Hormonal factors and risk of recurrent venous thrombosis: the prevention of recurrent venous thromboembolism trial.

BACKGROUND In some but not all studies, men with venous thrombosis had a higher risk of recurrence than women. Information on women with initial hormone-related thrombosis is scant. OBJECTIVE We assessed the incidence of recurrent thrombosis by gender, and among women using exogenous hormones or pregnant/postpartum at the time of index thrombosis. PATIENTS/METHODS A total of 508 men and women with one or more previous venous thrombosis episodes were observed while participating in a randomized trial of low-intensity warfarin or placebo for 2.1 years. Index thrombosis events during treatment with postmenopausal hormones, oral contraceptives, or during pregnancy, or the puerperium were considered to be hormone-related events. RESULTS Among 268 men the 3-year probability of recurrent thrombosis was 18.4% (95% confidence intervals; CI 12.3-24.4). Among 109 women without hormone-related thrombosis, the rate was 15.0% (95% CI 6.3-23.8). Among 129 women with hormone-related thrombosis, the rate was 5.0% (95% CI 1.1-8.9). Adjusting for other risk factors and treatment assignment, women had a 39% lower thrombosis recurrence risk than men: hazard ratio (HR) 0.61 (95% CI 0.34-1.08). Women with hormone-related thrombosis had a 58% lower risk than men: HR 0.42 (95% CI 0.19-0.97); and a 46% lower recurrence risk than other women; HR 0.54 (95% CI 0.19-1.54). Women without hormone-related index events had a recurrence rate similar to men; HR 0.83 (95% CI 0.42-1.66). CONCLUSIONS In this trial population, women had a lower risk of recurrent venous thrombosis than men. This difference was explained by a low risk of recurrence among women with hormone-related index thrombosis.