Plasma

All biochemical tests are performed using some kind of body fluid, typically serum or plasma. Plasma is in essence the same as serum, but is obtained by centrifuging the blood without clotting. Plasma is obtained by centrifugation before clotting occurs, and the act of centrifugation packs the denser blood cells and platelets to the bottom of the centrifuge tube, leaving the liquid plasma resting above, making extraction of the plasma simpler.