Factor V Leiden (FAK-tur five LIDE-n) is a mutation of one of the clotting factors in the blood. This mutation can increase your chance of developing abnormal blood clots, most commonly in your legs or lungs.

Most people with factor V Leiden never develop abnormal clots. But in people who do, these abnormal clots can lead to long-term health problems or become life-threatening.

Both men and women can have factor V Leiden. Women who carry the factor V Leiden mutation 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.

The factor V Leiden mutation does not itself cause any symptoms. Since factor V Leiden is a risk for developing blood clots in the leg or lungs, the first indication that you have the disorder may be the development of an abnormal blood clot.

Some clots do no damage and disappear on their own. Others can be life-threatening. Symptoms of a blood clot depend on what part of your body is affected.

A clot in a deep vein

This is known as deep vein thrombosis (DVT), which most commonly occurs in the legs. A DVT may not cause any symptoms. If signs and symptoms do occur, they can include:

• Pain
• Swelling
• Redness
• Warmth

A clot that travels to your lungs

Known as a pulmonary embolism, this occurs when a portion of a DVT 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

When to see a doctor

Seek medical attention immediately if you have signs or symptoms of either a DVT or a pulmonary embolism.

If you have factor V Leiden, you inherited either one copy or, rarely, two copies 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.

People who have inherited factor V Leiden from only one parent have a 5 percent chance of developing an abnormal blood clot by age 65. Factors that increase this risk include:

• Two faulty genes. Inheriting the genetic mutation from both parents instead of just one can significantly increase your risk of abnormal blood clots.
• Immobility. Extended periods of immobility, such as sitting during a long airplane flight, can increase the risk of leg clots.
• Estrogens. Oral contraceptives, hormone replacement therapy and pregnancy can make you more likely to develop blood clots.
• Surgeries or injuries. Surgeries or injuries such as broken bones can increase your risk of abnormal blood clots.
• Non-O blood type. Abnormal blood clots are more common in people who have blood types of A, B or AB compared with those with blood type O.

Factor V Leiden can cause blood clots in the legs (deep vein thrombosis) and lungs (pulmonary embolism). These blood clots can be life-threatening.

Your doctor may suspect factor V Leiden if you've had one or more episodes of abnormal blood clotting or if you have a strong family history of abnormal blood clots. Your doctor can confirm that you have factor V Leiden with a blood test.

Doctors generally prescribe blood-thinning medications to treat people who develop abnormal blood clots. This type of medicine usually isn't needed for people who have the factor V Leiden mutation but who have not experienced abnormal blood clots.

However, your doctor might suggest that you take extra precautions to prevent blood clots if you have the factor V Leiden mutation and are going to have surgery. These precautions might include:

• A short course of blood thinners
• Leg wraps that inflate and deflate to keep blood moving in your legs
• Compression stockings
• Going for walks soon after surgery

Some precautions to help reduce your risk of blood clots include:

• Keep your legs moving. When your legs remain still for hours, your calf muscles don't contract, which normally helps blood circulate. If you're on a long plane trip, raise your toes up and down and rotate your ankles every hour or so. Drink extra water to prevent dehydration, and avoid alcohol. On a car trip, take periodic breaks and walk around.
• Consider compression stockings. These types of socks, which usually come up to the knees, help improve blood circulation in your legs. Ask your doctor if they might be a good option for your situation.
• Be cautious with estrogen. Oral contraceptives or estrogen replacement therapy can increase the risk of blood clots on their own, so be sure to discuss the risks and the benefits of estrogen-containing medications with your doctor if you have factor V Leiden.

Prevent excessive bleeding

If your factor V Leiden requires you to take anticoagulant medication, here are some steps that might help you prevent injury and avoid excessive bleeding:

• Avoid playing contact sports or engaging in other activities that could result in physical injury. Regular noncontact exercise, such as walking or swimming, is still recommended for good health.
• Use a soft toothbrush and waxed floss.
• Avoid shaving cuts by using an electric razor.
• Be cautious with household tasks involving knives, scissors and other sharp tools.

Your doctor may refer you to a specialist in genetic disorders (geneticist) or a specialist in blood disorders (hematologist) for testing to determine whether the cause of your blood clots is genetic and, specifically, whether you have factor V Leiden.

Here's some information to help you prepare for your appointment.

• List any symptoms you're experiencing, including any that may seem unrelated to the reason for which you scheduled the appointment.
• List your health history, including your history of blood clots. Include any family history of blood clots or known family members with factor V Leiden.
• Make a list of all medications, vitamins or supplements you're taking, along with the dose for each.
• List questions to ask your doctor.

For factor V Leiden, some basic questions to ask your doctor include:

• What kinds of tests do I need?
• Do I need to see a specialist?
• Does my factor V Leiden need to be treated?
• Do I need to take medication to prevent additional blood clots?
• What types of side effects can I expect from the medication?
• Do I need to limit my activity in any way?
• If I have children, do they need to be tested?
• Do you have any brochures or other printed material that I can take with me? What websites do you recommend?

If your doctor recommends genetic testing, some questions you might want to ask the genetic specialist include:

• How accurate is this test?
• What are the risks of the test?
• What information will come out of the test?
• What will a positive or negative result tell me?
• Can the results of the test affect my ability to obtain health insurance?
• Is an uncertain result possible, and what would that mean?
• What are my treatment options if a mutation is found?
• Could other family members be affected?
• Should my children be tested?
• What measures are in place to protect my privacy?
• How experienced is the lab at performing this test?
• How long will it take to get results back?