Is it advisable to screen patients for the most common genetic thrombophilias before starting hormonal contraception?
Shannon Bates: The question of screening for thrombophilias is one that has been raised a number of times. I think there are important considerations. First of all, if we talk about screening for the most common thrombophilias – factor V Leiden and the prothrombin gene mutation – they are what we call relatively common, that is, they are present in between 1% and 5% of the population. If we are talking about general population screening, that is still a lot of screening to pick up one case. What we know is that even in women who have a family history of venous thromboembolism and who are heterozygotes for the factor V Leiden mutation the annual risk of venous thrombosis while on oral contraceptives is still quite low – it is 0.5% per year. So, what we do not want to do is screen general populations for thrombophilia, because the yield is low.
You can screen according to family history if you know that there is a family history of thrombophilia or venous thromboembolism. But before you screen, you need to advise the patient of the potential drawbacks of having genetic screening, so in some cases there can be insurance implications. You also need to discuss with the patient whether or not results of that genetic testing are going to make a difference to her choice. If, for example, the annual risk of thrombosis if you are heterozygotes for the factor V Leiden mutation and have a positive family history of venous thrombosis is 0.5% per year, and it is 0.2% per year regardless of whether you have the mutation but have a positive family history, the question with those lower-risk thrombophilias becomes, how much benefit are you actually getting?
Where you want to make sure you screen is those families with a history of thrombosis who have the high-risk thrombophilias – antithrombin deficiency, protein C deficiency, protein S deficiency – because the limited data that we have suggest that if those women take combined oral contraceptives, they have an annual risk of venous thrombosis of about 4% to 5% in the first year. And those are women whom you would for sure not want taking combined oral contraceptives. From my perspective, at a risk of 0.5% per year that is still a discussion that you might have with the patient and they still might choose to use contraceptives. So, before you do the screening, have a discussion with the patient, because we need to recognize that genetic screening does have some drawbacks both to the individual patient in terms of insurability and the society in terms of costs.