Factor V Leiden

What factor V Leiden is

Blood clotting is a chain reaction of proteins called clotting factors. Factor V speeds up clot formation. To keep clots in check, the body uses a protein called activated protein C, which cuts factor V and switches it off.

In factor V Leiden, a single change in the F5 gene alters the exact spot where protein C cuts. The faulty factor V is broken down about 10 times more slowly, so it keeps promoting clotting for longer. This is called activated protein C resistance. It is named after the Dutch city of Leiden, where it was discovered in 1994.

Factor V Leiden is one type of inherited thrombophilia. The second most common is the prothrombin gene mutation, and some people have both.

Heterozygous vs homozygous

You inherit one copy of the F5 gene from each parent.

Each child of a heterozygous parent has a 50% chance of inheriting the gene.

Symptoms: what a clot feels like

Factor V Leiden itself causes no symptoms. Problems arise only if a clot forms, most often in a vein:

• Deep vein thrombosis: swelling, pain, warmth and redness, usually in one calf or thigh.
• Pulmonary embolism: sudden breathlessness, sharp chest pain on breathing in, fast pulse, coughing up blood, fainting.
• Unusual sites: veins of the brain (cerebral venous thrombosis), liver or gut.
• Superficial thrombophlebitis: a tender, red cord along a vein near the skin.

Factor V Leiden does not meaningfully raise the risk of heart attacks or arterial strokes in most people.

What raises clot risk further

Clots usually happen when factor V Leiden combines with another trigger:

• Estrogen-containing birth control pills, patches or rings, which raise risk substantially in carriers
• Hormone replacement therapy, especially oral estrogen
• Pregnancy and the 6 weeks after birth
• Surgery, injury, casts and immobility
• Long-haul travel of more than about 4 hours
• Cancer, obesity and older age
• A second clotting disorder, such as the prothrombin mutation or antiphospholipid syndrome

Who should be tested

Testing is not recommended for everyone, because a positive result rarely changes care for people who have never had a clot. It may be considered for:

• a clot at a young age (under about 50) or without an obvious trigger
• a clot in an unusual site, such as the brain or abdomen
• recurrent clots
• a strong family history of clots, especially before starting estrogen or during pregnancy planning
• relatives of a known carrier, when the result would change decisions about hormones or pregnancy

The tests are an APC resistance blood test and a DNA test for the F5 gene change. The DNA test is accurate even while you are taking blood thinners or have a current clot.

Treatment

If you have never had a clot

No daily treatment is needed. Instead:

• Avoid estrogen-containing contraception; progestin-only methods and IUDs are safer options.
• Ask about clot prevention (blood-thinner injections, compression and early walking) before surgery or during long hospital stays.
• On long flights, walk around regularly, flex your calves, stay hydrated, and ask your doctor about compression stockings.
• Keep a healthy weight and do not smoke.

If you have had a clot

A DVT or pulmonary embolism is treated with anticoagulants (blood thinners) such as apixaban, rivaroxaban, low molecular weight heparin or warfarin, usually for at least 3 months. Whether to continue long term depends mostly on whether the clot had a clear temporary cause, how many clots you have had and your bleeding risk. Being a heterozygous carrier by itself usually does not change the length of treatment. Homozygous carriers and people with repeat clots are more often advised to stay on treatment indefinitely.

Factor V Leiden and pregnancy

Pregnancy multiplies clot risk, and the weeks after delivery are highest risk. Plans depend on your genes and history:

• Past clot: blood-thinner injections (low molecular weight heparin) are usually recommended during pregnancy and for 6 weeks after.
• Heterozygous with no past clot: usually monitoring in pregnancy, with injections after birth if there is a family history or other risk factors.
• Homozygous, or combined with another thrombophilia: injections are often recommended after delivery and sometimes during pregnancy.

The link with miscarriage and other pregnancy complications is weak, and routine treatment for that reason alone is not recommended. Warfarin is avoided in pregnancy. Breastfeeding is safe with heparin and warfarin.

Outlook

Most people with factor V Leiden live a normal life without ever having a clot. Knowing you carry it helps you and your doctors avoid avoidable risks, recognize clot symptoms early and plan pregnancy, surgery and contraception safely.

Frequently asked questions

Is factor V Leiden serious?

For most carriers, no. The majority of people with one copy never have a clot. It becomes significant when combined with other risks such as estrogen, pregnancy, surgery or immobility, or when a person carries two copies. A clot itself, especially a pulmonary embolism, can be serious and needs urgent care.

Can I take birth control pills with factor V Leiden?

Estrogen-containing pills, patches and rings are generally not recommended, because together with factor V Leiden they raise clot risk sharply. Progestin-only pills, the implant, hormonal IUDs and copper IUDs are safer choices. Discuss options with your doctor or gynecologist.

Does factor V Leiden cause symptoms on its own?

No. The gene change does not cause pain, fatigue or other daily symptoms. Symptoms appear only if a blood clot forms, such as leg swelling and pain from a DVT or sudden breathlessness and chest pain from a pulmonary embolism.

Should my family be tested if I have factor V Leiden?

Testing relatives is optional and most useful when the result would change a decision, for example a sister considering estrogen birth control or planning pregnancy. Children are usually not tested until they are old enough for these decisions. Talk to your doctor or a genetic counselor.

Do I need blood thinners for life?

Not usually. Carriers who have never had a clot do not need blood thinners. After a first clot with a clear trigger, treatment usually stops after about 3 to 6 months. Lifelong treatment is considered after unprovoked or repeated clots, or for homozygous carriers.

Can I fly with factor V Leiden?

Yes. On flights longer than about 4 hours, walk the aisle regularly, do calf exercises, drink water and avoid sleeping pills. Your doctor may recommend fitted compression stockings, and occasionally a preventive blood-thinner dose if you have had clots before.