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Blood plasma is the liquid component of blood, in which the blood cells are suspended. 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, blood proteins, and inorganic electrolytes. It serves as transport medium for glucose, lipids, hormones, metabolic end products, carbon dioxide and oxygen. (Oxygen transport capacity of plasma is much lower than that of the hemoglobin in the red blood cells; it may increase under hyperbaric conditions.) Plasma is the storage and transport medium of clotting factors and its protein content is necessary to maintain the oncotic pressure of the blood.
Laboratory use of plasma and serum
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 or use. This blood product derivative is known as fresh frozen plasma (FFP).
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.
Fresh frozen plasma
Fresh frozen plasma (FFP) is prepared from a single unit of blood. It is frozen after collection and can be stored for one year from date of collection. FFP contains all of the coagulation factors and proteins present in the original unit of blood. It is used to treat coagulopathies from warfarin overdose, liver disease, or dilutional coagulopathy. FFP that has been stored more than a standard length of time is re-classified as simply "frozen plasma", which is identical except that the coagulation factors are no longer considered completely viable.
Dried plasma
Dried plasma packages used by Britain and US military during WWII
Dried plasma was developed and first used during WWII. Prior to the United States involvement in the war, liquid plasma and whole blood were used. The "Blood for Britain" program during the early 1940s was quite successfull (and popular stateside). Nontheless the decision was made to develop a dried plasma package for the armed forces because it reduced breakage and made transport, packaging, and storage much simpler. [1]
The resulting Army-Navy dried plasma package came in two tin cans containing 400 cc bottles. One bottle contained enough distilled water to completely reconstitute the dried plasma contained in the other bottle. In about three minutes, the plasma would be ready to use and could stay fresh for around four hours. [2]
By the end of the war the American Red Cross had provide enough blood for over six million plasma packages. Most of the surplus plasma was returned stateside for civilian use. Serum albumin replaced dried plasma for combat use during the Korean War. [3]
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