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Prolonged clot lysis time increases the risk of a first but not recurrent venous thrombosis

The role of the fibrinolytic system in the development of venous thrombosis (VT) is unclear. We studied the risk of first and recurrent VT associated with reduced fibrinolysis, as measured by clot lysis time (CLT). We also studied the relationship between CLT and thrombin generation to determine if any relationship between CLT and VT was affected by thrombin generation. Analyses were performed in the Thrombophilia Hypercoagulability Environmental risk for Venous Thromboembolism Study, a two‐centre population‐based case‐control study, including 579 patients and 338 controls, with patients followed from the event to determine incidence of recurrent VT. Hypofibrinolysis was associated with a 1·8‐fold increased risk of a first VT [95% confidence interval (CI) 1·2–2·7]. Adjustment for sex, age, study location and Endogenous Thrombin Potential (ETP) did not change the result. The risk of VT was 2·9‐fold increased when the 90th percentiles of prolonged CLT and high ETP were combined, with the highest risk for unprovoked first events (Odds Ratio = 4·2, 95% CI 1·3–13·5). In the follow‐up study the Hazard Ratio for a recurrent VT associated with hypofibrinolysis was 1·5 (95% CI 0·9–2·6). A weak dose response effect was observed in relation to prolongation of CLT and recurrent VT. Although hypofibrinolysis constitutes a risk factor for a first VT, an association with recurrence is, at best, weak.