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Blood Test Tracker

Factor V Leiden (R506Q)

A DNA test for a common inherited change in the F5 gene that lets blood clot a little more easily. Most people who carry it never develop a problem clot.

AI-verified against cited sources. Awaiting expert reviewAlso called FVL, R506QLOINC 21668-9

What it is

Factor V is one of the proteins your blood uses to form clots. Factor V Leiden is an inherited change in the F5 gene (a G-to-A change, also written R506Q or G1691A) that makes factor V harder for your body's natural clot-limiting protein, activated protein C, to switch off. As a result, clots can form more easily. The test is a DNA test on a blood sample, and the result is usually not detected (no copies), heterozygous (one copy) or homozygous (two copies). Among people with European ancestry, about 3 to 8 in 100 carry one copy and about 1 in 5,000 carry two. It is less common in other groups.

Why it matters

Carrying factor V Leiden raises the chance of blood clots in the legs or lungs (called venous thromboembolism), but most carriers never have one. In the general population about 1 in 1,000 people develop an unwanted clot each year. One copy raises that to about 3 to 8 in 1,000, and two copies may raise it to as high as 80 in 1,000. Only about 10 in 100 carriers ever develop an unwanted clot. Other things add to the chance, including age, obesity, injury, surgery, smoking, pregnancy, and estrogen used for birth control or for menopause symptoms. Women who carry it are also 2 to 3 times more likely to have repeated miscarriages or a pregnancy loss, although most pregnancies go well. Your result can be one piece of the picture when you and your clinician think about surgery, pregnancy or hormone-containing medicines, or when someone in your family has had a clot.

If it’s high

A result with one or two copies means you carry the inherited change. It does not mean you have a clot or will have one. The chance goes up with two copies and when other risk factors are present. Your clinician weighs the result together with your own and your family's history of clots.

Common causes

  • Inherited from one parent (one copy) or from both parents (two copies)
  • Common in people with European ancestry: about 3 to 8 in 100 carry one copy

What can help

  • Ask your clinician how the result fits with your own and your family's history of clots
  • Mention the result before surgery, long stretches in bed, pregnancy, or starting hormone-containing medicines
  • Ask whether any other clotting tests make sense for you
  • Ask whether close relatives should talk with their own clinicians about testing

What can affect your result

  • The result is fixed at birth, so age, food, fasting and time of day do not change it
  • Other inherited clotting changes, such as the prothrombin G20210A variant, can add to the chance of a clot
  • Circumstances such as surgery, pregnancy, injury, long stretches without moving, and estrogen-containing medicines affect whether a clot forms
  • Ancestry: the change is most common in people of European descent and less common in other groups
  • A past bone marrow transplant from a donor can interfere with the DNA test
  • A previous bone marrow transplant from a donor will interfere with the test, because the DNA may come from the donor's cells (Mayo Clinic Laboratories).
  • The DNA test looks only for this one change. It will not find activated protein C resistance caused by anything else (Mayo Clinic Laboratories).
  • Clotting-based activated protein C resistance tests can be thrown off by a lupus anticoagulant or very high heparin levels (Mayo Clinic Laboratories). The DNA test reads the gene itself instead.

Your result is fixed at birth and does not change.

When to talk to a clinician

Contact a clinician promptly if you have swelling, pain, warmth or redness in one leg, or sudden unexplained shortness of breath. Call 911 or your local emergency number for chest pain, trouble breathing, coughing up blood or fainting.

Questions you could ask

  • Why was this test ordered for me, and what will we do differently with the result?
  • Do I need any other clotting tests alongside it?
  • How does this result fit with my own and my family's history of blood clots?
  • Does it change how we think about surgery, long trips, pregnancy or hormone-containing medicines for me?
  • Should any of my relatives talk with their own clinicians about testing?

Questions people ask

I carry one copy. Does that mean I will get a blood clot?

No. Only about 10 in 100 people who carry factor V Leiden ever develop an unwanted clot. One copy raises the yearly chance from about 1 in 1,000 to about 3 to 8 in 1,000. Your own chance also depends on other factors, such as surgery, pregnancy, hormones and family history.

Should my relatives be tested?

It depends. A review in Genetics in Medicine says that, because there is no evidence that finding it early reduces illness or death, testing at-risk family members should be decided case by case. Your clinician or a genetic counselor can help you think about it.

Does this change decisions about birth control, pregnancy or surgery?

It can be part of those conversations, alongside your own history. The American Society of Hematology (2023) strongly recommends against testing everyone before they start combined birth control pills, but supports testing in some specific situations. Talk with your clinician about how a result would fit your situation. Do not change any medicine on your own.

Show the science and sources

For clinicians and researchers

In current HGVS numbering the variant is F5 c.1601G>A, p.Arg534Gln (rs6025); it is better known by its older names, G1691A and R506Q. The change gives factor V a poor anticoagulant response to activated protein C (Kujovich 2011). Mayo Clinic Laboratories offers a clotting-based activated protein C resistance assay, with reflex to the DNA test (F5DNA, direct variant analysis, reference result "Negative", LOINC 21668-9). It recommends the coagulation-based resistance ratio as the first screening assay and notes that the DNA test cannot find resistance caused by other mechanisms. Carrier frequency is 3 to 8% in people of European ancestry, with about 1 in 5,000 homozygous (MedlinePlus Genetics). Venous thromboembolism rises from about 1 in 1,000 per year to 3 to 8 in 1,000 with one copy and as high as 80 in 1,000 with two copies, and only about 10% of carriers ever have an unwanted clot (MedlinePlus Genetics). The mutation has at most a modest effect on recurrence after a first clot, is linked to a 2 to 3 times higher relative risk of pregnancy loss, and long-term anticoagulation is not routinely recommended for heterozygotes without a clot history (Kujovich 2011). A meta-analysis found the highest clot risk in people with both non-O blood group and factor V Leiden (odds ratio 7.60, 95% CI 3.21-17.99) (Dentali 2012). The 2023 American Society of Hematology guideline makes a strong recommendation against testing the general population before starting combined oral contraceptives, makes conditional recommendations for testing in selected situations (for example a clot linked to nonsurgical major transient or hormonal risk factors, a family history of antithrombin, protein C or protein S deficiency, or pregnant women with a family history of a high-risk thrombophilia), and conditionally recommends against testing in most other situations (Middeldorp 2023). ACOG Practice Bulletin 197 reviews screening and management of inherited thrombophilias in pregnancy and notes limited evidence to guide screening (ACOG 2018).

LOINC 21668-9 (primary), 91712-0, 21667-1. Specimen: whole-blood.

Sources

  1. Factor V Leiden thrombophilia, MedlinePlus Genetics, National Library of Medicine
  2. Factor V Leiden thrombophilia, Genetics in Medicine (2011)
  3. Factor V Leiden (R506Q) Mutation, Blood (test F5DNA), Mayo Clinic Laboratories (Extended Catalog, testcatalog.org)
  4. Activated Protein C Resistance V, with Reflex to Factor V Leiden, Blood and Plasma (test APCRR), Mayo Clinic Laboratories (Extended Catalog, testcatalog.org)
  5. American Society of Hematology 2023 guidelines for management of venous thromboembolism: thrombophilia testing, Blood Advances (2023)
  6. ACOG Practice Bulletin No. 197: Inherited Thrombophilias in Pregnancy, Obstetrics & Gynecology (2018)
  7. Non-O blood type is the commonest genetic risk factor for VTE: results from a meta-analysis of the literature, Seminars in Thrombosis and Hemostasis (2012)
  8. dbSNP record for rs6025 (F5 c.1601G>A, p.Arg534Gln) (NCBI Variation Services API), NCBI dbSNP
  9. LOINC 21668-9 and 91712-0 with answer lists (NLM Clinical Tables API), National Library of Medicine / Regenstrief Institute

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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