Packed red blood cells

Bag of packed red blood cells.

In transfusion medicine, packed red blood cells (sometimes called stored packed red blood cells or simply packed cells) are red blood cells (RBC, also called erythrocytes) that have been collected, processed, and stored in bags as blood product units available for blood transfusion. The collection may be from a "whole blood" (WB) donation followed by component separation, or by RBC apheresis (sometimes called "double-red," due to the potential to donate two units' worth at once that way). The processing (often termed "manufacture," since the end result is deemed a biologic biopharmaceutical product) and the storage can occur at a collection center and/or a blood bank. RBCs are mixed with an anticoagulant and storage solution which provides nutrients and aims to preserve viability and functionality of the cells (limiting their so-called "storage lesion"), which are stored at refrigerated temperatures for up to 42 days (in the US), except for the rather unusual long-term storage in which case they can be frozen for up to 10 years. The cells are separated from the fluid portion of the blood after it is collected from a donor, or during the collection process in the case of apheresis. The product is then sometimes modified after collection to meet specific patient requirements.

The product is typically abbreviated RBC, pRBC, PRBC, and sometimes StRBC or even LRBC (the latter being to indicate those that have been leukoreduced, which is now true for the vast majority of RBC units). The name "Red Blood Cells" with initial capitals indicates a standardized blood product in the United States. Without capitalization, it is simply generic without specifying whether or not the cells comprise a blood product, patient blood, an etc. (with other generic terms for it being "erythrocyte" and "red cell").

RBCs are used to restore oxygen-carrying capacity to the blood of a patient that is suffering from an anemia due to trauma or other (perhaps chronic) medical problems, and are by far the most common blood component 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.

Compatibility testing

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.

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. 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.

Collection, processing, and use

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.

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, although this is becoming increasingly universal throughout the blood supply (over 80% in the US, 100% in Europe). 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, and is also important for immunocompromized patients. 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. In some patients, use of RBCs that are much fresher is important; for example, US guidelines call for blood less than seven days old to be used for neonatals, to "ensure optimal cell function". However, the phenomenon of RBC storage lesion and its implications for transfusion efficacy are complex and remain controversial (see blood bank and blood transfusion articles).

With the addition of glycerol or other cryoprotectants, RBCs can be frozen and thus stored for much longer (this is not common). 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. Since frozen RBCs have glycerol added, the added glycerol must be removed by washing the red blood cells using special equipment, such as the IBM 2991 cell processor in a similar manner to washing RBCs.

See also

• Blood bank
• Blood donation