Diseases & Conditions

Factor V Leiden

Factor V Leiden is a common inherited genetic disorder in which your blood has an increased tendency to form clots (thrombophilia), usually in your veins.

Although blood clots can form at any age, for most people the increased risk of clotting doesn't begin until adulthood. Most people with factor V Leiden never develop abnormal clots. However, some people with factor V Leiden develop clots that lead to long-term health problems or are life-threatening.

Both men and women can have factor V Leiden, but women may have an increased tendency to develop blood clots during pregnancy or when taking the hormone estrogen.

If you have factor V Leiden, medications can lessen your risk of developing blood clots and help you avoid potentially serious complications.

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 form as a result of chemical reactions between specialized blood cells (platelets) and proteins in your blood (clotting factors). Anti-clotting factors control excessive formation of blood clots. One of the clotting proteins is factor V. People with factor V Leiden have a genetic mutation that results in factor V protein responding more slowly to the anti-clotting factors.

In the normal clotting process, anti-clotting proteins combine to help break up factor V to keep it from being reused and forming clots when clotting isn't needed. However, the factor V Leiden mutation keeps the anti-clotting proteins from breaking down factor V, which keeps it in the blood longer and increases the chance of clotting.

If you have factor V Leiden, you either inherited one copy of the defective gene (heterozygous), which slightly increases your risk of developing blood clots, or you inherited two copies, one from each parent (homozygous), which significantly increases your risk.

Doctors generally use blood-thinning (anticoagulant) medications, such as warfarin (Coumadin), heparin or low molecular weight heparin to treat people who develop blood clots.

If you've been tested because of a family history of factor V Leiden, and have not had any blood clots, then your doctor probably won't prescribe blood thinners unless you're in a high-risk situation, such as being hospitalized for surgery or other medical reasons. In that case, you may receive preventive doses of blood thinners during your hospital stay.

If you have a thrombosis, standard initial treatment involves a combination of heparin and warfarin. Your doctor will discontinue the heparin after the initial treatment and continue the warfarin. How long you need to take medication will depend on the circumstances of your thrombosis.

• Heparin. This anticoagulant medication works more quickly than warfarin does, but until recently, it had to be administered through a vein and monitored closely. Newer forms of heparin — enoxaparin (Lovenox) and dalteparin (Fragmin) — are known as low molecular weight heparin, which you can inject yourself. Heparin is considered safe to take during pregnancy.
• Warfarin (Coumadin). This anticoagulant comes in pill form, so it's easier to take than heparin is. Rarely, warfarin can cause birth defects, so it isn't usually recommended during pregnancy, especially not during the first trimester. A doctor may prescribe warfarin after the first trimester of pregnancy, but only if the benefits of using it outweigh the risks.

Pregnancy considerations
Anticoagulation therapy is particularly complex during pregnancy. The therapy is expensive, requires regular injections and carries some significant risks of side effects.

Ongoing monitoring
If you're taking anticoagulant medication, your doctor will monitor your dose with blood tests to be sure your blood is capable of clotting enough to stop your bleeding if you bruise or cut yourself.