of the platelets (85%) and leukocytes [70, 74]. Because the WB centrifugation step involves higher g‐forces, the red cells are more tightly packed and more plasma is obtained along with the buffy coat. However, to obtain most of the buffy coat, it is necessary to remove some of the red cells, and so there is a loss of about 20–25 mL of red cells [70]. To obtain a platelet concentrate, the buffy coat is centrifuged using low g‐forces, and the platelets are separated from the leukocytes and red cells. Thus, in this method, the first step is a hard spin and the second step a soft spin—the opposite of the PRP method. It is thought that the use of the soft spin in the second centrifugation may result in platelets that function better than those obtained by the PRP method, in which the second centrifugation is a hard spin when there is less WB to “cushion” the platelets [74, 75]. The effectiveness of the second centrifugation step is improved if several units of buffy coat are pooled, usually in groups of six [74]. When units of buffy coat are pooled for the second centrifugation, they may be suspended in an artificial platelet preservation (platelet additive) solution that improves the separation and the quality of platelets during storage [76]. Also during the second centrifugation step, the platelets are passed through a filter as they are separated, thus removing most of the leukocytes and producing a leukocyte‐depleted platelet component.
Storage conditions and duration
Platelets prepared by either method can be stored in either plasma or PAS. Platelets stored at 20–24°C maintain functional effectiveness for several days [13, 77–80]. Many variables affect the quality of platelets during storage. In addition to temperature, these other variables include the anticoagulant–preservative solution, storage container, type of agitation, anticoagulant, and volume of plasma [72, 81–83]. Gentle horizontal agitation is preferable to end‐over‐end agitation [83]. If continuous agitation is interrupted, platelets stored for up to 5 days maintain appropriate in vitro characteristics for up to 24 hours of interruption of agitation [84]. The composition, surface area, and size of the storage container influence the ability for carbon dioxide to diffuse out and oxygen to enter the platelet concentrate, and storage containers specifically designed to optimize platelet quality are now used routinely [85, 86].
Maintenance of the pH greater than 6.0 is the crucial factor indicating satisfactory platelet preservation. This combination of storage container, agitation, preservative solution, temperature, and the use of about 50 mL of plasma provides satisfactory preservation of platelets for up to 7 days [85, 87]. However, several instances of bacterial contamination of platelet concentrates stored for this period were reported [87, 88], and the storage time was reduced to the 5 days currently used [24]. The problem of bacterial contamination still exists (see Chapter 16), although recently with additional point of care testing for bacterial detection, the FDA approved storage of apheresis platelets up to 7 days [89] (see Chapter 7). The storage container needs to be FDA approved for apheresis platelet storage up to 7 days. In addition, large‐volume delayed sampling and secondary cultures can be used to extend platelet storage to 7 days [89].
Several platelet concentrates are usually pooled to provide an adequate dose for most patients (see Chapter 10). For some patients, the volume of plasma in the final pooled component is too large, and plasma must be removed prior to transfusion. This involves another centrifugation step after the platelets have been pooled that causes a loss of 15% to as much as 55% of the platelets [90, 91].
Leukodepletion of platelets
The leukocyte content of the platelet concentrates is an important issue (see Chapters 10, 11, and 16). The conditions used to centrifuge WB influence the leukocyte content of the platelet concentrate, but most platelet concentrates contain 108 or more leukocytes. Filters are available that remove most of the leukocytes in the platelet concentrate. The filters can be used at the bedside, or preferably before the platelets are stored. All platelets should be leukodepleted.
Filters are available for leukodepletion of platelets, as well as red cells. This is necessary if it is hoped to prevent alloimmunization or cytomegalovirus transmission in patients receiving platelet transfusion [53]. The platelet filters result in a loss of about 20–25% of the platelets and have a rate of failure in achieving fewer than 5 × 106 leukocytes of about 5–7% [53].
5.5 Granulocytes
Granulocytes for transfusion are prepared by cytapheresis (see Chapter 6). Some investigators have prepared granulocytes from fresh WB by sedimentation with hydroxyethyl starch to obtain doses of 0.25 × 109. This is below the 1–3 × 109 desired for transfusion even to a neonate [92]. The possibility of obtaining granulocytes from units of WB is usually raised in a crisis; the blood bank often does not have procedures to prepare the cells, and it is not possible logistically to test the blood for transmissible disease. Thus, preparation of granulocytes from units of fresh WB is not recommended.
5.6 Irradiation of blood components
The techniques and clinical indications for irradiating blood components are described in Chapter 10.
5.7 Hematopoietic stem and progenitor cells
Hematopoietic stem cells are being obtained from bone marrow, peripheral blood, and cord blood. Collection of marrow and umbilical cord blood is described in Chapter 19 and peripheral blood stem cells in Chapter 6. Stem cells from these different sources are undergoing an increasing variety of cellular engineering methods that produce new blood components with exciting therapeutic potential.
5.8 Plasma derivatives
General
Procedures for the fractionation of plasma were developed during the 1940s in response to World War II (see Chapter 1). A large pool of plasma, often up to 10,000 L or 50,000 donor units, is processed using cold ethanol fractionation. In cold ethanol, different plasma proteins have different solubilities, which allow their separation. This large‐scale separation and manufacturing process results in the isolation of several proteins from plasma that are prepared for therapeutic use. These are called “plasma derivatives” (Table 5.10). The major derivatives have been albumin, immune serum, immune globulin, and coagulation factor VIII concentrate. Until the late 1980s, techniques were not available to sterilize some blood derivatives after manufacture. Thus, because of the large number of units of donor plasma in each pool, the chance of contamination of the pool with viruses (i.e., hepatitis and HIV) was high and the risk for disease transmission from these nonsterilized blood derivatives was high (see Chapter 17). This risk was accentuated because much of the plasma that serves as the raw material for the manufacture of blood derivatives was obtained from paid donors, a group known to provide blood with an increased likelihood of transmitting disease [93, 94]. Initially, only albumin and immune globulin carried no risk for disease transmission—albumin because it was sterilized by heating, and immune globulin because none of the known infectious agents was contained
