent meta-analyses found that a factor V Leiden allele was associated with a significant three- to fivefold increased risk of fetal growth retardation [Dudding & Attia 2004 , Howley et al 2005].
In contrast, two larger case-control studies found no significant association between factor V Leiden and fetal growth retardation [Infante-Rivard et al 2002 , McCowan et al 2003].
In several prospective studies of unselected pregnant women, the mutation did not increase the risk of fetal growth retardation [Lindqvist et al 1999 , Murphy et al 2000 , Dizon-Townson et al 2005].
Placental abruption. The data on the risk of placental abruption are limited and conflicting. Factor V Leiden was found in 22%-30% of women with placental abruption compared to 3%-6% of control women (odds ratio range: 5-12) [Wiener-Megnagi et al 1998 , Kupferminc et al 1999 , Facchinetti et al 2003].
Several other studies found no significant association [Lindqvist et al 1999 , Alfirevic et al 2001 , Prochazka et al 2003].
The clinical expression of factor V Leiden thrombophilia is influenced by four factors:
The number of factor V Leiden alleles
Factor V Leiden heterozygotes. The relative risk of venous thrombosis is increased approximately three - to eightfold in individuals who are heterozygous for the factor V Leiden allele. Lower relative risks are reported in heterozygotes identified from general population screening [Juul et al 2004 , Heit et al 2005].
Factor V Leiden homozygotes. The relative risk of venous thrombosis is increased 18- to 80-fold in individuals who are homozygous. Although homozygotes have a higher thrombotic risk and tend to develop thrombosis at a younger age, the risk is much lower than that associated with homozygous protein C or S deficiency.
Coexisting genetic abnormalities. The presence of at least one factor V Leiden allele increases the risk associated with other inherited and acquired thrombophilic disorders (including protein C deficiency, protein S deficiency, and antithrombin deficiency), the prothrombin 20210G>A gene mutation, and hyperhomocystinemia [Ridker, Hennekens et al 1997]. The combination of factor V Leiden heterozygosity and most thrombophilic disorders has a supra-additive effect on overall thrombotic risk.
Prothrombin thrombophilia . Individuals with either a single factor V Leiden allele or prothrombin gene mutation had a four- to fivefold increase in thrombotic risk, in contrast to double heterozygotes who had a 20-fold increase in relative risk, illustrating the multiplicative effect of these two factors on overall thrombotic risk [Emmerich et al 2001]. A prothrombin 20210G>A allele was four- to fivefold more common in symptomatic factor V Leiden homozygotes with VTE than in controls with no thrombotic history [Ehrenforth et al 2004].
Acquired thrombophilic disorders
Hyperhomocysteinemia. In the Physicians' Health Study, individuals with either at least one factor V Leiden allele or hyperhomocystinemia had a three- to fourfold increased risk of idiopathic thrombosis, but the relative risk increased 22-fold in individuals with both abnormalities [Ridker, Hennekens et al 1997].
High factor VIII levels. Factor V Leiden heterozygotes with high factor VIII levels (>150% of normal) had a two- to threefold higher incidence of VTE than those with a factor V Leiden allele alone [Lensen et al 2001].
Malignancy. Cancer patients have an increased risk of VTE. A heterozygous factor V Leiden mutation increased the risk of VTE in patients with malignancy in several studies, although the results did not achieve statistical significance in one small study [Pihusch et al 2002 , Blom et al 2005a , Kennedy et al 2005]. A large population-based case-control study found that factor V Leiden heterozygotes with malignancy had a twofold higher risk of VTE than cancer patients without the mutation, and a 12-fold higher risk than those with neither risk factor [Blom et al 2005a].
Cancer patients with a heterozygous factor V Leiden or prothrombin gene mutation had a 20-fold higher risk of developing an upper-extremity thrombosis than cancer patients with neither prothrombotic mutation [Blom et al 2005b].
Circumstantial risk factors. Other predisposing factors include: travel, central venous catheter use, pregnancy, oral contraceptive use, hormone replacement therapy (HRT), selective estrogen receptor modulators (SERMs), organ transplantation, age, and surgery. These predisposing factors are associated with the first thrombotic episode in at least 50% of individuals with a factor V Leiden allele.
In a retrospective study of a large cohort of symptomatic factor V Leiden homozygotes, the initial VTE was associated with circumstantial risk factors in 81% of women and 29% of men [Ehrenforth et al 2004]. Oral contraceptives and pregnancy were the most common predisposing factors in symptomatic women. Thirteen percent of major surgeries were complicated by VTE, suggesting a nearly 20-fold increase in risk. Leg trauma was associated with a ninefold increased risk of a first VTE, which occurred in 15% of factor V Leiden homozygotes compared to 1.8% of control individuals without the mutation.
VTE after travel. The combination of air travel and thrombophilia, including factor V Leiden, was associated with a 16-fold increased risk of VTE [Martinelli, Taioli et al 2003].
Central venous catheters. Individuals heterozygous for factor V Leiden have a two- to threefold increased risk of central venous catheter-related thrombosis [van Rooden et al 2004]. A factor V Leiden allele increases the risk of central venous catheter-associated thrombosis in individuals with advanced or metastatic breast cancer and those undergoing allogeneic bone marrow transplantation [Fijnheer et al 2002 , Mandala et al 2004].
Pregnancy. A factor V Leiden allele is associated with a five- to 16-fold increase in thrombotic risk during pregnancy and the puerperium, when compared to non-pregnant women without thrombophilia. A factor V Leiden mutation was confirmed by DNA testing in 20%-46% of women with pregnancy-associated venous thrombosis [Bokarewa et al 1996 , Hirsch et al 1996 , Hallak et al 1997 , Grandone et al 1999 , Gerhardt et al 2000 , Hiltunen et al 2007]. For example, in one study, factor V Leiden thrombophilia was found in 44% of women with a history of venous thrombosis during pregnancy, compared to 8% of matched controls, with a corresponding ninefold increase in thrombotic risk [Gerhardt et al 2000].
Two recent meta-analyses found that a heterozygous factor V Leiden mutation is associated with an eightfold increased risk of pregnancy-related VTE [Biron-Andreani et al 2006 , Robertson et al 2006]. The overall risk is likely higher in women with coexisting acquired or circumstantial risk factors. One study found the combination of a factor V Leiden allele and advanced maternal age (>35 years) and obesity (BMI >30) conferred a 44-fold and 75-fold increased risk, respectively, compared to younger and normal weight women without the mutation [Hiltunen et al 2007].
Women with multiple or homozygous thrombophilic defects have the highest risk of pregnancy-associated VTE. The risk of pregnancy-related VTE is increased 20- to 40-fold in women with homozygous factor V Leiden [Martinelli et al 2001 , Gerhardt et al 2003 , Robertson et al 2006].
The risk of thrombosis during pregnancy was increased more than 100-fold in women with both a factor V Leiden allele and the prothrombin gene mutation, illustrating the marked increase in overall risk when thrombophilic mutations are combined [Gerhardt et al 2000]. In studies of thrombophilic families, VTE complicated 4% of pregnancies in women doubly heterozygous for factor V Leiden and the prothrombin gene mutation, and 16% of pregnancies in factor V Leiden homozygotes, compared with 0.5% of those in unaffected relatives [Martinelli et al 2001 ; Middeldorp, Libourel et al 2001]. The prevalence of pregnancy-related VTE was 9% in a series of unselected homozygous women [Pabinger et al 2000].
Although presence of a factor V Leiden allele increases the relative risk of VTE during pregnancy and the puerperium, the true risk in asymptomatic heterozygotes is not well defined. The results of the following studies suggest that although factor V Leiden heterozygosity is an independent risk factor, the absolute incidence of thrombosis during pregnancy is low.
In several retrospective studies and meta-analyses, the estimated risk of VTE during pregnancy and the puerperium in factor V Leiden heterozygotes was in the range of one in 125 to 400 pregnancies [McColl et al 1997 , Gerhardt et al 2000 , Gerhardt et al 2003 , Robertson et al 2006].
In contrast, women with homozygous factor V Leiden or combined thrombophilia have a much higher probability of VTE, in the range of one in 20 to one in 100 pregnancies [Martinelli et al 2001 , Gerhardt et al 2003 , Robertson et al 2006].
Oral contraceptive use. The use of oral contraceptives substantially increases the risk of venous thromboembolism (VTE) in women heterozygous for a factor V Leiden allele. A heterozygous mutation is found in 20%-35% of women with a history of venous thrombosis during oral contraceptive use [Hirsch et al 1996 , Schambeck et al 1997]. In the Leiden Thrombophilia study, the risk of venous thrombosis was increased fourfold in oral contraceptive users, and sevenfold in women with a heterozygous factor V Leiden mutation. However, the risk was increased 35-fold in heterozygous women who used oral contraceptives, indicating a multiplicative rather than additive effect on overall thrombotic risk.
The supra-additive effect of a factor V Leiden allele and oral contraceptives was confirmed in other studies and a meta-analysis, with odds ratios ranging from 11 to 41 for the combination of both risk factors [Martinelli, Taioli et al 1999 , Legnani et al 2002 , Sidney et al 2004].
A meta-analysis found the combination of factor V Leiden and oral contraceptives conferred a 16-fold increase in relative thrombotic risk, which was fivefold higher than that observed with either risk factor alone [Wu et al 2005]. Heterozygous women who use oral contraceptives have a 30-fold higher risk of cerebral vein thrombosis than non-users without the mutation [Martinelli, Battaglioli et al 2003].
The corresponding risk is increased more than 100-fold in women homozygous for the factor V Leiden allele who use oral contraceptives. The risk of VTE is also markedly increased in oral contraceptive users who are doubly heterozygous for factor V Leiden and the prothrombin gene mutation, with reported odds ratios ranging from 17 to 110 [Mohllajee et al 2006].
Women with inherited thrombophilic disorders, such as factor V Leiden thrombophilia, tend to develop thrombotic complications sooner, with a much higher risk of thrombosis during the first year of oral contraceptive use [Bloemenkamp et al 2000].
Oral contraceptives containing the third-generation progestagen desogestrel are associated with a twofold higher risk of venous thromboembolism than second-generation preparations, with an especially high risk in factor V Leiden heterozygotes. The risk was increased 50-fold in factor V Leiden heterozygotes who used third-generation preparations containing desogestrel, compared to women without the factor V Leiden allele who were not using oral contraceptives.
Despite the marked increase in relative risk, the absolute incidence of VTE may still be low because of the low baseline risk in young healthy women. For example, the combination of factor V Leiden and oral contraceptives is estimated to result in an additional 28 VTE events per 10,000 women per year. Long-term use of oral contraceptives in asymptomatic factor V Leiden heterozygotes without complications has been reported, underscoring the multifactorial etiology of VTE [Girolami et al 2004].
Unopposed progestin contraception carries a much lower risk of thrombosis than estrogen-containing contraceptives, although the risk in thrombophilic women is not well defined. A retrospective study found that oral progestin alone did not increase the risk of VTE in high-risk women with a history of thrombosis and/or thrombophilia, including 28 women with factor V Leiden [Conard et al 2004].
However, no prospective studies confirm the safety of progestin-alone contraception in women with factor V Leiden.
Hormone replacement therapy (HRT). Multiple studies have confirmed a significant (two- to fourfold) increase in relative risk of VTE in current users of HRT compared to non-users [Daly et al 1996 , Grodstein et al 1996 , Jick et al 1996 , Perez-Guthann et al 1997 , Hulley et al 1998 , Varas-Lorenzo et al 1998 , Grady et al 2000 , Rossouw et al 2002].
The landmark Women's Health Initiative (WHI) randomized trial of estrogen and progesterone HRT versus placebo in postmenopausal women found that HRT was associated with a twofold increased risk of VTE [Rossouw et al 2002]. In a parallel WHI trial of estrogen-only HRT in women who had a hysterectomy, estrogen replacement increased the risk of VTE, although the risk was statistically significant only for DVT (hazard ratio 1.47) [Anderson et al 2004 , Curb et al 2006].
Most of the observational studies of HRT excluded women with known thrombophilia. Based on the known interaction with estrogen, the use of HRT is expected to significantly increase the risk of VTE in women with a factor V Leiden allele. Evidence is now compelling that women with factor V Leiden who use HRT have a markedly increased risk of developing VTE. In one study, the combination of HRT use and activated protein C resistance was associated with a 13-fold increase in relative thrombotic risk compared to that found in women with neither risk factor [Lowe et al 2000]. Reinvestigation of this same group of women for prothrombotic mutations (factor V Leiden or the prothrombin gene mutation) demonstrated a 15-fold increased risk of venous thrombosis in HRT users with a heterozygous factor V Leiden mutation [Rosendaal et al 2002].
In another study of postmenopausal women with coronary heart disease, factor V Leiden heterozygotes who used HRT had a 14-fold higher thrombotic risk than non-users without the mutation. The estimated absolute incidence of VTE in women with coronary heart disease and factor V Leiden who used HRT was 15 VTE events per 1000 women per year, compared to two VTE events per 1000 women per year for non-users with a normal genoype [Herrington et al 2002]. A meta-analysis of the data from these confirmed that factor V Leiden heterozygotes who use HRT have a 13-fold higher risk of VTE [Wu et al 2005].
In a nested case-control study of the WHI, factor V Leiden heteroz