Factor V Leiden Thrombophilia

APC Resistance Assay

The original APC resistance assay involves performing an aPTT on the individual's plasma in the presence and absence of exogenous APC; the two results are expressed as a ratio (aPTT +APC/aPTT-APC). The APC-resistant phenotype is characterized by a minimal prolongation of the aPTT in response to APC and a correspondingly low ratio.

The modified "second generation" assay involves diluting the patient's plasma in factor V-deficient plasma containing a heparin neutralizer (which increases the specificity and sensitivity of detection of factor V Leiden thrombophilia to almost 100%) [Kadauke et al 2014].

Note: The second generation APC resistance assay: (1) is cost effective, highly accurate, and detects causes of APC resisance other than factor V Leiden thrombophilia and (2) is used to detect "pseudohomozygous" factor V Leiden thrombophilia (defined as heterozygosity for both the factor V Leiden variant and a second F5 pathogenic variant that causes a factor V deficiency) (see Genotype-Phenotype Correlations) or "pseudo wild-type" factor V Leiden thrombophilia (defined as the combination of a Leiden variant with a normal APC resistance ratio) [Prüller et al 2013, Kadauke et al 2014, Van Cott et al 2016].

Molecular genetic testing is recommended in individuals receiving direct thrombin inhibitors or direct factor Xa inhibitors, which may interfere with results of the APC resistance assay [Kadauke et al 2014], and in individuals with the following laboratory findings:

• Positive APC-resistance assay values: to confirm the diagnosis and to distinguish factor V Leiden variant heterozygotes from homozygotes
• Borderline APC resistance assay values: to confirm the diagnosis
• Very low APC resistance assay values to differentiate:
  • Leiden variant heterozygotes
  • Leiden variant homozygotes
  • "Pseudohomozygotes"
• Strong lupus inhibitors and a markedly prolonged baseline aPTT

Factor V Leiden Variant Testing

The growing consensus is that factor V Leiden variant testing should not be performed on a routine basis and should only be considered when the results will affect clinical management [Chong et al 2012, Canadian Agency for Drugs and Technologies in Health 2015, Stevens et al 2016] for the following reasons:

• No randomized controlled trial has evaluated the effect of thrombophilia testing on the rate of recurrence after a first VTE.
• Analysis of a large cohort of indviduals with VTE suggested that thrombophilia testing at the time of the first VTE did not reduce the risk of recurrence [Coppens et al 2008].
• Not testing patients with VTE for inherited thrombophilia is included in the "Choosing Wisely Campaign" recommended by several professional societies [Hicks et al 2013, Hillis et al 2015].

Factor V Leiden variant testing may be considered in the following individuals when the results of testing would affect clinical management:

• Persons with a first unprovoked VTE who are planning to stop anticoagulation
• Female relatives of persons with VTE or hereditary thrombophilia considering estrogen contraception or hormone replacement
• Female relatives of persons with VTE or hereditary thrombophilia contemplating prophylactic anticoagulation during pregnancy

Factor V Leiden variant testing should not be performed on the following individuals:

• Adults with VTE provoked by major transient risk factors
• Adults with unprovoked VTE while on long-term anticoagulation
• Persons with arterial thrombosis
• Women with unexplained pregnancy loss
• Neonates and children with asymptomatic central venous catheter-related thrombosis
• Asymptomatic adult family members of individuals known to have a Leiden variant

Factor V Leiden variant testing should not be performed in the following circumstances:

• Routine testing:
  • During pregnancy
  • Prior to the use of oral contraceptives, hormone replacement therapy (HRT), or selective estrogen receptor modulators (SERMs)
  • In asymptomatic children
• Prenatal or newborn testing

Molecular genetic testing approaches can include targeted analysis for the factor V Leiden variant (see Table 1) or a multigene panel that includes the factor V Leiden variant and other genes of interest (see Differential Diagnosis). Note: The genes included and sensitivity of multigene panels vary by laboratory and over time. For an introduction to multigene panels click here. More detailed information for clinicians ordering genetic tests can be found here.

Clinical Manifestations of Factor V Leiden Thrombophilia

Venous thromboembolism (VTE) is the primary clinical manifestation of factor V Leiden thrombophilia. Deep vein 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.

Pulmonary embolism (PE). Some evidence suggests that PE is less common than DVT in individuals with the Leiden variant. Multiple studies and a meta-analysis consistently found a higher risk of DVT than PE in individuals with a Leiden variant [Dentali et al 2012, van Langevelde et al 2012]. The different effect on the risk of DVT and PE is referred to as "the factor V Leiden paradox"; the mechanism is still not well understood.

Thrombosis in unusual locations such as cerebral veins and splanchnic veins may also occur, but less commonly.

Risk for VTE in adults. The clinical expression of the Leiden variant varies [Campello et al 2016]: many individuals with the Leiden variant never develop thrombosis [Heit et al 2005] and those who do typically experience their first thrombotic event as adults; however, some have recurrent thromboembolism before age 30 years.

Heterozygosity for the Leiden variant is not associated with an increase in mortality or reduction in normal life expectancy even in the presence of a history of VTE [Hille et al 1997, Heijmans et al 1998, Pabinger et al 2012].

While individuals homozygous for the Leiden variant have a higher risk for thrombosis than heterozygotes, the clinical course of an acute thrombotic episode is not more severe or more resistant to anticoagulation in homozygotes than in heterozygotes.

Heterozygotes. The relative risk for venous thrombosis is increased approximately three- to eightfold in Leiden variant heterozygotes. Lower relative risks are reported in heterozygotes identified from general population screening [Juul et al 2004, Heit et al 2005].

The risk for VTE is increased in Leiden variant heterozygotes:

• Three- to eightfold [Rosendaal & Reitsma 2009]
• Four- to fivefold in two large meta-analyses [Gohil et al 2009, Simone et al 2013]

Despite the increase in relative risk, the overall annual incidence of a first VTE is low in heterozygotes, approximately 0.5% [Middeldorp 2011].

A heterozygous Leiden variant was associated with the following:

• A sixfold increased risk for primary upper-extremity thrombosis (not related to malignancy or a venous catheter) [Martinelli et al 2004]
• A sixfold increased risk of superficial vein thrombosis not associated with varicose veins, malignancy, or autoimmune disorders [Martinelli et al 1999]
• Increased risk of venous thrombosis at unusual sites [Martinelli et al 2014]
• A fourfold increased risk of cerebral venous thrombosis [Dentali et al 2006]

A Leiden variant:

• May increase the risk of splanchic vein thrombosis;
• Was associated with an 11-fold increased risk of Budd-Chiari syndrome in case-control studies [Janssen et al 2000];
• Confers a threefold increased risk of portal vein thrombosis (meta-analysis by Dentali et al [2008]).

Homozygotes. Compared to heterozygotes, homozygotes have a higher thrombotic risk and tend to develop thrombosis at a younger age.

The risk for VTE is increased in Leiden variant homozygotes:

• Nine- to 80-fold [Rosendaal & Reitsma 2009]
• Nine- to 12-fold [Gohil et al 2009, Simone et al 2013]

Risk for VTE in children. The cause of VTE in children is multifactorial and results from the interaction between acquired clinical risk factors (see Table 2), one or more underlying medical conditions, and an inherited predisposition to thrombophilia [Klaassen et al 2015, van Ommen & Nowak-Göttl 2017].

The most important clinical risk factor for thrombosis in children is a central venous catheter (CVC). A Leiden variant was associated with CVC-related VTE in some [Neshat-Vahid et al 2016] but not all studies [Thom et al 2014].

A Leiden variant significantly increased the risk of cerebral venous thrombosis in children (odds ratio 2.74); see meta-analysis by Kenet et al [2010].

A Leiden variant was also reported to increase the risk of neonatal cerebral vein thrombosis [Kenet et al 2010, Laugesaar et al 2010].

In a prospective study, asymptomatic children – family members of symptomatic probands with the Leiden variant who were themselves heterozygous or homozygous for the Leiden variant – had no thrombotic complications during follow up that averaged five years [Tormene et al 2002]. Thus, the available data suggest that asymptomatic children with a Leiden variant are at low risk for thrombosis except in the setting of strong circumstantial risk factors (see Table 2).

Risk for VTE in pregnancy. Normal pregnancy is associated with a five- to tenfold increased risk of developing VTE.

Women heterozygous for the Leiden variant have a five to eight times greater risk of pregnancy-related VTE than women without the variant [Robertson et al 2006, Bleker et al 2014, Gerhardt et al 2016]. The risk is higher in women from families with a history of thrombosis and in women older than age 34 years. The highest risk of VTE occurs during the first six weeks post partum.

While heterozygosity for the Leiden variant increases the relative risk for pregnancy-associated VTE, the absolute risk is low in the absence of other predisposing factors. VTE is estimated to occur in 1% of pregnancies in women who are Leiden variant heterozygotes. The absolute risk increases to 3% in those with a positive family history of VTE [Bleker et al 2014, Campello et al 2016].

In women homozygous for the Leiden variant the relative risk is increased 17- to 34-fold [Robertson et al 2006, Gerhardt et al 2016]. The absolute risk of developing pregnancy-related VTE is estimated at 2.2%-4.8% of pregnancies. The risk is higher (14%) in homozygotes with a positive family history and in those older than age 34 years [Bleker et al 2014, Gerhardt et al 2016].

Women doubly heterozygous for the Leiden variant and the 20210G>A F2 variant are reported to have an eight- to 47-fold increased relative risk of pregnancy-related VTE [Jacobsen et al 2010, Gerhardt et al 2016]. The probability of VTE during pregnancy and the puerperium is lower (5.5%) in doubly heterozygous women younger than age 35 years than in older women (8.2%) [Gerhardt et al 2016].

Recurrent Thrombosis

In adults heterozygous for a Leiden variant alone. Evidence suggests that a heterozygous Leiden variant has at most a modest effect on risk for recurrent thrombosis after initial treatment of a first VTE.

A modest, approximately 1.5-fold increased risk of VTE recurrence was identified in several meta-analyses [Marchiori et al 2007, Segal et al 2009]; however, in several prospective cohort studies of unselected individuals with a first VTE the recurrence risk was not increased in Leiden variant heterozygotes [Christiansen et al 2005, Lijfering et al 2010].

The reported risk may be higher in studies of families prone to thrombosis than in unselected individuals. In a prospective study of families with a strong history of thrombosis, the incidence of recurrent VTE was 3.5 per 100 person-years in persons with the Leiden variant (heterozygotes and homozygotes) [Vossen et al 2005]; however, a large family study found the rate of recurrent VTE in relatives with a Leiden variant to be similar to those reported in the general population (7% after 2 years, 11% after 5 years, and 25% after 10 years) [Lijfering et al 2009].

In Leiden variant homozygotes and heterozygotes with other risk factors. Risks for recurrent VTE in Leiden variant homozygotes and double heterozygotes for the Leiden variant and the F2 20210G>A variant vary widely between studies.

Similar rates of VTE recurrence for both Leiden variant homozygotes and heterozygotes were found in a recent study [Perez Botero et al 2016], whereas an earlier systematic review found that homozygosity for the Leiden variant conferred a 2.6-fold increased risk of recurrent VTE [Segal et al 2009].

Not all studies found a high risk for recurrence in Leiden variant homozygotes and double heterozygotes even when the analysis was restricted to those with a first unprovoked VTE [Lijfering et al 2010].

The risk of VTE is unknown in individuals with the rare combination of Leiden variant and prothrombin 20210G>A variants (i.e., homozygous Leiden variant / heterozygous 20210G>A, homozygous Leiden variant / homozygous 20210G>A). In a retrospective study, the risk of recurrence was 13% at one year and 22% at five years, similar to that expected in individuals with an unprovoked VTE without thrombophilia. However, the recurrence risk in this group was significantly higher than expected after a first VTE provoked by a transient risk factor (6% at 1 year and 26% at 5 years) [Lim et al 2016].

In children. In children, inherited thrombophilia appears to have at most a modest effect on the risk of recurrence, similar to findings in adults [Klaassen et al 2015].

In pregnant women. During pregnancy women with a prior history of VTE have an increased recurrence risk, ranging from 0% to 15% in published studies. The risk is higher in women with a prior unprovoked episode or an estrogen-related VTE, and in those with coexisting genetic or acquired risk factors (Table 2). No studies have specifically evaluated the risk for recurrent VTE in pregnant women who have the Leiden variant.

In subgroup analysis of a prospective study of the safety of withholding anticoagulation during pregnancy in 125 women with a history of VTE, Brill-Edwards et al [2000] found the following:

• Women with a prior unprovoked VTE and thrombophilia (especially factor V Leiden thrombophilia) had the highest recurrence rate during pregnancy (20% of pregnancies).
• A Leiden variant was associated with an increased risk of antepartum recurrence (odds ratio 10).
• Women with either thrombophilia or a prior unprovoked VTE (but not both) had recurrence rates of 13% and 7.7%, respectively.

Obstetric Complications

It is unlikely that a factor V Leiden thrombophilia (i.e., heterozygosity or homozygosity for the Leiden variant) is a major factor contributing to pregnancy loss and other adverse pregnancy outcomes (preeclampsia, fetal growth restriction, and placental abruption). At most, factor V Leiden thrombophilia is one of multiple largely unknown genetic and environmental predisposing factors contributing to these complications.

Pregnancy loss. Available data suggest that Leiden variant heterozygosity is at most a weak contributor to recurrent or late pregnancy loss. A meta-analysis evaluating only prospective cohort studies reported a slightly increased risk of pregnancy loss in women with the Leiden variant (4.2%) compared to those without the variant (3.2%) (odds ratio 1.52) [Rodger et al 2010]. A meta-analysis found that heterozygosity for the Leiden variant is associated with a twofold increased risk for a late unexplained fetal loss and a fourfold higher risk for loss in the second trimester compared to the first trimester [Robertson et al 2006]. Although maternal homozygosity for the Leiden variant was associated with stillbirth, presence of the Leiden variant was not associated with stillbirths in the subset of stillbirths resulting from placental insufficiency [Silver et al 2016].

Preeclampsia, fetal growth restriction, and placental abruption may also involve impaired placental perfusion; however, their association with inherited thrombophilia is more controversial [Greer et al 2014]. For example:

• A systematic review focused on prospective cohort studies found no significant association of preeclampsia or placental abruption with factor V Leiden thrombophilia [Rodger et al 2010].
• A Danish case-cohort study found that heterozygosity for the Leiden variant increased the risk of severe preeclampsia (odds ratio 1.6), severe fetal growth restriction (odds ratio 1.5), and symptomatic placental abruption (odds ratio 1.7) [Lykke et al 2012].

Such conflicting results may reflect the varying diagnostic and selection criteria, different ethnic groups, and small number of cases included. However, given that preeclampsia and placental abruption are heterogeneous disorders, it is unlikely that a single thrombophilic variant (such as the Leiden variant) plays a major causal role.

Arterial Thrombosis NOT Convincingly Associated with the Leiden Variant

The available evidence indicates that presence of a Leiden variant is not a major risk factor for arterial thrombosis of any sort, including myocardial infarction and stroke in fetuses, children, and adults, the majority of which occur in the presence of established cardiovascular risk factors (including hypertension, hyperlipidemia, diabetes mellitus, and smoking). For more information click here (pdf).

Clinical Expression of Factor V Leiden Thrombophilia

In addition to the number of Leiden variants (discussed above), the clinical expression of factor V Leiden thrombophilia is influenced by family history, coexisting genetic abnormalities, acquired thrombophilic disorders, and circumstantial risk factors.

A positive family history. Individuals with a Leiden variant who have a first-degree relative with a history of thrombosis had a threefold increased risk for VTE compared to those with a Leiden variant with a negative family history [Bezemer et al 2009]. The risk was increased to fivefold in those with a relative with a VTE before age 50 years and to 18-fold with two or more affected relatives. The family history had additional value in predicting risk even in those with a Leiden variant, suggesting the presence of unknown genetic risk factors.

Coexisting genetic abnormalities. The combination of Leiden variant heterozygosity and most other thrombophilic disorders (including protein C deficiency, protein S deficiency, antithrombin deficiency, and the F2 thrombophilia variant [c.*97G>A, commonly known as 20210G>A]) has a supra-additive effect on overall thrombotic risk [Ridker et al 1997a] (see Differential Diagnosis). For example:

• Individuals heterozygous for either the Leiden variant or the F2 thrombophilia variant had a four- to fivefold increased thrombotic risk, compared to double heterozygotes, who had a 20-fold increased thrombotic risk [Emmerich et al 2001].
• The F2 thrombophilia variant was four- to fivefold more common in Leiden variant homozygotes with VTE than in controls without a Leiden variant and with no thrombotic history [Ehrenforth et al 2004].

Acquired thrombophilic disorders include antiphospholipid antibody (APLA) syndrome, paroxysmal nocturnal hemoglobinuria, myeloproliferative disorders, and increased levels of clotting factors. Despite the following observations, the effect of these acquired disorders on Leiden variant-associated thrombotic risk is not well defined.

• Leiden variant heterozygotes with factor VIII levels greater than150% of normal had a two- to threefold increased incidence of VTE than heterozygotes for a Leiden variant alone [Lensen et al 2001]. The reason for the association of high FVIII levels with VTE is unknown; factor VIII, an acute phase reactant, is high in the presence of inflammation and estrogen. Although suspected, a genetic basis has not been identified.
• A Leiden variant was reported to contribute to increased risk for thrombotic complications in persons with polycythemia vera and essential thrombocytosis [Trifa et al 2014].

Circumstantial Risk Factors for VTE

Circumstantial risk factors for VTE in Leiden variant heterozygotes or homozygotes are summarized in Table 2. Other risk factors that to date have not been studied in Leiden variant heterozygotes are the newer forms of combined hormonal contraception, transdermal and vaginal ring contraception, for which the risk of VTE is at least as great as the risk associated with combined oral contraceptives (COCs) [Dore et al 2010, Lidegaard et al 2012].

Malignancy. To what extent inherited thrombophilia increases the risk of VTE in persons with cancer remains controversial [Decousus et al 2007, Pabinger et al 2015]. Because malignancy is such a strong thrombotic risk factor, it may obscure the effect of mild thrombophilic disorders including factor V Leiden. Thrombophilia status was not considered in recent guidelines for prophylaxis and treatment of VTE in patients with cancer [Farge et al 2013].

Combined oral contraceptive (COC) use substantially increases the relative risk for VTE in women heterozygous for the Leiden variant.

The supra-additive effect of both a Leiden variant and use of COC was confirmed in multiple studies in which odds ratios for VTE ranged from 11 to 41 [Wu et al 2005, Dayan et al 2011, Bergendal et al 2014, van Vlijmen et al 2016]. For women who are either homozygous for the Leiden variant or doubly heterozygous for the Leiden variant and the prothrombin 20210G>A variant the odds ratios for VTE ranged from 17 to 110 [Mohllajee et al 2006, van Vlijmen et al 2016].

Despite the marked increase in relative risk for VTE, the absolute incidence of VTE may be low because of the low baseline risk for VTE in young women.The incidence of VTE in COC users with either a Leiden variant or the prothrombin 20210G>A variant ranged 0.49 to 2.0 VTE/100 pill-years compared to 0.19 to 0/100 pill-years in COC users without these variants. The absolute VTE risk is substantially higher in COC users doubly heterozygous for the Leiden variant and the prothrombin 20210G>A variant or homozygous for either variant (0.86 vs 0.19/100 pill years) [van Vlijmen et al 2011, van Vlijmen et al 2016].

The thrombotic risk of COC is at least as high in women older than age 50 years as in younger users [Roach et al 2013]. However, since the incidence of VTE increases with age, the absolute risk for VTE in women older than age 50 years is much higher than in younger COC users.

Oral hormone replacement therapy (HRT) is associated with a two- to fourfold increased relative risk for VTE in healthy postmenopausal users of HRT compared to non-users [Renoux et al 2010, Eisenberger & Westhoff 2014]. Data comparing the VTE risk of combined estrogen/progestin HRT and unopposed estrogen are inconclusive [Eisenberger & Westhoff 2014].

The risk increases with higher estrogen doses and may differ with the particular estrogen and progestin components [Renoux et al 2010, Canonico et al 2011, Smith et al 2014].

The risk of HRT is threefold increased in postmenopausal women with a factor V Leiden or prothrombin 20210G>A variant than in HRT users without thrombophilia [Roach et al 2013].

Transdermal HRT. Multiple observational studies consistently found that transdermal HRT did not increase the risk of VTE [Canonico et al 2010, Sweetland et al 2012, ACOG 2013a]. There is also evidence that transdermal estrogen is associated with a lower thrombotic risk than oral estrogen in women with inherited thrombophilic variants including the Leiden variant [Canonico et al 2010]. Women with a Leiden variant using transdermal estrogen had a risk similar to that of non-users with the variant. Among women with a Leiden variant the use of oral estrogen was associated with a fourfold increased risk for VTE over transdermal estrogen [Straczek et al 2005]. However, no prospective randomized trials have confirmed the safety of transdermal HRT in women with inherited thrombophilia.

Selective estrogen receptor modulators (SERMS). The risk for VTE in women with the Leiden variant who use SERMS is not well defined. Limited data suggest that SERMs such as tamoxifen and raloxifene are associated with a twofold increased risk for VTE, similar to the risk for HRT [Barrett-Connor et al 2006]. The thrombotic risk conferred by tamoxifen was higher than raloxifene in trials for primary prevention of breast cancer [Nelson 2013]. In light of the interaction of factor V Leiden with HRT, the risk is likely higher than that associated with SERMS alone.

Treatment of VTE in Adults

The management of individuals with factor V Leiden thrombophilia depends on the clinical circumstances.

The first acute thrombosis should be treated according to standard guidelines [Kearon et al 2012, Kearon et al 2016]. For initial treatment of VTE, current guidelines suggest a new oral anticoagulant (dabigatran, edoxaban, rivaroxaban, or apixaban) over warfarin because of a lower bleeding risk and greater convenience [Kearon et al 2016]. Of note, low molecular-weight heparin (LMWH) is given before dabigatran and edoxaban but not before rivaroxaban or apixaban.

For patients not treated with one of the new oral anticoagulants, administration of warfarin is started concurrently with LMWH or fondaparinux (except during pregnancy) and monitored with the international normalized ratio (INR). A target international normalized ratio (INR) of 2.5 (therapeutic range 2.0-3.0) provides effective anticoagulation, even in individuals homozygous for the Leiden variant [Kearon et al 2008, Tzoran et al 2017]. LMWH and warfarin therapy should be overlapped for at least five days, and until the INR has been within the therapeutic range on two consecutive measurements over two days.

LMWH and warfarin are both safe in breastfeeding women (see Pregnancy Management for issues with anticoagulants).

The duration of oral anticoagulation therapy should be based on an assessment of the risks for VTE recurrence and anticoagulant-related bleeding. Recurrence risk is determined by the clinical circumstances of the first event (provoked or unprovoked), adequacy of early treatment, and individual risk factors.

• Heterozygosity for the Leiden variant alone is not an indication for long-term anticoagulation in the absence of other risk factors, according to the American College of Chest Physicians guidelines on antithrombotic therapy and prevention of thrombosis [Kearon et al 2012, Kearon et al 2016] as well as other clinical guidelines and expert opinion [Baglin et al 2010, Bauer 2010, National Clinical Guideline Centre 2012].
• Anticoagulation for at least three months is recommended for persons with DVT and/or PE associated with a transient (reversible) risk factor [Kearon et al 2012, Kearon et al 2016].

Long-term oral anticoagulation is recommended for individuals with a first or recurrent unprovoked (i.e., idiopathic) proximal DVT of the leg or pulmonary embolism (PE) who have a low or moderate bleeding risk [Kearon et al 2016]. The decision should be based on an assessment of potential risks and benefits regardless of Leiden variant status [EGAPP Working Group 2011].

Long-term anticoagulation is occasionally considered in individuals homozygous for the Leiden variant or with multiple thrombophilic disorders, particularly in the presence of additional risk factors (e.g., obesity) as the potential benefits from long-term anticoagulation may outweigh the bleeding risks [De Stefano & Rossi 2013].

Treatment of VTE in Children

The treatment recommendations for adults (which concluded that presence of a Leiden variant should not influence the intensity or duration of anticoagulation) are generally followed in children as well [Chalmers et al 2011, Heleen van Ommen & Middeldorp 2011, Monagle et al 2012].

Children with a first VTE should receive initial treatment with either unfractionated heparin (UFH) or LMWH for at least five days. LMWH is favored over warfarin for continued therapy, especially in very young children and those with complex medical problems. Recommendations on the duration of antithrombotic therapy are based on the nature of the thrombotic event (spontaneous or provoked) [Chalmers et al 2011, Monagle et al 2012].

Anticoagulation is recommended:

• For at least three months following a VTE provoked by a clinical risk factor that has resolved;
• At least three months and until the risk factor has resolved in children with an ongoing but potentially reversible risk factor;
• For 6-12 months after a first unprovoked VTE.

Expert opinion emphasizes the importance of a careful risk/benefit assessment in each individual [Heleen van Ommen & Middeldorp 2011].

Prevention of Thrombosis During Pregnancy

No consensus exists on the optimal management of factor V Leiden thrombophilia during pregnancy; guidelines are derived from studies in non-pregnant individuals [Baglin et al 2010, Bates et al 2012]. All women with inherited thrombophilia should undergo individualized risk assessment in order to base decisions about anticoagulation on the number and type of thrombophilic defects, coexisting risk factors, and personal and family history of thrombosis.

For pregnant women with a prior single episode of VTE provoked by a transient risk factor not related either to pregnancy or to the use of estrogen, clinical vigilance during pregnancy is suggested [Bates et al 2012].

LMWH is the preferred antithrombotic agent for prophylaxis and treatment during pregnancy [Bates et al 2012, ACOG 2013b].

The oral direct thrombin inhibitor dabigatran and the direct factor Xa inhibitors rivaroxaban, apixaban, and edoxaban are contraindicted during pregnancy and breastfeeding because of (1) absence of data on fetal and neonatal safety and (2) animal studies that showed reproductive toxicity [Ageno et al 2012].

Prophylactic anticoagulation during pregnancy is recommended for all women:

• With a history of unprovoked VTE including those heterozygous for the Leiden variant. LMWH should be given during pregnancy, followed by a six-week course of anticoagulation post partum [Bates et al 2012, ACOG 2013b];
• Heterozygous for the Leiden variant with a prior pregnancy or estrogen-related thrombosis who are also at increased risk for recurrence [Pabinger et al 2005, Bates et al 2012, ACOG 2013b].

Prophylactic anticoagulation during pregnancy is suggested for asymptomatic women who:

• Are homozygous for the Leiden variant;
• Are double heterozygotes for the Leiden variant and the prothrombin 20210G>A variant;
• Have other combined thrombophilic defects;
• Also have a positive family history for VTE.

In the absence of a positive family history for VTE, antepartum clinical vigilance and postpartum prophylaxis with LMWH is suggested as the greatest thrombotic risk is in the initial postpartum period [Bates et al 2012, ACOG 2013b].

Prophylactic anticoagulation during pregnancy is not routinely recommended in asymptomatic women heterozygous for the Leiden variant with no history of thrombosis. All women with a Leiden variant should be warned about potential thrombotic complications and counseled regarding the risks and benefits of anticoagulation during pregnancy [Bates et al 2012, ACOG 2013b].

Prevention of Thrombosis During the Postpartum Period

A six-week course of postpartum prophylaxis with LMWH is recommended for [Bates et al 2012, ACOG 2013b]:

• All women heterozygous for the Leiden variant with a prior history of VTE;
• Women heterozygous for the Leiden variant and a positive family history of VTE;
• All asymptomatic homozygous women.

Other

Unexplained pregnancy loss. Current consensus guidelines and expert opinion recommend against the use of antithrombotic therapy outside of clinical trials in women with inherited thrombophilia and unexplained pregnancy loss because of the absence of high-quality evidence confirming benefit [Baglin et al 2010, Bates et al 2012, ACOG 2013b, Middeldorp 2013, Skeith et al 2016].

Pregnancy complications. Current guidelines recommend against antithrombotic prophylaxis for women with inherited thrombophilia and a history of other pregnancy complications such as preeclampsia or placental abruption [Bates et al 2012, ACOG 2013b].