When Are Platelet and Plasma Transfusion Indicated?

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Platelet concentrates can be prepared from whole blood using platelet-rich plasma (PRP) and buffy coat (BC) methods or by single-donor apheresis. When platelets are prepared from whole blood, approximately 450 mL of blood is drawn and immediately mixed with citrate anticoagulant. This volume of blood will be separated into a unit of red cells, platelets, and plasma. Platelets prepared using this technique are often referred to as whole blood “random donor” platelets (WB-RDP) because the human leukocyte antigen (HLA) type of the donor is unknown. Each WB-RDP contains on average 5.5 × 10 ¹⁰ platelets suspended in approximately 50 to 60 mL of the donor’s plasma. This volume of plasma is required to maintain platelet viability during storage. The two methods used to isolate the platelets from whole blood are described in Fig. 24.1 . Both the PRP and BC techniques use a two-step centrifugation process, but the sequence of the step for the BC method is reversed. The PRP method is the method of choice in the United States, whereas the BC method is often preferred in European countries. The PRP method yields approximately 5.0 to 7.5 × 10 ¹⁰ platelets, corresponding to 60% to 75% of the platelets found in the whole blood unit before separation. Four to six PRP units are combined to create a platelet “pool” that is more convenient to transfuse than single PRP units. Platelets pooled in blood banks must be transfused within 4 hours of preparation because of the risks for bacterial contamination during the pooling process. When platelets are manufactured by the BC method, whole blood is first centrifuged at high force to create a BC layer where platelets and leukocytes reside. During high-speed centrifugation, white cells initially sediment with red cells, whereas platelets remain in the supernatant plasma. Red cells are packed closely together and rapidly fall to the bag bottom. This process forces plasma and white cells upward to the plasma interface, and the platelets eventually accumulate on this interface. The settling of platelets on the red cell interface may explain the lesser degree of platelet activation when platelets are prepared by the BC method compared with PRP. The resultant BC, consisting of platelets and white cells, is removed along with small portions of the lower plasma layer and the upper red cell layer. The BC is then centrifuged at low g force to separate the platelets from leukocytes and red cells. Top and bottom bag systems facilitate separation by allowing the platelets in the BC to remain relatively undisturbed in the primary separation bag (with the plasma and red cells removed from the top and bottom ports, respectively). In practice, four to six BC bags are pooled, diluted in plasma, and centrifuged at low speed. The resulting platelet-rich supernatant is then transferred to a larger-volume storage bag. An important feature of the BC method is the ability to select the optimal platelet storage additive solution as the diluent, thus decreasing side effects from the infusion of large volumes of plasma and improving platelet metabolism during storage. Platelets collected by cell separators are generally referred to as “apheresis platelets” or “single-donor platelets (SDP) by apheresis.” Donor blood is obtained from a catheter in an arm vein and passed through an apheresis device in which platelets are separated from other cellular components by differential centrifugation. Most apheresis systems directly collect platelets as PRP. Approximately 4 or 5 L of donor blood is processed during the approximately 2-hour collection process. US standards require that an apheresis platelet product contain at least 3 × 10 ¹¹ platelets suspended in approximately 200 mL of the donor’s plasma. Consequently, one apheresis SDP approximates four to eight WB-RDPs. Plateletpheresis products are stored at 20°C to 24°C for up to 5 days, the same as platelets prepared by other methods. Apheresis-derived platelets, such as BC- and PRP-prepared platelet concentrates, contain few red cells, making cross-matching unnecessary, but there are enough red cells in an SDP unit for some programs, in an attempt to prevent active alloimmunization, to consider providing Rh immunoglobulin to RhD negative recipients who receive RhD positive SDP, especially women of child-bearing potential. Transfusing apheresis-derived platelets that are ABO incompatible typically does not produce measurable hemolysis in adults. Some centers titer group O platelets to avoid the uncommon situation in which the platelet donor carries an exceptionally high-titer anti-A that could cause hemolysis if infused to a group A recipient. This situation is more dangerous when group O apheresis-derived platelets are given to group A infants and small children. Platelet concentrates undergo alterations during collection, processing, and storage that adversely affect their structure and function. These changes, commonly referred to as the “platelet storage defect” or “platelet storage lesion,” are important because they are associated with decreased post-transfusion in vivo survival of the transfused platelets and efficacy.

Stored platelet concentrates have been studied using a wide variety of techniques, including platelet morphology by microscopy, pH, lactate dehydrogenase (LDH), platelet activation markers, osmotic recovery, platelet aggregation, and extent of shape change. Only limited data are available to understand the relationship between storage, change in porphology and function of the platelets, their clinical efficacy, and the risks associated with the transfusion (e.g., thrombosis and immune and nonimmune responses).

It has long been a principle of blood component production that they should contain as few white cells as possible, because leukocytes increase the risk for complications. Because removal of white cells by post-storage filtration does not remove biologically active substances released by white cells during storage, leukocyte reduction is now achieved by pre-storage filtration of platelet concentrates or by apheresis protocols, which use size and density differences between platelets and white cells to remove white cells during platelet collection. ^, Although the properties and effectiveness of BC, PRP, and apheresis platelets are similar, ^, different standards on the platelet concentrates are required in the United States and Europe ( Table 24.1 ).

Platelet concentrates are currently stored at 22°C (±2°C) and with constant agitation. Depending on the country, the storage duration varies between 3 and 7 days to limit the risk for bacterial proliferation and progressive metabolic changes. In recent years, alternatives to room temperature storage have been studied ( Table 24.2 ). Those options include lyophilized (freeze-drying), cryopreserved (80°C), and cold-stored platelets (4°C). The concept of cold-stored platelets is not new because this method was abandoned after the publication of a study by Murphy and Gardner in 1969, which demonstrated that the half-life of cold-stored platelets after transfusion was markedly reduced in blood circulation (1.3 vs. 3.9 days) and that the platelets underwent morphologic changes during cold storage. Those changes are known as storage lesions and include irreversible disc-to-sphere transformation, apoptosis, signs of activation with increased expression of both P-selectin and GPIba, and increased production of thromboxane A2. To date, there is no clinically available alternative to allogeneic platelet concentrates. In recent years, we have seen a resurgence of interest toward the use of cold-stored platelets for bleeding management. The hypothesis is now that because cold-storage lesions include preactivation of the platelets, cold-stored platelets might have a better hemostatic profile for treating active hemorrhage compared with room temperature platelets. ^, In 2017 the US Food and Drug Administration (FDA) stated that apheresis platelets stored at 4°C for up to 72 hours could be used for the treatment of active hemorrhage. Two recent studies looked at the characteristics of cold-stored platelets. In a study by Nair and colleagues published in 2017, the authors demonstrated that cold-stored platelets form a significantly stiffer and stronger blood clot with more crosslinks and with thinner, denser, and straighter fibers compared with room temperature–stored platelets. In another study published by Johnson and colleagues, the authors described the biochemical and morphologic changes platelets undergo during cold storage. Cold-stored platelets had a lower metabolic activity, using adenosine triphosphate (ATP) mainly for shape changes from disc to sphere. According to the maintained metabolic reserves and shape-change capacity, cold-stored platelets were at least noninferior compared with room temperature–stored platelets. Both studies agree that cold-stored platelets are a useful alternative to room temperature–stored platelets and could be even more effective for the management of massive hemorrhage, where speed and strength of clot formation is a priority. A recent pilot trial supports the feasibility of cold-stored platelets for up to 14 days and provides critical guidance for future pivotal trials in high-risk cardiothoracic bleeding patients. It is, however, important to keep in mind that the administration of highly hemostatic cold-stored platelets will be limited to acute bleeding clinical scenarios where hemostasis needs to be restored rapidly and after a proper assessment of the risk for thrombotic complications in large clinical trials. The prophylactic administration of those platelets in nonbleeding patients with severe thrombocytopenia is contraindicated because the administration of platelets with a more physiologic cycle of activation should be preferred.

Research activities focusing on alternatives to platelet transfusion have increased in the last decades. The objective would be to develop an effective “off the shelf” product, with optimal storage properties, that can mimic the biochemical platelet mechanisms even after prolonged storage but with no need for blood group typing and a reduced risk for side effects. Although some research laboratories are working on the development of platelet-like particles and artificial or synthetic platelets, ^, there is still a long way to go before those alternative to allogeneic platelets can be tested and used in vivo.