Blood plasma
Blood plasma is a component of blood. It is the liquid in which blood cells are suspended. Blood plasma contains proteins, nutrients, metabolic end productss, hormones, and inorganic electrolytes.Serum is the same as blood plasma except that clotting factors (such as fibrin) 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 or heparin is added to the blood specimen immediately after it is obtained. (Usually the anticoagulant is already in the vacutainer when the patient is bled.) The specimen is then centrifuged to separate plasma from blood cells. Plasma can be frozen below -80°C nearly indefinately 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.
Tests of coagulation (such as the INR and APTT) require all clotting factors to be preserved. Serum, therefore, is useless for these tests. A citrated vacutainer is usually used, as the anticoagulant effects of citrate can be reversed when 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.
Laboratory use of plasma and serum
| Cardiovascular system - Blood |
| Red blood cells - White blood cells - Platelets - Blood plasma |
| White blood cells |
| Granulocytes (Neutrophil granulocytes, Eosinophil granulocytes, Basophil granulocytes) - Lymphocytes - Monocytes |