The incidence of venous thromboembolism among Factor V Leiden carriers: A community-based cohort study

While Factor V (FV) Leiden is a risk factor for venous thromboembolism (VTE), the incidence of VTE among FV Leiden carriers is uncertain. The objective of the study was to estimate the overall age-specific and pregnancy-related VTE incidence and the relative risk among FV Leiden carriers. In a community-based sample of 3424 south-eastern Minnesota residents, 230 (6.7%) were genotyped as FV Leiden carriers; 220 carriers (mean age = 68 years) could be matched to a non-carrier on age, gender, ethnicity and length of medical history. We performed a retrospective cohort study of carriers and non-carriers by reviewing the complete medical records in the community for demographic and baseline characteristics, pregnancies and live births, and first lifetime VTE. Over 14 722 person-years, 24 (10.9%) carriers developed VTE [overall incidence = 163 (95% CI 104, 242) per 100 000 person-years]. VTE incidence rates for ages 15-29, 30-44, 45-59 and ≥60 years were 0, 61, 244 and 764 per 100 000 person-years, respectively (cumulative VTE incidence at age 65 years = 6.3%). VTE incidence for carriers did not differ significantly from that for non-carriers except for those ≥60 years old (relative risk = 3.6; 95% CI 2.0, 6.0). There were 311 live births among 130 women carriers; no VTE events occurred during pregnancy or postpartum [incidence = 0(95% CI 0, 1186) per 100 000 women-years]. Most FV Leiden carriers do not develop VTE. Among all carriers, those ≥ 60 years old are at the highest risk for VTE. The incidence of VTE among asymptomatic women carriers during pregnancy is low and insufficient to warrant prophylaxis.