Serum is the yellowish, clear liquid that precipitates on the surface of the blood clot after the plasma has clotted. Compared to plasma, serum has fewer of the various clotting factors involved in the clotting process. The most common method to obtain patient’s serum is by centrifugation, because human blood, including various blood cells as well as plasma, also contains various solute components. The density of the aforementioned substances is not the same, and when centrifugation is performed, they appear to be significantly stratified due to the different densities of the various substances. When centrifugation is performed at 2500-3000 revolutions per minute, the serum is obtained at the top of the tube, in the middle are white blood cells and platelets, etc., and at the bottom are red blood cells, so the patient’s serum can be obtained by centrifugation.