In transfusion medicine, packed red blood cells are red blood cells that have been separated from whole blood for transfusion purposes. The product is typically abbreviated RBC or PRBC, and sometimes LRBC for leukoreduced products. The name "Red Blood Cells" with initial capitals indicates a standardized product in the United States.[1]
RBCs are used to restore oxygen carrying capacity to the blood of a patient that is suffering from anemia due to trauma or other medical problems, and are one of the most important blood components used in transfusion medicine. Historically they were transfused as part of whole blood, but in modern practice the RBCs and plasma components are transfused separately. The process of identifying a compatible blood product for transfusion is complicated and giving incompatible RBCs to a patient can be fatal.[2]
The RBCs are mixed with an anticoagulant and usually a storage solution which provides nutrients and preserves the functionality of the living cells, which are stored at refrigerated temperatures. The cells are separated from the fluid portion of the blood either after it is collected from a donor or during the collection process by apheresis. The product is sometimes modified after collection to meet specific patient requirements.
Contents |
Red blood cells are the component of blood that carry oxygen from the lungs to the rest of the body. If the body does not have enough red blood cells, due to blood loss, anemia, or other medical conditions, then RBCs are transfused to improve the oxygen carrying capacity. RBCs are also used in exchange transfusions where the patient's abnormal cells are replaced with healthy donor cells.[3] One unit of PRBCs typically will raise the hematocrit by 3-4% and the blood hemoglobin concentration by 1 g/dl.[4]
The exact criteria for when a patient requires a transfusion are often set by a standards organization such as a hospital transfusion committee. Typical triggers for transfusion include low patient hemoglobin test values and active severe bleeding. Because of the possible complications with RBCs and sometimes because of scarcity of donor blood components, other treatments are preferred for anemia when the patient's condition is not critical. Iron supplements and vitamins or erythropoietin can be used to help the patient's body produce more red blood cells.
Donor RBCs must be compatible with the recipient to avoid potentially life-threatening reactions. Incompatible cells will be destroyed by the patient's immune system and the free hemoglobin released from the damaged cells is toxic, particularly to the kidneys. This is called a hemolytic transfusion reaction, which is often immediately apparent during the transfusion but may also take several days to develop.[5] Other complications of transfusion, such as allergic reactions or volume overload, can also be dangerous. Transfusion related acute lung injury (TRALI) is not usually associated with RBCs, though cases have been reported.
To avoid transfusion reactions, the donor and recipient blood are tested, typically ordered as a "type and screen" for the recipient. The "type" in this case is the ABO and Rh type, specifically the phenotype, and the "screen" refers to testing for atypical antibodies that might cause transfusion problems. The typing and screening are also performed on donor blood. The blood groups represent antigens on the surface of the red blood cells which might react with antibodies in the recipient.
The ABO blood group system has four basic phenotypes: O, A, B, and AB. In the former Soviet Union these were called I, II, III, and IV, respectively. There are two important antigens in the system: A and B. Red cells without A or B are called type O, and red cells with both are called AB. Except in unusual cases like infants or seriously immunocompromised individuals, all people will have antibodies to any ABO blood type that isn't present on their own red blood cells, and will have an immediate hemolytic reaction to a unit that is not compatible with their ABO type. In addition to the A and B antigens, there are rare variations which can further complicate transfusions, such as the Bombay phenotype.
The Rh blood group system consists of nearly around 50 different antigens, but the one of the greatest clinical interest is the "D" antigen, though it has other names and is commonly just called "negative" or "positive." Unlike the ABO antigens, a recipient will not usually react to the first incompatible transfusion because the adaptive immune system does not immediately recognize it. After an incompatible transfusion the recipient may develop an antibody to the antigen and will react to any further incompatible transfusions. This antibody is important because it is the most frequent cause of hemolytic disease of the newborn. Incompatible red blood cells are sometimes given to recipients who will never become pregnant, such as males or postmenopausal women, as long as they do not have an antibody, since the greatest risk of Rh incompatible blood is to current or future pregnancies.[6]
For RBCs, type O negative blood is considered a "universal donor" as recipients with types A, B, or AB can almost always receive O negative blood safely. Type AB positive is considered a "universal recipient" because they can receive the other ABO/Rh types safely. These are not truly universal, as other red cell antigens can further complicate transfusions.
There are many other human blood group systems and most of them are only rarely associated with transfusion problems. A screening test is used to identify if the recipient has any antibodies to any of these other blood group systems. If the screening test is positive, a complex set of tests must follow to identify which antibody the recipient has by process of elimination. Finding suitable blood for transfusion when a recipient has multiple antibodies or antibodies to extremely common antigens can be very difficult and time consuming.
Because this testing can take time, doctors will sometimes order a unit of blood transfused before it can be completed if the recipient is in critical condition. Typically two to four units of O negative blood are used in these situations, since they are unlikely to cause a reaction.[7] A potentially fatal reaction is possible if the recipient has pre-existing antibodies, and uncrossmatched blood is only used in dire circumstances. Since O negative blood is not common, other blood types may be used if the situation is desperate.
Most frequently, whole blood is collected from a blood donation and is spun in a centrifuge. The red blood cells are denser and settle to the bottom, and the majority of the liquid blood plasma remains on the top. The plasma is separated and the red blood cells are kept with a minimal amount of fluid. Generally an additive solution of citrate, dextrose, and adenine is mixed with the cells to keep them alive during storage. This process is sometimes done as automated apheresis where the centrifuging and mixing take place at the donation site.[8]
Red Blood Cells are sometimes modified to address specific patient needs. The most common modification is leukoreduction, where the donor blood is filtered to remove white cells. The blood may also be irradiated, which destroys the DNA in the white cells and prevents graft versus host disease, which may happen if the blood donor and recipient are closely related. Other modifications, such as washing the RBCs to remove any remaining plasma, are much less common.
With additive solutions, RBCs are typically kept at refrigerated temperatures for up to 42 days.[9] In some patients, use of RBCs that are much fresher is important. With the addition of glycerol or other cryoprotectants, RBCs can be frozen for much longer. Frozen RBCs are typically assigned a ten year expiration date, though older units have been transfused successfully. The freezing process is expensive and time-consuming and is generally reserved for rare units such as ones that can be used in patients that have unusual antibodies.