There are a number of inherited (genetic) blood conditions that may lead to blood clots in veins. The most common is called factor V (five) Leiden. When you have factor V Leiden, you may be at an increased risk of having a clot in your vein.

Factor V Leiden is very common, occurring in about 5 percent of people in the United States. It is present, however, in 20-40 percent of people with blood clots in their veins.

Causes of blood clotting
Your body normally forms blood clots when a blood vessel is injured. Our bodies naturally keep the clotting process in check. However, if one of the mechanisms that help stop clotting is not working correctly, it may result in abnormal clotting. Inherited disorders contribute to about one-half of all clotting episodes that occur in a vein. Factor V Leiden is the most common, but not the only inherited abnormality contributing to blood clotting.

Inherited abnormalities are especially suspected if clotting occurs in someone who is relatively young, if the clotting has occurred in many sites, or if clotting is in unusual sites.
 
Non-inherited risk factors for clotting in veins include:

• Pregnancy
• Oral contraceptive use
• Extreme obesity
• Cancer
• Trauma
• Major surgery
• Prolonged bed rest or inactivity
• Presence of a lupus anticoagulant
• Possibly hormone replacement therapy
• Incidence of clotting increases with age

Clotting in a vein
Most clots in a vein are deep vein thrombosis (DVT) and usually occur in a leg. In the United States, about one-half million people are treated each year for DVT. When blood clots form in veins, they get in the way of blood flow and cause swelling, pain and warmth (called phlebitis). Occasionally, a part of the clot may break off and travel in the blood stream to the lungs. This is called a pulmonary embolus (PE) and can be life threatening. Symptoms of PE can include shortness of breath with or without activity, increased and/or irregular heart rate, chest pain, sudden loss of consciousness or coughing up blood.

Factor V Leiden test
Your physician may request that you have a factor V Leiden test for any of the following reasons:

• You have a clot in a vein
• You have a family or personal history of clotting in the veins
• You have another inherited clotting disorder
• You have a documented family history of factor V Leiden

During a factor V Leiden test, about two teaspoons of blood is drawn from a vein in your arm. Please check with your insurance company about coverage for this test. Most insurance companies will provide coverage.

Identifying a genetic cause of clotting is important because it:

• etablishes reason for clotting
• identifies persons and families at increased risk for future clotting
• reduces incidence of future clotting through preventive measures
  

Inheriting factor V Leiden
Genes are in the center of each of our cells and come in pairs. If factor V Leiden is identified in a person, it means that one gene out of the pair is altered (this is called a mutation) and the other gene is working correctly. Both males and females with one altered factor V gene have an increased chanced of clotting and a 50 percent chance of passing that gene on to each of his/her children. If a child inherits two working genes, he or she does not have an increased chance of clotting and cannot pass on an altered factor V gene to any of his/her children.
 
Risk for clotting with factor V Leiden
In the general population, one person in 1,000 has a DVT each year. The risk for clotting is increased sevenfold in those who inherit one altered factor V gene, and is increased eightyfold in those who receive two altered factor V genes (one from each parent). These rates are in comparison to people without an inherited gene.

Factor V Leiden in women
Women who are pregnant or are using oral contraceptives that contain estrogen are at an increased risk for clotting. Women who use oral contraceptives and who have factor V Leiden are at a thirtyfold increased risk for clotting. Studies have shown that some women who have had miscarriages in the first and second trimester have factor V Leiden. A high proportion of women who develop venous clotting during pregnancy or the period immediately following delivery have factor V Leiden. Women who have a history of clotting may need to take medication to prevent clotting during pregnancy and the immediate post-delivery period.
 
Women who have inherited two altered genes for the factor V Leiden mutation (one from each parent) should consider receiving medication during pregnancy to help prevent clotting, regardless of whether they have experienced a previous clotting episode. This group of women would also be encouraged not to use oral contraceptives in the future.

Management of individuals with factor V Leiden
People who have inherited two altered genes for factor V Leiden with or without a history of clotting should receive medication to help prevent clotting at high-risk times: pregnancy, major surgery, prolonged bed rest or inactivity or trauma. These people would require more prolonged courses of treatment after a clotting episode.

Individuals with one altered gene and a history of vein clotting should receive medication to prevent clotting at high-risk times and possibly a longer treatment course after a clotting episode. Individuals with one altered gene do not have a personal or family history of thrombosis may still be given medication in high-risk situations. Genetic counseling and testing is available to you and your family.

Avoiding risk factors
If you or a family member has the factor V Leiden mutation, it is important to avoid other risk factors that may be under your control, including:

• Periods of prolonged inactivity. Examples may include long car rides/plane trips, illnesses that confine you to bed, and having a cast or an injury to your leg. It is important for you to try to move your legs frequently and avoid sitting for longer than one and a half to two hours at a time.
• Oral contraceptive use and hormone-replacement therapy. Talk to your primary care provider or specialist about possible risks and benefits of therapy before taking the medication.
• Pregnancy. If you are planning to become pregnant and you or a family member has a history of clotting, tell your primary care provider or specialist of the factor V Leiden mutation so that you may receive appropriate care.