There are several indications for leukocyte removal. The most frequent reason is development of an immune reaction to antigens on “foreign” transfused leukocytes that constitutes the great majority of febrile nonhemolytic transfusion reactions and by far the most common overall transfusion reaction, especially in multitransfused patients. Second, removal of leukocytes helps prevent microaggregates (miniclots) of WBC, fibrin, platelets, and RBC debris that form in stored blood. These microaggregates have been implicated as one cause for adult respiratory distress syndrome. Another indication is to prevent immunization and subsequent reaction to class II (D-locus) HLA antigens present on lymphocytes. This may also cause nonhemolytic febrile reactions, but assumes greatest importance in patients who may need tissue transplants. Another indication is to help prevent or delay sensitization to platelets, which carry HLA class I (A, B, C loci) antigens. Leukocyte removal also helps prevent transmission of cytomegalovirus that infects lymphocytes. This is most important in neonates and in immunocompromised patients. However, the most common use of leukocyte removal is in multiply transfused patients with febrile reactions.

Leukocyte removal is most easily accomplished in older blood units (in which some degree of spontaneous leukocyte microaggregation takes place). The original and still used method is centrifugation of the blood unit and removal of the leukocyte-rich layer (“buffy coat”). This removes about 70%-80% of the leukocytes. If the remaining blood is passed through a 20-40 micron depth-type microaggregate filter, this increases WBC removal to 90%-94%. Special granulocyte filters are now commercially available that can remove as much as 99.9% of the leucocytes; these filters can be used at the patient’s bedside. About 25% of the RBCs are lost during special leukocyte filtration. Age of the blood does not matter. When the object is to prevent immunization, there are differences in performance between commercially available filters.

Other methods of leukocyte removal are the washing of red cells or as a side effect of preparing frozen RBCs. These are discussed separately since these methods accomplish other purposes besides only leukocyte removal.

Washed red blood cells. Washed RBCs are packed cells that have received several washes with saline, followed by centrifugation. This removes more than 90% of the WBCs with most of the platelets and plasma proteins. About 10%-20% of the RBCs are lost. Indications for washed cells are relatively few. Cell washing removes donor antibodies and is useful in IgA immune reactions. Washed RBCs are traditionally used in patients with paroxysmal nocturnal hemoglobinuria. Washed cell methods remove 70%-93% of total WBCs and are reported to cause fewer leukoagglutinin reactions than ordinary centrifuged leukocyte-poor RBCs. However, current leukocyte filters remove more leukocytes than cell washing. The saline wash process reduces but does not completely eliminate the risk of viral hepatitis.

Washed RBCs must be discarded if not transfused within 24 hours after preparation (washing). No more units should be ordered than definitely will be used.

Frozen red blood cells. Fresh citrate-anticoagulated RBCs may have their storage life greatly prolonged by freezing. Glycerol is added to packed RBCs to protect them during freezing; this substance prevents intracellular fluid from becoming ice. The blood is slowly frozen and is maintained at below-zero temperatures until ready for use. Thereafter it must be thawed, after which the glycerol is removed (to avoid osmotic hemolysis), and the cells are suspended in saline. This technique will maintain packed RBCs for up to 5 years. Advantages to the blood bank include the ability to maintain a much larger inventory of blood types without fear of outdating and better control over temporary fluctuations in donor supply or recipient demand. It also permits stockpiling of rare RBC antigen types. Advantages to the patient include approximately 95% elimination of leukocytes, platelets, and plasma proteins, thus removing sources of immunization and febrile reactions; removal of most potassium, ammonium, and citrate, three substances that might be undesirable in large quantities; and considerable reduction of risk for viral hepatitis. Unfortunately, risk of viral hepatitis or HIV-I (HTLV-III) infection is not completely eliminated. Transfused frozen RBCs contain less plasma than washed RBCs.

Disadvantages include considerably greater cost; significant time lost in thawing and preparing the RBCs (1 hour or more); equipment limitation on the number of units that can be prepared simultaneously, which might cause difficulty in emergencies; the fact that once thawed, cells must be used within 24 hours (by current AABB rules; some data suggest this period could be extended to 7 days using a special plastic bag closed system to remove the glycerine); and the presence of variable amounts of free Hb, which might be of sufficient quantity to be troublesome if tests are needed for possible hemolytic transfusion reaction.

Frozen RBCs are becoming more widely available as larger blood banks acquire the necessary equipment. In addition to use in persons with rare blood types, current practice favors their use in some circumstances with greater risk of certain types of immunohematologic sensitization. These include circumstances in which reactions to plasma proteins may occur (e.g., IgA-deficient persons or persons already sensitized to IgA or other serum proteins). Some advocate frozen RBCs as therapy for paroxysmal nocturnal hemoglobinuria. The new leukocyte filters are more effective in removing WBCs than are frozen RBCs. RBCs containing sickle Hb are difficult to deglycerolize and require special techniques.

Irradiated red blood cells. Donor lymphocytes can induce the graft-vs.-host reaction (Chapter 11) in recipients who are severely immunocompromised or are blood relatives of the donor. Gamma radiation (at doses of 25 Gy, equivalent to 2500 rads) affects lymphocytes but not RBCs, granulocytes, or platelets. Such radiation substantially reduces the risk of graft-vs.-host reaction. Radiation of this magnitude will not inactivate viral hepatitis or HIV-I. If blood product irradiation is necessary, all blood products that contain any lymphocytes must be irradiated; these include whole blood, the various RBC preparations (including frozen RBCs), platelet concentrates, and granulocyte concentrates.