Background: Resistance to activated protein C due to the factor V R506Q (Leiden) mutation is the most common clotting abnormality in patients with venous thromboembolism.
Objective: To evaluate the risk for venous thromboembolism associated with the factor V Leiden mutation or with resistance to activated protein C in the general population.
Design: Cross-sectional survey.
Setting: General community of Vicenza, Italy.
Patients: A population-based sample of 15 109 white persons 18 to 65 years of age who were randomly selected from the census list.
Measurements: Sequential validated approach based on participants' reports and Doppler ultrasonography. Resistance to activated protein C was investigated in all participants; 2134 participants with resistance to activated protein C were screened for the factor V Leiden mutation.
Results: Carriers of the factor V Leiden mutation had a relative risk of 3.3 (95% CI, 1.7 to 6.1) for venous thromboembolism before 65 years of age. The fraction of cases attributable to the factor V Leiden mutation was 6.6%. By 65 years of age, 5.7% of carriers of the mutation had had venous thromboembolism, mostly after surgery. Participants with a reduced response to activated protein C were at higher risk even if they did not carry the mutation (odds ratio, 1.7 [CI, 1.0 to 2.7]); the attributable risk for venous thromboembolism was 5.1%.
Conclusions: The factor V Leiden mutation and resistance to activated protein C are important, independent risk factors for venous thromboembolism. Screening strategies for the factor V Leiden mutation in patients undergoing surgery or experiencing major trauma cannot be recommended, but phenotypic evaluation of resistance to activated protein C should be encouraged in patients with venous thromboembolism.