The prothrombin 20210A allele is the most prevalent genetic risk factor for venous thromboembolism in the Spanish population.

We investigated the prevalence of the new recently reported mutation in the prothrombin gene (20210 A) in a sample of 116 unrelated patients with venous thromboembolism. We found 20 heterozygous carriers (17.2%, CI 95% 10.4-21.1). In comparison, we observed 13 carriers among 201 healthy unmatched controls (6.5%, CI 3.5-10.8). The 20210 A mutation seems to increase the risk of venous thrombosis 3-fold (odds ratio 3.1, 95% CI 1.4-6.6). Considering only patients with a first event (n = 62) the OR was 2.0 (p = 0.18, NS) while those with recurrent events (n = 54) showed an OR of 5.9 (95% CI 2.5-14.4). A majority of heterozygous patients (55%) presented a second thrombophilic factor and 60% of affected females had their first event before 30 years of age, while on oral contraceptive treatment. The prevalence found in this study for healthy people is the highest reported to date. The 20210 A variant appears to be the most prevalent genetic risk factor among patients with thrombosis in our geographical area.