This Medical Monday series is about risk factors for venous thromboembolism, VTE (pulmonary embolism and DVT). The post ‘Are you on the red or blue team?‘ outlines the difference between arterial and venous clots.
We are going to cover
Other risk factors; obesity, pregnancy, smoking and alcohol
What is a family history?
If you have had a DVT or PE I suspect that the doctor asked if you have a family history of blood clots. You may have wondered why. The main reason is that there is good evidence that having a family history of venous thrombosis increases your risk of a clot, however working out the risk and even getting a good family history can be hard. I ask my patients if they have a family history of blood clots, often they will say that they have no idea as they have never discussed this with their relatives. It is not uncommon for somebody to say there is no family history but at their next visit tell me they have been asking the family and have found out that a distant Aunt or a grandparent had a blood clot. The other difficulty is confirming the type of clot; many people will tell me their mother had a stroke and father had a heart attack, but these are not relevant as they are not venous thromboses. Or a patient may tell me their mother had swollen legs and varicose veins, she may have a history of a DVT but it is hard to know.
Venous thrombosis is not necessarily an easy condition to track through families as the genetic disorders have variable penetrance. For example some conditions, like severe haemophilia, have high penetrance, that means virtually every case will be identified; all boys with haemophilia will have a bleed at some time. On the other hand something like the factor V Leiden mutation, which is associated with blood clots, has a low penetrance; many people have this condition and never have a blood clot. This means your family might have a genetic disorder linked to blood clots, but no other family member has ever had a clot.
Does a family history increase my risk of a blood clot?
Most studies have shown that the risk of a thrombosis is increased if you have a first degree relative, either a parent or sibling (brother or sister), with a confirmed DVT or PE. A family history is a very strong predictor of a first event. A large population study from Sweden showed the risk of a first clot was more than twice the normal population if you had one affected sibling and more than 10 fold higher with more than 2 affected siblings. This suggests there is a strong genetic component to venous thrombosis, but surprisingly we are only able to identify a genetic disorder in small proportion of patients. This is an area of ongoing research.
Surprisingly a positive family history has only a modest impact on your risk of a second clot. The relative risk of a second clot is only about 1.2 times higher than for somebody without a family history. The reason for this is that the major factor that contributes to a second clot is the fact you have already had your first clot, family history has only a proportionately small impact.
If you have already had a blood clot, having a positive family history has little impact on your risk of a second event, but can be important for your family. It means your siblings and children all have an increased risk of a blood clot, and that the risk is even higher if more than one relative is affected. This can be particularly important if you have daughters; it can influence their decision to take an oral contraceptive and can increase their risk of a clot in pregnancy.
Although you may not want to tell everybody that you have had a blood clot, it is worth making sure your family know that they have a risk. It may mean they are diagnosed faster if they ever present with the symptoms of a DVT or PE.