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  • Title: Factor V Leiden: should we screen oral contraceptive users and pregnant women?
    Author: Vandenbroucke JP, van der Meer FJ, Helmerhorst FM, Rosendaal FR.
    Journal: BMJ; 1996 Nov 02; 313(7065):1127-30. PubMed ID: 8916702.
    Abstract:
    The factor V Leiden mutation is the most common genetic risk factor for deep vein thrombosis: it is present in about 5% of the white population. The risk of deep vein thrombosis among women who use oral contraceptives is greatly increased by the presence of the mutation. The same seems to be true of the risk of postpartum thrombosis. Several authors have called for all women to be screened before prescription of oral contraceptives and during pregnancy. Such a policy might deny effective contraception to a substantial number of women while preventing only a small number of deaths due to pulmonary emboli. Moreover, in pregnancy the ensuing use of oral anticoagulation prophylaxis might carry a penalty of fatal bleeding that is equal to or exceeds the risk of death due to postpartum thrombosis. It might pay, however, to take a personal and family history of deep vein thrombosis when prescribing oral contraceptives or at a first antenatal visit to detect women from families with a tendency to multiple thrombosis. The authors' recent reports of an increased risk of venous thrombosis in oral contraceptive (OC) users with the factor V mutation raised questions about the potential benefits of screening for this coagulation defect before OCs are prescribed or during pregnancy. The factor V Leiden mutation, the most common genetic risk factor for deep vein thrombosis, is prevalent in about 5% of Whites. According to the authors' research, the incidence of deep vein thrombosis of the legs increases from 0.8/10,000 women-years among women who are neither OC users nor carriers of the mutation, to 3.0/10,000 women-years among OC users who are not carriers, to 5.7/10,000 women-years among nonusers who are carriers, to 28.5/10,000 women-years among women who are both OC users and carriers of the mutation. The case fatality of venous thrombosis among women under 40 years of age is about 2%. Thus, a pulmonary emboli mortality rate of 5.7/100,000 women/year can be expected among OC users with factor V. Were screening to be initiated, about 20,000 women positive for factor V would be denied OCs in order to prevent 1 death. Routine screening for this mutation during pregnancy, since the pulmonary emboli risk is greatest among postpartum women, could result in more fatal bleeding attributable to anticoagulation prophylaxis than prevented fatal emboli. Taking a personal and family history of deep vein thrombosis when prescribing OCs or at the first prenatal visit will detect women with a tendency toward multiple venous thrombosis and is recommended as an alternative to mass screening for factor V.
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