Summary. Elevated plasminogen activator inhibitor 1 (PAI-1) levels are associated with venous thromboembolism, although their significance is unclear. PAI-1 levels are influenced by a PAI-1 promoter dimorphism (4G/5G), the 4G allele being associated with increased PAI-1 activity. We investigated whether the 4G allele influenced thrombotic risk by studying 99 symptomatic factor V (FV) Leiden heterozygotes and 99 healthy subjects. The 4G allele was more prevalent among cases than among healthy subjects (x 2 8´00, P 0´005) and the odds ratio (OR) for thrombosis associated with either heterozygosity or homozygosity for the 4G allele was 2´43 (P 0´011). We conclude that carriership of the 4G allele was more prevalent in patients who already carried factor V Leiden than in control subjects without factor V Leiden.
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