The Factor V Leiden (FVL, rs6025) and prothrombin G20210A (PTGM, rs1799963) polymorphisms are two of the most well-studied genetic risk factors for venous thromboembolism (VTE). However, double heterozygosity (DH) for FVL and PTGM remains poorly understood, with prior studies in marked disagreement about the thrombosis risk conferred by the DH genotype. Utilizing multi-dimensional data from the UK Biobank (UKB) and the FinnGen biorepositories, we evaluated the clinical impact of DH …