Factor V Leiden is an inherited condition with a risk of thrombosis.
Heterozygous
If you have one parent with the mutation and you have inherited factor V Leiden from them, your risk of a blood clot is 5 to 7 times higher than normal.
Homozygous
If both parents have Factor V Leiden and you have inherited two copies of the mutation, your risk of a blood clot is 40 times higher than normal.
Should my family be tested?
If you have Factor V Leiden, should your family be tested? Unfortunately the answer is not straight forward.
In theory
In medicine we have a mantra that:
You should only do a test if the result will alter management.
We should not do tests just to know the answer.
Based on theory alone, there is no point screening family members as the result will not change management.
Case study
Here is an example to explain this. You are 40 years old and have your first DVT. You are found to have the factor V Leiden mutation.
Your parents are in their 60s and you have a brother and sister in their 40s. Is there any reason for them to test? No.
- If they have FV Leiden and have never had a clot, they do not need any treatment
- If they have FV Leiden and they have had a clot they will be treated like anybody else. The decision to continue treatment is not affected by having FV Leiden.
You have an 8-year-old son and a 12-year-old daughter.
- Your son – There is no benefit testing. Knowing he has factor V Leiden will not alter his treatment in the future. He will have a higher risk of a blood clot than the normal population, but there is nothing you can do to prevent that.
- Your daughter – This is more controversial as your daughter may want to take the oral contraceptive in the future and may want children. You might believe that testing would be helpful here.
- Oral contraceptive use – Does testing change management? No, because the most important thing to consider when deciding if she could use an oral contraceptive is the family history. Current recommendations are that women with a first degree relative (you in this case) with a history of DVT or PE should avoid an oestrogen containing oral contraceptive. Based on your history alone, without any testing, your daughter should avoid the pill.
- Pregnancy – Women with Factor V Leiden and no previous history of thrombosis do not need any intervention. Your doctor might consider anticoagulant prophylaxis after delivery because of your family history.
In Practice
Like many things in medicine, practice differs from the theory. Many families have been screened for the Factor V Leiden mutation. I can totally understand why. If somebody in your family has an inherited condition it is nice to know if you have the same thing, in fact better to know that you don’t have it!
Practice differs between countries. Far more genetic testing is done in the US and Australia than in New Zealand. Cost is a factor, genetic testing is not cheap. Testing is discouraged if it is not going to change things.
Word of Caution
These are some of the arguments used for testing,
If a young woman tests negative for FV Leiden, she can use an oestrogen containing pill.
This would seem logical but needs to be considered carefully. Some experts believe that the oral contraceptive should be avoided in young women with a family history of FV Leiden with thrombosis, even if they test negative.
The reasoning goes something like this. Only a small proportion of people with factor V Leiden develop blood clots, which raises the question, is there some other factor in affected families that increases their risk of clots developing? Some families have multiple affected members with thrombosis where as other families with the mutation may have only one affected members. Perhaps some other factor is inherited in these families. Testing negative for factor V Leiden may not exclude this other factor.
Knowing I have FV Leiden helps me take precautions when flying long distance.
Everybody should take precautions flying long distance; wear flight socks, drink plenty of fluid and get up occasionally. If you have never had a blood clot, you do not need an anticoagulant when flying, even if you have FV Leiden. The role of low dose aspirin remains controversial.
Knowing I have FV Leiden helps me take precautions for surgery.
All patients should be assessed for their risk of thrombosis and risk of bleeding before having surgery especially major procedures. If you have a family history of thrombosis, your anaesthetist may take extra precautions. Some thrombosis risk scores do include thrombophilia (including FV Leiden) in the score, but there is no clear evidence that Factor V Leiden increases the risk of post operative thrombosis.
What do I do?
I like to discuss the options. I point out that testing is not going to change management and generally try to avoid unnecessary tests. However, if I have a patient who is very worried about the condition and feels more comfortable with more information, I will support testing. In this situation it is important to provide sufficient counselling to explain the significance of the results. I certainly try to avoid blanket testing of all family members when a new patient is identified with factor V Leiden.
I know some laboratory reports suggest family screening when a case of factor V Leiden is detected. I believe it would be more appropriate for the report to state that family screening should be discussed with your doctor.
If you have been screened as part of a family study and have a different view, please feel free to share.
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