Plasma and serum are both components of blood, but they have obvious differences, as follows: First, the color is different, plasma is the remaining part of whole blood after separating out the blood cells and is light yellow, while serum refers to the colorless and transparent liquid that is sucked out on the surface of the blood clot after the plasma has coagulated, so the color is different. Second, the density is different, because the plasma contains various coagulation factors, so the plasma density is higher than the serum density. Third, the composition is different because plasma contains various coagulation factors, from clotting XIII to fibrinogen. In contrast, serum does not contain clotting factors because all clotting factors are involved in blood clotting. Also, because calcium ions are also involved in the clotting process, the concentration of calcium ions in serum is lower compared to plasma.