Sex difference in risk of recurrent venous thrombosis and the risk profile for a second event

Background: The risk of recurrent venous thrombosis is higher in men than in women, and this is so far unexplained. We set out to determine the influence of age, time between first and second event, type of first event, oral contraception, pregnancy and surgery. Methods: We performed a prospective follow-up study of 474 patients with a first objective diagnosis of deep vein thrombosis, aged 18-70 years (Leiden Thrombophilia Study cohort). Results: During 3477 person-years of follow-up, 90 recurrences occurred. The overall incidence rates of recurrence (IRs) were 40.9 per 1000 person-years in men and 15.8 per 1000 person years in women. Men with an unprovoked first event had the highest risk of recurrence, with almost one-third experiencing a second unprovoked event within 8 years (IR 41.2 per 1000 person-years). This risk was three-fold lower in women [IR 14.2 per 1000 person-years; hazard ratio 2.8 (95% confidence interval 1.4-5.7)]. Age at diagnosis had little effect on recurrence rate, and nor had time elapsed since the first event. In women, almost half of the recurrences were provoked and were mainly related to oral contraceptive use or pregnancy. Conclusions: The higher recurrence rate in men than in women is not the result of differences in the environmental or transient risk factors that we studied. The risk profile for a second thrombotic event is clearly different from that of a first.