Comparison of thrombotic risk between 85 homozygotes and 481 heterozygotes carriers of the factor V Leiden mutation: retrospective analysis from the Procare Study

Factor V Leiden is the most common risk factor in thrombophilia. However, whether homozygosity confers a higher risk than heterozygosity remains questionable. The first phase of the Procare Study was to collect clinical and biological data from individuals carrying the R506Q mutation. To date, 566 individuals have been included in 23 French centres. Data from 85 homozygotes were compared with those of 481 heterozygotes. Mean age at first thrombosis was earlier for homozygous females versus heterozygous females (P = 0.02) or versus homozygous males (P < 0.005). Thrombosis occurred more frequently spontaneously in homozygotes than in heterozygotes [odds ratio, 4.1; 95% confidence interval (CI), 2.27–7.42]. Idiopathic events occurred earlier in homozygous females compared with males (P < 0.001). In our homozygous population, the 20210A allele of the prothrombin gene may constitute one associated risk condition. Recurrence after a first event was statistically more frequent in homozygotes than in heterozygotes (odds ratio, 1.95; 95% CI, 1.08–3.53). Therefore, homozygosity seems to confer a higher risk of thrombosis than heterozygosity, particularly in women.