Factor V Leiden is a mutation of one of the clotting factors in the blood called factor V. This mutation can increase your chance of developing abnormal blood clots (thrombophilia), usually in your veins.
Most people with factor V Leiden (FAK-tur five LIDE-n) never develop abnormal clots. But some do develop clots that lead to long-term health problems or become life-threatening.
Both men and women can have factor V Leiden. Women may have an increased tendency to develop blood clots during pregnancy or when taking the hormone estrogen.
If you have factor V Leiden and have developed blood clots, anticoagulant medications can lessen your risk of developing additional blood clots and help you avoid potentially serious complications.
Most people who have factor V Leiden never develop signs or symptoms. The first indication that you have the disorder may be the development of a blood clot (thrombosis).
Some clots do no damage and disappear on their own. Others can be life-threatening. Symptoms of a blood clot depend on where it forms and whether and where it travels.
A clot in a deep vein
This is known as deep vein thrombosis (DVT). Deep vein thrombosis may not cause any symptoms. If signs and symptoms do occur, they commonly affect your legs, including swelling of your ankles and feet. Other signs and symptoms may include:
- Significant swelling
A clot closer to the surface of your skin
This is referred to as superficial venous thrombosis, phlebitis or thrombophlebitis. Signs and symptoms usually include:
- Tenderness or pain, often in or around the vein with the blood clot
A clot that travels to your lungs
Known as a pulmonary embolism, this occurs when a deep vein clot breaks free and travels through the right side of your heart to your lung, where it blocks blood flow. This can be a life-threatening situation. Signs and symptoms may include:
- Sudden shortness of breath
- Chest pain when breathing in
- A cough that produces bloody or blood-streaked sputum
- Rapid heartbeat (tachycardia)
When to see a doctor
Seek medical attention immediately if you:
- Have signs or symptoms of a pulmonary embolism, such as chest pain or discomfort.
- Have signs or symptoms of deep vein thrombosis, such as leg pain and swelling.
See a doctor if you:
- Have a family history of blood clots or if family members have factor V Leiden. Ask your doctor about the risks and benefits of genetic testing for the disorder.
- Have had one or more blood-clotting incidents without an apparent cause, especially if you're under 50.
A blood clot (thrombus) normally forms to stop the bleeding when an artery or vein is damaged, such as when you experience a cut. Clots are formed by chemical reactions between specialized blood cells (platelets) and proteins in your blood (clotting factors). Anti-clotting factors prevent an excessive formation of blood clots.
Normally, factor V is a clotting protein. Anti-clotting proteins break up factor V, keeping it from forming clots when clotting isn't needed.
Factor V Leiden makes it harder for anti-clotting proteins to break up factor V. This keeps factor V in the blood longer and increases the chance of clotting.
If you have factor V Leiden, you inherited either one copy (heterozygous) or, rarely, two copies (homozygous) of the defective gene. Inheriting one copy slightly increases your risk of developing blood clots. Inheriting two copies — one from each parent — significantly increases your risk of developing blood clots.
A family history of factor V Leiden increases your risk of inheriting the disorder. The disorder is most common in people who are white and of European descent.
Factor V Leiden can be associated with a variety of serious and potentially serious clotting complications, including:
- Pregnancy complications. Most women with factor V Leiden have normal pregnancies. But the mutation has been linked with an increased risk of miscarriage and possibly other complications during pregnancy, including pregnancy-induced high blood pressure (preeclampsia), slow fetal growth and early separation of the placenta from the uterine wall (placental abruption). If you're a pregnant woman with factor V Leiden, be sure your doctor monitors you carefully throughout your pregnancy.
- Deep vein thrombosis (DVT). People with factor V Leiden have an increased risk of developing deep vein thrombosis compared with someone without the mutation, though the overall risk of developing deep vein thrombosis is still low.
- Pulmonary embolism. Deep vein thrombosis puts you at risk of a clot breaking off and traveling to your lungs or, rarely, your brain. A pulmonary embolism can be fatal. It's important to watch for signs and symptoms of a pulmonary embolism, such as shortness of breath or chest pain, and to seek prompt medical attention.