The purpose of anticoagulation is to prevent recurrence of VTE and its consequences. However, the treatment carries a risk of bleeding. Therefore, anticoagulation should normally continue only if the risk of recurrent VTE, or its consequences, outweighs the risk of bleeding (or other adverse effects) due to anticoagulant therapy .
A transient risk factor increases the risk of thrombosis briefly; the risk is reversible and, after the event, thrombotic risk abates. An example is surgery; this transient risk factor is associated with a very low VTE recurrence risk after adequate treatment (<1% at 1 year and 3% at 5 years after surgery). Surgery-induced VTE requires only 3 months of anticoagulation .
For a first VTE following a non-surgical event such as pregnancy, major trauma, or significant immobilisation after medical illness, the risk of recurrent VTE is estimated to be higher (5% after 1 year and 15% after 5 years), but 3 months of anticoagulation is regarded as adequate .
If a person suffers a first VTE without any identifiable risk factor, the event is classified as unprovoked. The importance of identifying unprovoked VTE lies in the recurrence rates—unprovoked VTE has a significant recurrence risk of at least 10% after 1 year and at least 30% at 5 years . Consequently, most patients require extended and perhaps indefinite anticoagulation.
Persons with malignancy have a higher incidence of recurrent VTE and bleeding complications while receiving anticoagulation therapy than those with unprovoked VTE . Recurrence is three times higher in patients with VTE and cancer than those with VTE without cancer . Vitamin K antagonists (VKAs) may not be the optimal treatment for these complex patients, who often undergo procedures and who have periodic chemotherapy-induced thrombocytopenia. NICE guidelines consider cancer patients with VTE as a special risk group and note that there is evidence that alternative anticoagulants other than VKAs may be a better choice for long-term anticoagulation of cancer patients .