The risk of venous thrombosis in individuals with a history of superficial vein thrombosis and acquired venous thrombotic risk factors.

Superficial vein thrombosis (SVT) increases the risk of venous thrombosis fourfold to sixfold. As most individuals with SVT do not develop venous thrombosis, additional risk factors may explain the risk of developing a venous thrombosis. In the Multiple Environmental and Genetic Assessment of risk factors for venous thrombosis study, we assessed the risk of venous thrombosis in individuals with previous SVT and a mild thrombotic risk factor (smoking or overweight/obesity), a strong risk factor (surgery, hospitalization, plaster cast immobilization, or malignancy), or a reproductive factor in women (oral contraception, postmenopausal hormone therapy, or pregnancy/puerperium). Individuals with previous SVT alone had a 5.5-fold (95% confidence interval [CI], 4.4-6.8) increased risk of venous thrombosis. This was 9.3 (95% CI, 7.2-12.1) combined with a mild thrombotic risk factor, 31.4 (95% CI, 14.6-67.5) with a strong risk factor, and 34.9 (95% CI, 19.1-63.8) in women with a reproductive risk factor. The highest separate risk estimates were found for SVT with surgery (42.5; 95% CI, 10.2-177.6), hospitalization (49.8; 95% CI, 11.9-209.2), or oral contraception (43.0; 95% CI, 15.5-119.3 in women). In conclusion, the risk of venous thrombosis is markedly increased in individuals with previous SVT who have an acquired thrombotic risk factor.