High levels of factor IX increase the risk of venous thrombosis

Elevated plasma levels of factor VIII (> 150 IU/dL) are an important risk f actor for deep vein thrombosis (DVT). Factor VIII is the cofactor of factor IXa in the activation of factor X, The risk of thrombosis in individuals w ith an elevated factor IX level is unknown. This study investigated the rol e of elevated factor IX levels in the development of DVT. We compared 426 p atients with a first objectively diagnosed episode of DVT with 473 populati on controls. This study was part of a large population-based case-control s tudy on risk factors for venous thrombosis, the Leiden Thrombophilia Study (LETS), Using the 90th percentile measured in control subjects (P-90 = 129 U/dL) as a cutoff point for factor IX levels, we found a 2- to 3-fold incre ased risk for individuals who have factor IX levels above 129 U/dL compared with individuals having factor IX levels below this cutoff point. This ris k was not affected by adjustment for possible confounders (age, sex, oral c ontraceptive use, and high levels of factor VIII, XI, and vitamin K-depende nt proteins). After exclusion of individuals with known genetic disorders, we still found an odds ratio (OR) of 2.5 (95% confidence interval [CI]: 1.6 -3.9). The risk was higher in women (OR: 2.6, CI: 1.6-4.3) than in men (OR: 1.9, CI: 1.0-3.6) and appeared highest in the group of premenopausal women not using oral contraceptives (OR: 12.4, CI: 3.3-47.2). These results show that an elevated level of factor IX is a common risk factor for DVT. (C) 2 000 by The American Society of Hematology.