While the prothrombin G20210A variant is found in approximately 2% of the U.S. population, prothrombin variants associated with antithrombin resistance are estimated to affect less than 1% of individuals.13, 14However, the true prevalence of antithrombin resistance may be underestimated due to limitations in standard thrombophilia testing protocols. This evidence concerns the gene F2 and Rare hereditary thrombophilia.