A lateral flow device for analyzing a whole blood sample is provided. More specifically, the lateral flow device contains a porous membrane that defines a barrier zone for separating red blood cells from blood plasma (includes plasma in which clotting factors haven been removed), which may then flow to a subsequent detection zone for analysis. The barrier zone is formed from a blood cell barrier composition that includes an unsaturated aliphatic fatty acid or an ester thereof. Without intending to be limited by theory, the present inventors believe such unsaturated aliphatic fatty acid molecules undergo autoxidation in the presence of air and hemoglobin to release peroxides (e.g., hydrogen peroxide) via oxidative saturation of double bonds. In turn, the released peroxides are believed to induce the formation of echinocytes or crenated blood cells. The crenated red blood cells are distorted in shape and less flexible and malleable than normal red blood cells, making them less able to penetrate into the pores of the porous membrane of the lateral flow device. Consequently, the stiffer, less flexible cells cannot move easily into the porous and become trapped at the surface of the membrane, while the liquid components of the sample flow and penetrate through the membrane to the detection zone for analysis.