A filter medium includes two porous zones arranged in the form of a stack that include (i) a first porous zone, 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 (ii) a second porous zone, called “asymmetric”, presenting pore size that decreases transversely to its thickness, the portion of higher pore size being positioned towards the prefiltration zone.