Risk of recurrent venous thrombosis related to past provoking risk situations

Strategies targeting at classifying the risk for recurrent venous thrombosis are needed. We previously hypothesized, by studying a cohort of patients, that those who had ‘survived’ risk situations for venous thrombosis without developing it would, after a first venous thrombosis, have a low recurrence risk. Therefore, we re-evaluated the same cohort, now with a longer follow-up. Patients, after a first confirmed venous thrombosis event, were followed for an average of 43 months after suspension of anticoagulation. Patients with indication for indefinite anticoagulation were not included. The primary endpoint was objective recurrent venous thrombosis. Recurrent venous thrombosis was recorded in 9% of 378 eligible patients. Patients with a provoked first event and positive past risk situations for venous thrombosis had an incidence rate of recurrence of 1.26 (95% CI, 0.60–2.31) per 100 patient-years. The incidence rate ratio (IRR) of this subgroup compared with patients with a provoked event without other past risk situations for venous thrombosis was 0.8 (95% CI 0.2–2.9). This IRR was 2.8 (95% CI, 1.2–6.5) in patients with an unprovoked event and positive past risk situations and 7.1 (95% CI, 3.0–17.1) in patients with an unprovoked event and no past risk situations. When only idiopathic first events were evaluated the IRR was 2.5 (95% CI, 1.1–5.9) for patients without past risk situation compared with those with these history. In this study, asking a patient about past exposure to risk factors for venous thrombosis long before the occurrence of a first venous thrombosis occurred, could be used to classify patients with a first unprovoked venous thrombosis at higher risk for recurrence of venous thrombosis.