We describe the clotting process in stented aneurysms with a lattice Boltzmann (LB) model. In addition to model the haemodynamic properties of blood flow, our model includes the transport of idealized red blood cells and platelets as well as an aggregation mechanism at low shear rate. This model simulates the capabilities of a stent to initiate and maintain clotting inside an aneurysm. From the clotting fraction observed in simulation, we define a new stent performance metric and compare it with the so-called velocity reduction and shear rate reduction metrics that are based on a pure haemodynamic description.