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Whether an episode of venous thromboembolism (VTE) was unprovoked or provoked by an environmental (or acquired) risk factor and, if it was provoked, whether the provoking factor was transient or persistent, has important prognostic and treatment implications [1, 2]. If thrombosis was provoked by a major transient risk factor, such as recent surgery, there is a very low risk of recurrence after stopping therapy [1]. At the other extreme, if thrombosis was provoked by a persistent and progressive risk factor, such as metastatic cancer, there is a high risk of recurrence after stopping therapy [3, 4]. Patients with neither an important transient nor persistent provoking risk factor for thrombosis, who are often referred to as having ‘unprovoked’VTE, have an intermediate risk of recurrence after stopping therapy [1, 2].

Because of the implications for risk of recurrence and how long patients should be treated, it is often important to …