A prospective study of asymptomatic carriers of the factor V Leiden mutation to determine the incidence of venous thromboembolism

Background: The factor V Leiden mutation is a common genetic defect associated with an increased risk for venous thromboembolism. The clinical implications for asymptomatic carriers of this mutation and, consequently, the usefulness of screening families in which a proband has both the mutation and venous thrombo embolism are unclear. Objective: To determine the incidence of venous thromboembolism in asymptomatic carriers of the factor V Leiden mutation. Design: Prospective cohort study. Setting: University hospitals in the Netherlands. Participants: 470 asymptomatic carriers of the factor V Leiden mutation (234 men, 236 women; mean age, 43 years [range, 15 to 88 years]), 12 of whom were homozygous. Carriers were identified by screening the first-degree relatives (>15 years of age) of 247 symptomatic probands. Measurements: Objectively diagnosed episodes of venous thromboembolism and the relationship between incidence and exposure to high-risk situations. Results: Nine venous thromboembolic events were observed in 1564 observation-years, resulting in an annual incidence of 0.58% (95% CI, 0.26% to 1.10%). The incidence of spontaneous venous thromboembolism was 0.26% (CI, 0.07% to 0.65%) per year; 3.5% (CI, 0.1% to 17.8%) per episode of surgery, trauma, or immobilization; 0.0% (CI, 0.0% to 19.5%) per pregnancy; 1.8% (CI, 0.4% to 5.2%) per year of oral contraceptive use; and 2.9% (CI, 0.8% to 15.3%) per year of use of hormone replacement therapy. Conclusions: The absolute annual incidence of spontaneous venous thromboembolism in asymptomatic carriers of the factor V Leiden mutation is low and does not justify routine screening of the families of symptomatic patients.