The most powerful risk factor for DVT is having suffered a DVT previously. Up to 25% of cases of DVT happen in people with a history of DVT. This happens for several reasons. Studies have shown that remnants of a previous thrombus are often seen within the specimens of new thrombi. These pieces of old thrombus may act as the starting point for a new DVT. Also, if a person has thrombophilia, this increased tendency to develop emboli persists and this person remains at risk of DVT; when other risk factors occur, such as acute illness, a DVT is triggered.
Figure: once someone has suffered a DVT, remnants of the clot may be left behind in the vein; this increases the risk of a future DVT.