<P> 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 . </P> <P> 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 . </P> <P> With additive solutions, RBCs are typically kept at refrigerated temperatures for up to 45 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). </P> <P> 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 . </P>

The packed cell volume of red blood cells per unit of blood is known as