Different risk of deep vein thrombosis and pulmonary embolism in carriers with factor V Leiden compared with non-carriers, but not in other thrombophilic defects. Results from a large retrospective family cohort study.

The term factor V Leiden (FVL) paradox is used to describe the different risk of deep vein thrombosis and pulmonary embolism that has been found in carriers of FVL. In a thrombophilic family-cohort, we estimated differences in absolute risks of deep vein thrombosis and pulmonary embolism for various thrombophilic defects. Of 2,054 relatives, 1,131 were female, 41 had pulmonary embolism and 126 deep vein thrombosis. Annual incidence for deep vein thrombosis in non-carriers of FVL was 0.19% (95%CI, 0.16-0.23), and 0.41% (95%CI, 0.28-0.58) in carriers; relative risk (RR) 2.1 (95%CI, 1.4-3.2). For pulmonary embolism these incidences were similar in carriers and non-carriers 0.07%, respectively; RR 1.0 (95% CI, 0.4-2.5). When co-inheritance of other thrombophilic defects was excluded the RR for deep vein thrombosis in FVL carriers was 7.0 (95%CI, 2.3-21.7) compared to non-carriers and 2.8 (95%CI, 0.5-14.4) for pulmonary embolism. For other thrombophilic defects no such effect was observed. Thus the FVL paradox was confirmed in our study. However, a similar paradox in carriers of other thrombophilic defects was not observed.