Factor V Leiden: What Your DNA Says About Blood Clot Risk

Up to 5% of people with European ancestry carry a genetic mutation they've never heard of - one that makes their blood more likely to clot when it shouldn't. It's called Factor V Leiden, and it's the most common inherited blood clotting disorder in the world (Kujovich, 2011).

If you've ever had an unexplained blood clot, or if blood clots run in your family, a single DNA variant - rs6025 - could explain why. Here's what the science says, what your results mean, and what you can actually do about it.

What Is Factor V Leiden?

Your blood uses a carefully balanced system to form clots when you're injured and dissolve them when you're healed. One key player is Factor V, a protein produced by your liver that helps convert prothrombin to thrombin during the clotting cascade (Dahlbäck, 2008).

Normally, a protein called activated protein C (APC) acts as a brake - it cuts Factor V at a specific site (amino acid 506) to shut down clotting when it's no longer needed (Bertina et al., 1994). Think of APC as the off switch for your clotting system.

Factor V Leiden is a single-letter change in your DNA: a G-to-A swap at nucleotide 1691 of the F5 gene. This replaces arginine with glutamine at position 506 - the exact spot where APC is supposed to cut. The result? APC can't turn off Factor V properly, so your blood keeps clotting longer than it should (Bertina et al., 1994).

The mutation was discovered in 1994 by researchers at the University of Leiden in the Netherlands, which is how it got its name (Bertina et al., 1994).

How Common Is Factor V Leiden?

Factor V Leiden prevalence varies dramatically by ancestry (Rees et al., 1995):

• European descent: 3–8% carry at least one copy
• Hispanic Americans: ~2%
• African Americans: ~1.2%
• East Asian and African populations: extremely rare (less than 0.5%)

Among people of European descent, roughly 1 in 20 carries one copy (heterozygous), while about 1 in 5,000 carries two copies (homozygous) (Kujovich, 2011). This makes it by far the most common inherited thrombophilia - more common than protein C deficiency, protein S deficiency, and antithrombin deficiency combined.

Up to 30% of patients who show up at hospitals with deep vein thrombosis (DVT) or pulmonary embolism (PE) test positive for Factor V Leiden (Rosendaal et al., 1995). That's a staggering overrepresentation for a variant carried by only 5% of the general population.

What Your Genotype Means for Blood Clot Risk

Factor V Leiden follows an autosomal dominant inheritance pattern with incomplete penetrance - meaning you only need one copy to be affected, but having the mutation doesn't guarantee you'll ever develop a clot (Kujovich, 2011).

Here's what the research shows for the SNP rs6025:

• GG (no mutation): Baseline risk. About 1 in 1,000 people per year develop a venous blood clot.
• GA (heterozygous - one copy): 4- to 8-fold increased risk of VTE. Lifetime VTE risk is approximately 5–10% in the general population, higher in thrombophilic families (Rosendaal et al., 1995; Middeldorp, 2011).
• AA (homozygous - two copies): ~20-fold increased risk compared to baseline. Lifetime VTE risk can reach 20% or more (Martinelli et al., 1998).

But here's the critical context: 90% of people with Factor V Leiden never develop an abnormal blood clot (MedlinePlus Genetics, 2024). The mutation increases relative risk significantly, but the absolute risk - especially for heterozygous carriers without other risk factors - remains relatively modest.

When Factor V Leiden Becomes Dangerous

Factor V Leiden rarely causes problems on its own. It becomes dangerous when combined with other risk factors that stack on top of the genetic predisposition (Rosendaal, 1999):

Hormonal factors:

• Oral contraceptives: Women with Factor V Leiden who take combined oral contraceptives have a 30- to 35-fold increased VTE risk compared to women without the mutation who don't use hormonal birth control (Vandenbroucke et al., 1994). This is the most clinically significant drug-gene interaction for this variant.
• Hormone replacement therapy (HRT): Estrogen-containing HRT approximately doubles VTE risk in carriers (Herrington et al., 2002).
• Pregnancy and postpartum: The postpartum period already increases clotting risk 5-fold; Factor V Leiden adds to this significantly (James, 2009).

Situational risk factors:

• Surgery, especially orthopedic procedures
• Prolonged immobility (long flights, bed rest)
• Cancer
• Obesity
• Smoking
• Advancing age

Combined thrombophilias:

• Carrying both Factor V Leiden and the prothrombin G20210A mutation (rs1799963) increases VTE risk more than either mutation alone - comparable to homozygous Factor V Leiden (Emmerich et al., 2001).

Factor V Leiden and Pregnancy

This variant deserves special attention during pregnancy. VTE affects about 1.2 per 1,000 deliveries, and it remains a leading cause of maternal mortality in developed countries (James, 2009).

For heterozygous carriers without a personal or family history of VTE, current guidelines from the American College of Obstetricians and Gynecologists (ACOG) and the American Society of Hematology (ASH) generally recommend:

• Clinical surveillance during pregnancy without routine anticoagulation
• Postpartum prophylaxis with low-molecular-weight heparin for 6 weeks may be considered if there's a strong family history (Bates et al., 2018)

For homozygous carriers or those with a prior VTE event, antepartum and postpartum anticoagulation is typically recommended (Bates et al., 2018).

It's important to note that despite earlier concerns, current evidence does not support a link between Factor V Leiden and recurrent pregnancy loss (ACOG Practice Bulletin No. 197, 2018).

Should You Get Tested?

The American Society of Hematology (ASH) and the American College of Medical Genetics (ACMG) recommend against routine screening of the general population for Factor V Leiden (Grody et al., 2001). However, testing may be appropriate if you have:

• An unprovoked DVT or PE, especially before age 50
• Recurrent VTE
• VTE in an unusual site (cerebral, portal, or hepatic veins)
• A first-degree relative with VTE before age 50
• VTE during pregnancy, postpartum, or while on oral contraceptives

Direct-to-consumer DNA tests from companies like 23andMe and AncestryDNA include rs6025 in their raw data files. If you've already been tested, you can check your Factor V Leiden status by uploading your raw data to a tool like GenomeInsight - no additional blood draw needed.

What You Can Do About It

If you discover you carry Factor V Leiden, here's what evidence-based medicine recommends:

For all carriers:

• Inform your healthcare provider before any surgery, hospitalization, or period of immobility
• Stay active - regular movement is one of the best clot-prevention strategies
• Stay hydrated, especially during long travel
• On flights over 4 hours, wear compression stockings and walk the aisle periodically
• Know the warning signs of DVT (leg swelling, pain, warmth, redness) and PE (sudden shortness of breath, chest pain, rapid heartbeat)

For women:

• Discuss contraceptive options with your doctor - progestin-only methods or non-hormonal options avoid the estrogen-related risk increase (van Vlijmen et al., 2011)
• If pregnant, ensure your OB/GYN knows your Factor V Leiden status for appropriate monitoring

For carriers with prior VTE:

• Work with a hematologist to determine appropriate anticoagulation duration
• Long-term or indefinite anticoagulation may be recommended after recurrent events (Kearon et al., 2016)

You do NOT need to:

• Avoid all physical activity (exercise actually reduces clot risk)
• Take daily aspirin prophylactically (not recommended for venous clot prevention)
• Panic - most carriers live completely normal lives

Key Takeaways

• Factor V Leiden is the most common inherited blood clotting disorder, affecting up to 5% of people with European ancestry.
• The mutation at rs6025 prevents activated protein C from properly shutting down the clotting cascade.
• Heterozygous carriers (one copy) have a 4–8x increased VTE risk; homozygous carriers (two copies) have ~20x increased risk.
• 90% of carriers never develop a blood clot - additional risk factors like oral contraceptives, surgery, or immobility are usually needed to trigger an event.
• The most dangerous combination is Factor V Leiden + estrogen-containing birth control (30–35x increased risk).
• You can check your Factor V Leiden status from existing DNA data - upload your raw data to GenomeInsight for a comprehensive analysis that includes thrombophilia variants.

Want to learn more about what your DNA reveals? Explore our pharmacogenomics guide to see how your genes affect drug response, or check out your full health risk profile with GenomeInsight. Sign up for our newsletter to get the latest genetics research delivered to your inbox.

References

ACOG Practice Bulletin No. 197. (2018). Inherited thrombophilias in pregnancy. Obstetrics & Gynecology, 132(1), e18–e34. https://doi.org/10.1097/AOG.0000000000002703

Bates, S. M., Rajasekhar, A., Engel, S., et al. (2018). American Society of Hematology 2018 guidelines for management of venous thromboembolism: Venous thromboembolism in the context of pregnancy. Blood Advances, 2(22), 3317–3359. https://doi.org/10.1182/bloodadvances.2018024802

Bertina, R. M., Koeleman, B. P., Koster, T., et al. (1994). Mutation in blood coagulation factor V associated with resistance to activated protein C. Nature, 369(6475), 64–67. https://doi.org/10.1038/369064a0

Dahlbäck, B. (2008). Advances in understanding pathogenic mechanisms of thrombophilic disorders. Blood, 112(1), 19–27. https://doi.org/10.1182/blood-2008-01-077909

Emmerich, J., Rosendaal, F. R., Cattaneo, M., et al. (2001). Combined effect of factor V Leiden and prothrombin 20210A on the risk of venous thromboembolism. Thrombosis and Haemostasis, 86(3), 809–816. https://doi.org/10.1055/s-0037-1616135

Grody, W. W., Griffin, J. H., Taylor, A. K., et al. (2001). American College of Medical Genetics consensus statement on factor V Leiden mutation testing. Genetics in Medicine, 3(2), 139–148. https://doi.org/10.1097/00125817-200103000-00009

Herrington, D. M., Vittinghoff, E., Howard, T. D., et al. (2002). Factor V Leiden, hormone replacement therapy, and risk of venous thromboembolic events in women with coronary disease. Arteriosclerosis, Thrombosis, and Vascular Biology, 22(6), 1012–1017. https://doi.org/10.1161/01.ATV.0000018300.48845.40

James, A. H. (2009). Venous thromboembolism in pregnancy. Arteriosclerosis, Thrombosis, and Vascular Biology, 29(3), 326–331. https://doi.org/10.1161/ATVBAHA.109.184127

Kearon, C., Akl, E. A., Ornelas, J., et al. (2016). Antithrombotic therapy for VTE disease: CHEST guideline and expert panel report. Chest, 149(2), 315–352. https://doi.org/10.1016/j.chest.2015.11.026

Kujovich, J. L. (2011). Factor V Leiden thrombophilia. Genetics in Medicine, 13(1), 1–16. https://doi.org/10.1097/GIM.0b013e3181faa0f2

Martinelli, I., Mannucci, P. M., De Stefano, V., et al. (1998). Different risks of thrombosis in four coagulation defects associated with inherited thrombophilia. Blood, 92(7), 2353–2358. https://doi.org/10.1182/blood.V92.7.2353

MedlinePlus Genetics. (2024). Factor V Leiden thrombophilia. National Library of Medicine. https://medlineplus.gov/genetics/condition/factor-v-leiden-thrombophilia/

Middeldorp, S. (2011). Is thrombophilia testing useful? Hematology ASH Education Program, 2011(1), 150–155. https://doi.org/10.1182/asheducation-2011.1.150

Rees, D. C., Cox, M., & Clegg, J. B. (1995). World distribution of factor V Leiden. The Lancet, 346(8983), 1133–1134. https://doi.org/10.1016/S0140-6736(95)91803-5

Rosendaal, F. R. (1999). Risk factors for venous thrombotic disease. Thrombosis and Haemostasis, 82(2), 610–619. https://doi.org/10.1055/s-0037-1615887

Rosendaal, F. R., Koster, T., Vandenbroucke, J. P., & Reitsma, P. H. (1995). High risk of thrombosis in patients homozygous for factor V Leiden. Blood, 85(6), 1504–1508. https://doi.org/10.1182/blood.V85.6.1504.bloodjournal8561504

van Vlijmen, E. F., Brouwer, J. L., Veeger, N. J., et al. (2011). Oral contraceptives and the absolute risk of venous thromboembolism in women with single or multiple thrombophilic defects. Archives of Internal Medicine, 171(6), 523–528. https://doi.org/10.1001/archinternmed.2011.64

Vandenbroucke, J. P., Koster, T., Briët, E., et al. (1994). Increased risk of venous thrombosis in oral-contraceptive users who are carriers of factor V Leiden mutation. The Lancet, 344(8935), 1453–1457. https://doi.org/10.1016/S0140-6736(94)90286-0

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