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Family Testing for Clotting Disorders

A “genetic thrombophilia,” or an inherited predisposition to develop blood clots, affects both you and your family members. This article provides information about: 
  • Informing relatives of inherited risks to develop a blood clot 
  • Advantages and disadvantages of genetic testing for the inherited thrombophilias

Understanding Genetic Risks

Genetic changes (also called mutations) in clotting factors, like factor V and factor II (prothrombin), are the most common, inherited, predisposing factors for blood clots; 5%-7% of Caucasians have factor V Leiden (a common change in the clotting factor V) and 2%-3% have a prothrombin 20210G/A mutation (a change in the clotting factor II). Other inherited thrombophilias include protein C deficiency, protein S deficiency, and antithrombin III deficiency. These conditions are less common, and all together affect only about 1% of the population.  

All of these conditions are inherited in an “autosomal dominant” pattern. Autosomal dominant inheritance means that only one gene mutation is required to have an increased risk, and there is a 50/50 chance that the mutation will be present in first-degree relatives (parent(s), brother(s), sister(s), and children). Inheritance is not dependent on gender. In almost all cases, this means that if you have tested positive for an inherited blood clotting disorder, at least one of your parents will also be positive. Other relatives, including siblings, children, aunts, uncles, and cousins, may also be at risk. The Genetics of Thrombophilia contains further information about the inheritance of clotting genes. 

Having a genetic predisposition does not mean that you will definitely have a blood clot or related condition (which may include recurrent pregnancy losses). Abnormal clotting of blood is a complex condition, in which genetic risks interact with non-genetic risks. Non-genetic risks may include:

  • smoking
  • using hormones
  • recent trauma or surgery
  • a sedentary lifestyle, or
  • recent immobility.

A blood clot will only happen if a combination of these factors is present. In fact, most people (up to 90%) with an inherited predisposition (like factor V Leiden and the prothrombin mutation) will never develop a blood clot. However, it is particularly important for those with genetic predispositions to avoid controllable risk factors that may cause them to develop a blood clot. 


Telling Your Relatives about Genetic Risks in the Family

Genetic risks, by nature, affect not only you, but your family members as well. Genetics professionals often recommend that information about genetic risks be shared with other at-risk relatives. 
Testing of other relatives
There is no right or wrong answer about who should have testing in a family. Individual decisions should be made based on a person’s situation and opinions. 
There are general advantages and disadvantages of testing that may be considered (below). Your family members may want to consult this list when discussing testing with a health care provider, such as a genetic counselor. 

Genetic testing may be an advantage in the following circumstances:

Testing may help women weigh the risks of estrogen-containing contraceptives. Women who have an inherited blood clotting disorder have a greater risk of developing a blood clot while using estrogen (a hormone normally produced by the body, which may also be supplemented). Use of estrogen-containing birth control pills increases the chance that any woman will develop a blood clot by about 5-fold. For women with a thrombophilia, like factor V Leiden, the risk of developing a blood clot is about 32 times higher if they are taking estrogen-containing birth control pills. Clearly, thrombophilia interacts with estrogen in some way. Each woman’s exact risk to develop a blood clot will vary. On average, approximately 1 out of 700 women who has factor V Leiden and uses estrogen-containing contraceptives will develop a blood clot each year. Genetic testing might be helpful for women who would choose not to use oral contraceptives due to this level of risk.
Testing may inform individuals about personal risk so they can make decisions about lifestyle changes that might lower risk. Knowing that one has an inherited thrombophilia may encourage a person to make lifestyle changes to decrease his or her risk for a blood clot. These changes might include:

  • losing weight
  • exercising
  • quitting smoking, or
  • leading a more active lifestyle.

Testing may encourage a relative, or his or her doctor, to take symptoms of a blood clot more seriously. Blood clots are frequently misdiagnosed, as the symptoms are similar to those seen with other conditions (i.e., leg pain may be mistaken for a muscle strain). Awareness of increased risk for clotting may help your relative, and his or her doctor, to take these symptoms more seriously. This could help ensure early diagnosis and treatment of a blood clot. 

Testing may affect health care management during pregnancy. All women have an increased chance to develop a blood clot while they are pregnant. However, women with an inherited thrombophilia are at greater risk. Women with an inherited thrombophilia may also be more likely to have a pregnancy loss, stillbirth, or certain complications of pregnancy. If there is a history of thrombophilia in the family, testing may be beneficial so that a woman can make informed decisions about using blood thinning medication during her pregnancy. 
Before surgery, knowing that a person has an inherited thrombophilia could influence whether a doctor prescribes blood thinning medication after surgery. Also, the dose or length of blood thinning treatment may be influenced by a person’s thrombophilia status. 
Testing may provide peace of mind. For family members at risk, knowing for certain whether they have inherited a thrombophilia may provide reassurance. 

People may choose not to get testing for several reasons. These might include:

Genetic testing may not change medical management. Many people who have inherited thrombophilia will never develop a blood clot. Since use of blood thinners can cause serious and life-threatening bleeding, physicians very rarely put individuals who have not had a blood clot on blood thinning medication. Thus, even if an individual knows that he or she has an inherited clotting abnormality, often no therapy for prevention is currently available if one has not already had a blood clot. Because genetic testing often does not result in a change of medical management, some people feel that testing is not very necessary or helpful. 
Testing positive may lead to feelings of guilt. Some parents may feel guilty that they may have passed on an inherited thrombophilia to their children, and therefore may prefer not to have testing since this would reveal the affected parent. Other relatives may not want testing because they would feel guilty if they did not have an inherited thrombophilia, particularly if their relative has been negatively affected as a result of having a genetic predisposition (this is called survivor guilt). 
Concerns about insurance discrimination. There are federal laws to help protect people from health insurance and job discrimination. (For information, see However, these laws have limitations, and it is possible that a person with inherited thrombophilia may have to (a) pay higher life, disability, or long-term care insurance premiums, and/or (b) get denied life or disability insurance if they test positive. 
Testing may be costly. Testing may or may not be covered by insurance, depending on the reason for testing and the individual’s insurance plan. 
Testing may raise questions about paternity. Testing could reveal a genetic pattern that cannot be explained by inheritance (for example: a person who has one copy of the prothrombin mutation could discover that neither parent is positive.) This may lead to questions about paternity. 
Individuals may be concern about labeling, infringement of privacy, and autonomy of children. There is some concern that testing children would lead to labeling. This could result in treating a child differently, due to concerns that he or she is at high-risk for illness. Making the choice to test a child who has never had a health problem may also raise concerns about privacy and autonomy. Testing a child may be an invasion of privacy and also interferes with autonomy, since the child is not able to make an independent decision about testing. 
Your children: where to go from here? If you have an inherited thrombophilia, you may wonder if your children have inherited it from you. You may be concerned and wonder if your children should be tested. 
In general, medical experts recommend that children make an independent, informed decision about testing. It is not routine for children, who are otherwise healthy, to have testing, unless they have been fully informed of the pros and cons and can make an independent choice. It is also important to consider whether or not testing will influence a child’s medical management. For example, testing may be more strongly encouraged for females when they are at an age where they may consider taking birth control pills, since this medication adds additional risk to the inherited risk. Testing is not as strongly indicated for boys because test results would not affect decisions about oral contraceptives or pregnancy. 

Right for Some, Not for Others 

There is no definite right or wrong answer when it comes to genetic testing for inherited thrombophilia. There are many complexities— understanding who is at risk, dealing with emotional responses to this information, and determining if there is benefit to genetic evaluation of relatives. It is important to discuss these issues with a knowledgeable health care provider who can assist you in identifying at-risk relatives and make recommendations regarding thrombophilia testing in your family. Genetic counselors are health care professionals who have experience and expertise in helping families with these issues. 
To find a genetic counselor in your area, visit This site allows you to search for genetic counselors by city, state, or hospital. 
Permission to reprint this material has been granted by the National Blood Clot Alliance (

For more information:

Go to the Bleeding and Clotting Disorders health topic.