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. (Oxygen transport capacity of plasma is lower compared to that of the hemoglobin in the red blood cells; it may increase 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 or heparin is added to the blood specimen immediately after it is obtained. (Usually the anticoagulant is already in the evacuated blood collection tube (e.g. Vacutainer or Vacuette) when the patient is bled.) The specimen is then centrifuged to separate plasma from blood cells. Plasma can be frozen below -80°C nearly indefinitely 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 an inert catalyst (such as glass beads or powder) to facilitate clotting as well as a portion of gel with a density designed to sit between the liquid and cellular layers in the tube after centrifugation, making separation more convenient.
Tests of coagulation (such as the INR and APTT) require all clotting factors to be preserved. Serum, therefore, is inappropriate for these tests. A citrated evacuated blood collection tube (e.g. vacutainer or Vacuette) is usually used, as the anticoagulant effects of citrate is dependent upon concentration and can be reversed for testing.
Serum is preferred for many tests as the anticoagulants in plasma can sometimes interfere with the results. Different anticoagulants interfere with different tests; using serum means the same sample can be used for many tests. In protein electrophoresis, using plasma causes an additional band to be seen, which might be mistaken for a paraprotein.