The invention relates to a filter medium (31) comprising two porous zones arranged in the form of a stack: a first porous zone (100), called “prefiltration”, treated with at least one reagent presenting affinity for red blood cells and leading to red blood cells being captured or agglutinated on the prefiltration zone; and a second porous zone (200), called “asymmetric”, presenting pore size that decreases transversaly to its thickness, the portion of higher pore size being positioned towards the prefiltration zone (100). The invention also relates to systems, devices, and methods making use of the medium.