Blood is made up of two fractions: blood cells, and the liquid in which they are suspended, blood plasma. Blood serum is blood plasma from which the clotting factors have been removed.
Plasma resembles whey in appearance (transparent with a faint straw colour). It is mainly composed of water, proteins, and mineral salts. It serves as transport medium for glucose, lipids, hormones, products of metabolism, carbon dioxide and oxygen. (It should be noted that the oxygen transport capacity of plasma is insignificant compared to that of the hemoglobin in the red blood cells; it may however become relevant under hyperbaric conditions.) It is the storage and transport medium of clotting factors and its protein content is necessary to maintain the oncotic pressure of the blood.
For purposes of laboratory tests, plasma is obtained from whole blood. To prevent clotting, an anticoagulant such as citrate is added to the blood specimen immediately after it is obtained. The specimen is then centrifuged to separate plasma from blood cells. Plasma can be frozen below -80oC for subsequent analysis.
For many biochemical laboratory tests, plasma and blood serum can be used interchangeably. Serum resembles plasma in composition but lacks the coagulation factors. It is obtained by letting a blood specimen clot prior to centrifugation. For this purpose, a serum separating tube (SST) can be used which contains glass beads to facilitate clotting.