Summary

The factor V Leiden mutation (FVL) leads to a seven-fold increasedrisk of venous thromboembolism (VTE). In thrombophilic families,25% of carriers have experienced thrombosis before the age of40 years. Aim of our study was to assess the association of FVL withVTE in first-degree family members of unselected symptomatic andasymptomatic carriers of FVL.We tested 197 relatives of consecutive thrombosis patients withFVL and 36 relatives of asymptomatic carriers on the presence of FVLand the occurrence of VTE.The incidence of VTE in relatives with FVL of symptomatic carrierswas 0.34%/year. This was similar to the incidence in relatives withFVL of asymptomatic carriers. Kaplan Meier analysis in relatives ofsymptomatic propositi showed that at the age of 58 years, thrombosisfreesurvival was reduced to 75% in carriers and 93% in non-carriers(P < 0.05). Carriers of FVL had a three times higher thrombotic riskthan non-carriers. In combination with environmental risk factors, FVLclearly adds to the risk of VTE. The thrombotic incidence rate in theseunselected relatives with FVL, however, is considerably lower than wasseen in carriers of thrombophilic families (1.7%/year). Therefore,special care should be paid to individuals with a positive family historyof venous thrombosis while exposed to these risk factors.