1 19 19 Blood Transfusion Practice When blood component therapy is administered for a sound medical indication, therapeutic benefit should result. Because adverse outcomes may follow hemotherapy, even when that therapy is indicated, transfusion should be undertaken only if the anticipated benefit outweighs the potential risks. Red Blood Cell Transfusion Physiologic Principles The primary indication for transfusion of red blood cells (RBCs) is to restore or maintain oxygen-carrying capacity to meet tissue demands. Since demand for oxygen varies greatly among different individuals in different clinical circumstances, a single laboratory measurement (the hematocrit or the hemoglobin) cannot accurately assess the need for transfusion. 1,2 Normal Oxygen Supply and Demand Tissues at rest have a constant demand for oxygen. See Table The oxygen content of blood (ml O 2 /ml blood) is determined by the hemoglobin concentration, the binding coefficient of oxygen for normal hemoglobin, the oxygen saturation of hemoglobin (%), and the quantity of oxygen dissolved in the plasma. This is described as: 02 content = (Hb 1.39 %sat) + (p ) Tissue oxygen consumption is calculated as the difference between oxygen delivery in the arterial blood and oxygen return by the venous blood: 0 2 consumption = Cardiac output Hb 1.39 (%satarterial %satvenous) which is expressed as (ml O2/min) = L/min g/l ml O2/g (% %) The oxygen saturation of arterial and venous hemoglobin varies with the partial pressure of oxygen dissolved in the 413
2 414 AABB Technical Manual Table Oxygen Demand by Body Organs Blood Flow (ml/min/100 g) Cardiac Output (%) Oxygen Consumption (ml/min/100 g) Brain Heart Kidneys Liver and GI tract Skeletal muscle Skin Remainder plasma. Under normal circumstances, the po 2 falls from 100 mm Hg in the arteries to 40 mm Hg in the veins as the tissues extract oxygen, and hemoglobin saturation falls from near 100% in the arteries to approximately 75% in the veins. Under normal circumstances, the oxygen extraction ratio is 0.25, ie, the hemoglobin gives up only 25% of its oxygen. When tissue demand for oxygen increases or supply of oxygen decreases, the tissues extract more oxygen from the plasma and from hemoglobin; this results in a lower venous po 2 and decreased oxygen saturation of the venous blood. Studies in primates suggest that a critical point of limited oxygen delivery is reached when oxygen extraction ratio approaches twice normal or Under normal resting conditions the body has a tremendous reserve of oxygen supply relative to demand. In the average adult, approximately 1000 ml/min is available to the tissues and only 250 ml/min is consumed. 02 supply = Cardiac output 02 contentarterial =5L/min [( %) + ( )] =5L/min 200 ml 02/L =1000mL0 2 /min 02 consumption = Cardiac output (02 contentarterial 02contentvenous) =5L (200 ml 02/L 150 ml 02/L) =5L 50 ml 0 2 /L = 250 ml 0 2 /min Measuring Adequate Oxygen Supply Because measuring the hemoglobin concentration or hematocrit does not assess the adequacy of oxygen delivery to the tissues, a value for hemoglobin cannot by itself serve as a clinical guide for blood transfusion. Clinical assessment of adequate oxygenation is based on the patient s cardiac performance, hemoglobin concentration, and current oxygen demand. For patients in an intensive care unit or in the operating room, direct measurement of the cardiac output and the systemic oxygen extraction via a pulmonary artery catheter can serve as a useful guide to the overall adequacy of oxygen supply. This approach provides a more physiologic indication for transfusion than measurement of the hematocrit. The mixed venous oxygen saturation and the extraction ratio provide data on total body oxygen consumption that must, like the hematocrit, be interpreted in light of the clinical situation.
3 Chapter 19: Blood Transfusion Practice 415 Treating Inadequate Oxygen Supply Tissue oxygen debt results when oxygen demand exceeds supply; tissues convert to anaerobic metabolism and produce increased quantities of lactic acid. Metabolic acidosis, in turn, impairs cardiac performance, further decreasing perfusion, and tissue oxygen delivery, resulting in greater tissue hypoxia. Oxygen supply reflects blood flow, gas exchange in the lungs, hemoglobin concentration, oxygen-hemoglobin affinity, and tissue demands for oxygen; all but the oxygen-hemoglobin affinity are subject to substantial variation. Red cell transfusion is an excellent means of raising the hemoglobin concentration; in the absence of bleeding or hemolysis, one unit of RBCs raises the average adult s hemoglobin concentration by 1 g/dl. Ways to improve oxygen supply relative to demand, independently of transfusion, include increasing tissue perfusion (maximizing cardiac performance), increasing the hemoglobin saturation (supplemental oxygen), and decreasing tissue oxygen demands (bed rest). The relationship among oxygen delivery, cardiac output, and hemoglobin concentration is shown in Fig For example, a patient with a hemoglobin of 6 g/dl and a cardiac index of 5 L/min/m 2 has abnormal oxygen delivery. Assuming that the oxygen extraction remains constant, oxygen delivery could be normalized by either a 3 g/dl increase in the hemoglobin concentration to 9 g/dl, or a combination of a smaller increase in hemoglobin coupled with an increase in cardiac index. Increased cardiac output is an important compensation for anemia. In addition, anemia results in increased oxygen extraction, which further increases oxygen delivery to tissues. Whole Blood Whole blood provides both oxygen-carrying capacity and blood volume expansion. It may be used for actively bleeding patients who have lost more than 25% of their blood volume acutely, or for patients undergoing exchange transfusions. 1 For actively bleeding patients, the goals of initial treatment should be to stop bleeding and to restore intravascular volume to prevent the development of hypovolemic shock. Efforts to achieve hemostasis and to restore volume by the infusion of crystalloid or colloid should be started immediately. In some clinical settings, a practitioner may request fresh whole blood. It is important to determine the practitioner s definition of fresh and the reason for the request. Whole Blood less than 24 hours old is rarely available in the United States because of the time needed for required testing. Attempts to obtain donor testing on a stat basis or to transfuse blood before all necessary tests have been completed carry risk of a transfusion complication that may outweigh any anticipated benefit. There should be a documented discussion with the requesting practitioner of the safety issues surrounding compliance with such a blood order. Red Blood Cells Red cell components are indicated for the treatment of anemia in normovolemic patients who require an increase in oxygen-carrying capacity and red cell mass. 1 The transfusion of red cells increases oxygen-carrying capacity with less expansion of blood volume than transfusion of whole blood. Patients who have chronic anemia or congestive heart failure, or who are elderly or debilitated, may not tolerate the increased volume load provided by whole blood. For many adult patients with operative blood loss of only ml, transfusion may be avoided by the administration of an adequate volume of crystalloid and/or colloid solutions; due to increased oxy-
4 416 AABB Technical Manual CI = 2L / m 2 CI = 3L / m 2 CI = 4L / m 2 CI = 5L / m 2 CI = 6L / m 2 CI = 7L / m 2 CI = 8L / m 2 HEMOGLOBIN (g / dl) OXYGEN DELIVERY (ml O 2 / min / m ) Figure Oxygen delivery, cardiac output, and hemoglobin concentration are interrelated. The shaded box represents normal values. An individual with a cardiac index (CI) of 5 L/m 2 and a hemoglobin level of 6 g/dl has inadequate oxygen delivery at 400 ml O 2 /min/m 2.Arise in either cardiac index or hemoglobin results in increased oxygen delivery to 600 ml/o 2/min/m 2.
5 Chapter 19: Blood Transfusion Practice 417 gen extraction, most tissues that are adequately perfused will not become ischemic even with a hemoglobin concentration as low as 7 g/dl. 1,4 The factors determining the oxygen extraction ratio should be considered before transfusions are given to stable patients with a low hematocrit. Patients at bed rest who are not febrile, who do not have congestive heart failure, and who are not hypermetabolic have low oxygen requirements and may tolerate anemia remarkably well. A National Institutes of Health consensus development conference on perioperative blood transfusion emphasized that preoperative transfusions should not be given simply to raise the hemoglobin concentration above 10 g/dl. 5 Transfusion does not improve wound healing, which depends on po 2 rather than total oxygen content of blood. 4 Patients with chronic anemia tolerate lower hematocrit values better than those with acute anemia because of cardiovascular compensation and increased oxygen extraction. However, the high oxygen needs of cardiac muscle precipitate angina in patients with anemia. In the absence of cardiac dysfunction or critical coronary artery disease, a hemoglobin concentration of 8 g/dl adequately meets the oxygen needs of most patients. While it is desirable to prevent unnecessary transfusions, anemic patients who are symptomatic should receive appropriate treatment. Anemia may cause symptoms of headache, dizziness, disorientation, breathlessness, pallor (not cyanosis), tachycardia, palpitations, or chest pain. Platelet Transfusion Physiologic Principles Hemostasis occurs in four major phases: the vascular phase, the formation of a platelet plug, the development of fibrin clot on the platelet plug, and the ultimate lysis of the clot. Platelets are essential to the formation of the primary hemostatic plug and provide the hemostatic surface upon which fibrin formation occurs. Deficiencies in platelet number and/or function can have unpredictable effects that range from clinically insignificant prolongation of the bleeding time to major life-threatening defects in hemostasis. Platelet plug formation results from the combined processes of adhesion, activation and release, aggregation, and procoagulant activity. Platelet adhesion to damaged endothelium is mediated largely by von Willebrand factor, which binds to a glycoprotein (Gp) receptor on the platelet surface termed Gp Ib. The process of activation and release causes a dramatic change in platelet shape, with extension of long cytoplasmic pseudopod-like structures; a change in the binding properties of membrane activation proteins; the secretion of internal granule contents; and the activation of several metabolic pathways. These changes have many effects, including the recruitment of additional platelets, which aggregate one to another as fibrinogen binds to a platelet surface structure termed Gp IIb/IIIa. Platelet members have potent procoagulant activity, which serves to localize the formation of fibrin clot. Assessing Platelet Function Decreased platelet numbers result from many conditions that decrease platelet production or increase destruction. Platelet function may be adversely affected by such factors as drugs, liver or kidney disease, sepsis, increased fibrin(ogen) degradation, cardiopulmonary bypass, and primary bone marrow disorders. Platelet hemostasis is best assessed by the medical history and physical examination. Patients with inade-
6 418 AABB Technical Manual quate platelet number or function may demonstrate petechiae, easy bruising, or mucous membrane bleeding. Laboratory investigation may include the platelet count and bleeding time, which measures both the vascular phase and the platelet phase of hemostasis. While the bleeding time may be a useful diagnostic test in the evaluation of patients with known or suspected abnormalities of platelet function, it is a poor predictor of bleeding 6 and is not a reliable indicator of the need for platelet transfusion therapy. 7 The measurement of platelet aggregation is useful in investigating abnormal platelet function but, like the bleeding time, is a poor predictor of clinical bleeding. Platelet Life Span and Kinetics Platelets normally circulate with a life span of 9.5 days. 8 Conditions that shorten platelet life span include splenomegaly, sepsis, drugs, disseminated intravascular coagulation (DIC), auto- and alloantibodies, endothelial cell activation, and platelet activation. Because the number (7-10,000/µL/day) of platelets consumed by routine plugging of minor endothelial defects is constant, the proportion of the total number of platelets required for maintenance functions goes up as the total number of platelets declines, and platelet life span decreases with progressive thrombocytopenia. 8 Platelets that have been properly collected and stored have a near normal life span when reinfused into the original donor. Platelets transfused to patients, however, frequently have reduced survival. The response to platelet transfusion is best assessed by observing whether bleeding stops, and by measuring posttransfusion platelet increment. The posttransfusion increment is generally measured between 10 minutes and 1 hour after the completion of the transfusion and is expressed as a corrected count increment (CCI). The CCI corrects for the number of platelets infused and the blood volume of the recipient: CCI at 1 hour = (Platelet count post Platelet count pre ) BSA (m 2 ) Number of units transfused or CCI at 1 hour = (Platelet countpost Platelet countpre) BSA (m 2 ) Number of platelets transfused (multiples of ) where BSA is the body surface area in square meters. A CCI above /µL (first equation) or above ,000/µL (second equation) suggests an adequate response to allogeneic platelet transfusion. Two consecutive poor CCIs suggest platelet refractoriness. Platelet Components Platelets A single unit of Platelets (PLTs) prepared from an individual unit of Whole Blood may be adequate for transfusion to neonates or infants, but for adults, 6-10 units are ordinarily pooled for transfusion at an approximate dose of 1 unit/10 kg patient body weight. Pooled PLTs are probably hemostatically equal to those prepared by apheresis, but the recipient is exposed to a larger number of allogeneic donors. Platelets, Pheresis Units of platelets prepared by apheresis technology have a platelet content similar to that of 6-8 units of pooled PLTs and, depending on the equipment used, may have a reduced leukocyte content. Platelets, Pheresis are usually transfused
7 Chapter 19: Blood Transfusion Practice 419 without matching for HLA or platelet antigens, and expose the recipient to only one donor per transfusion. For alloimmunized recipients and those refractory to platelet transfusions, the transfusion of a single unit that is HLAmatched or crossmatched may improve the clinical response. Transfusion Circumstances Therapeutic Platelet Transfusion The proper clinical indications for platelet transfusion are the subject of controversy. The decision to transfuse platelets depends on the cause of bleeding, the patient s clinical condition, and the number and function of the circulating platelets. Bleeding due to thrombocytopenia or abnormal platelet function is an indication for platelet transfusion. Platelet transfusions are most likely to be of benefit when thrombocytopenia is the primary hemostatic defect; other blood components may also be required in patients with multiple defects. Bleeding due to the defects in platelet function that follow cardiopulmonary bypass or ingestion of aspirin-containing compounds often responds to platelet transfusion, but other acquired defects (eg, that found in uremia) respond less well because the transfused platelets tend to acquire the same defect. Prophylactic Platelet Transfusion The traditional threshold of 20,000/µL or less for patients with chemotherapyinduced thrombocytopenia has been questioned. 9,10 Platelet transfusion therapy should be tailored to the individual patient; rigid thresholds for transfusion mistakenly assume that all patients carry the same risk of bleeding. Despite the widespread use of prophylactic platelet transfusions, few studies have actually documented their clinical benefit. One study comparing patients given prophylactic transfusions with patients transfused only for clinically significant bleeding found no difference in overall survival or deaths due to bleeding between the groups, 11 even though the prophylactic group received twice as many platelet transfusions. Controlled studies and longitudinal observations have not yielded definitive answers about the benefits of prophylactic transfusions and the risks of inducing alloimmunization Patients with cerebral leukostasis are at high risk for fatal intracranial hemorrhage. In contrast, many stable thrombocytopenic patients can tolerate platelet counts of less than 5000/µL with evidence of minor hemorrhage (eg, petechiae, ecchymoses, or epistaxis) but without serious bleeding. 15,16 Bleeding at any platelet count may be aggravated by fever, infection, or drugs. 8,17 Selection of Platelets ABO Matching Because ABO antigens are present on the platelet surface, recovery of group A platelets transfused into group O patients is somewhat decreased. 18 Transfusion of ABO-incompatible plasma present in platelet components may also result in a blunted posttransfusion platelet count increment. 19,20 It may be prudent to use ABO-matched platelets whenever feasible, but it is not usually necessary to delay needed transfusion in order to obtain ABO-compatible platelets. For infants, it is desirable to avoid administration of plasma that is incompatible with the infant s red cells. If platelet concentrates of an appropriate ABO type are not available, the plasma that contains ABO antibodies incompatible with the recipient s red cells can be removed (see Method 9.13). This is rarely necessary in adults or older chil-
8 420 AABB Technical Manual dren. If transfused ABO antibodies are detected in the recipient s circulation, it may become necessary to use group O red cells for transfusion. Matching for D The D antigen is not detectable on platelets and posttransfusion survival of platelets from D-positive donors is normal in recipients with anti-d. However, even with proper preparation, platelet concentrates may contain up to 0.5 ml of red cells and Platelets, Pheresis may contain up to 5 ml; D-negative individuals may become alloimmunized by the residual D-positive red cells in a platelet component. Concomitant immunosuppression may diminish the risk of alloimmunization when platelets are given to patients who are thrombocytopenic because of cytotoxic therapy. The benefit of Rh Immune Globulin (RhIG) in this setting should be weighed against the risk of hematoma formation as a result of the injection. Some transfusionists have used the subcutaneous route to inject a 50 µg dose of RhIG (immune prophylactic for 2.5 ml of D-positive cells), but this indication is not stated on the package insert. An intravenous injectable form of RhIG has been approved by the Food and Drug Administration; this is suitable for patients who are at high risk of hematoma formation after intramuscular injection. For immunologically normal D- negative females of childbearing potential, it is especially desirable to avoid administration of platelets from D-positive donors; however, if this is unavoidable, administration of RhIG should be considered. A full dose of RhIG, immunoprophylactic for up to 15 ml of D-positive red cells, would protect against the red cells in 30 units of D-positive platelet concentrates or 3 units of Platelets, Pheresis. Refractoriness to Platelet Transfusion Platelet refractoriness, defined as a poor increment following a dose of platelets, can result from either immune or nonimmune mechanisms. The antibodies that cause immune refractoriness (discussed in Chapter 25) may have either allo- or autoreactivity. The alloantibodies are directed against either platelet alloantigens or Class I HLA antigens. Autoantibodies occur in immune thrombocytopenic purpura (ITP) and in some patients after bone marrow transplantation. Nonimmune causes of the refractory state include splenomegaly, drugs (eg, amphotericin B), and accelerated platelet consumption. Identifying the cause(s) for an individual patient can be quite difficult, but provision of effective hemostatic support often depends on identifying the dominant cause of refractoriness. See Table 19-2 and the section on HLA-matched platelets. For refractory patients with broadly reactive alloimmunization, the use of leukocytereduced platelets may avert a febrile response but may not improve posttransfusion increments. HLA Matching HLA-matched Platelets, Pheresis may give good results in thrombocytopenic bleeding if HLA alloimmunization is causing refractoriness. Platelets manifest Class I HLA antigens, the expression of which results, at least in part, from adsorption of plasma antigens onto the platelet surface. A grading system for HLA-based donor selection is shown in Table Note that only with grade A and BU matches is the recipient spared exposure to foreign antigens. Grade BX donors have antigens that are said to be cross-reactive with those of the recipient. A useful source of HLA-identical or HLA-compatible donors may be the pa-
9 Chapter 19: Blood Transfusion Practice 421 Table Etiology and Management of Platelet Refractoriness Cause Management Immune HLA alloantibodies HLA-matched platelets or crossmatch-compatible platelets Platelet alloantibodies Platelet-antigen matched or crossmatch-compatible platelets Autoantibodies IVIG, corticosteroids, splenectomy Drug (eg, heparin) Stop offending drug Nonimmune Splenomegaly Treat cause Drug (eg, amphotericin) Stop offending drug if possible Consumption Treat cause Sepsis Treat cause tient s siblings. Siblings who are potential bone marrow donors, however, are generally not selected because of concern that the recipient may become immunized to other transplantation antigens. Children of the patient are characteristically twoantigen HLA mismatches. Platelets collected from blood relatives of the patient must be irradiated prior to transfusion 21 ; it may also be prudent to irradiate platelets collected from HLA-matched grade A or B donors. 22 Patients who appear to be broadly alloimmunized to multiple private antigens may, in fact, be immunized to a limited number of public antigens. The selection of donors based on matching for public antigens may be a more effective strategy than an attempt to coordinate cross-reactive groups. A scheme for donor selection that includes both public and private antigens has been proposed. 23 See Table Studies of posttransfusion increments indicate that success is most likely with donors who are grade A or BU matches and thus are matched for public antigens, and are also ABO-compatible with the recipient. Mismatching of antigens at the HLA-C locus is usually not important. 13 Table Degree of Matching for HLA-Matched Platelets Match Grade Description A 4-antigen match A1,3;B8,27 B No mismatched antigens present B1U 1 antigen unknown or blank A1,-;B8,27 B1X 1 cross-reactive group A1,3;B8,7 B2UX 1 antigen blank and 1 cross-reactive A1,-;B8,7 C 1 mismatched antigen present A1,3;B8,35 D 2 or more mismatched antigens present A1,32;B8,35 R Random A2,28;B7,35 Examples of Donor Phenotypes for a Recipient Who Is A1,3;B8,27
10 422 AABB Technical Manual Table Strategy for Selection of Platelet Donors Matched for Public and Private HLA Antigens Public Epitope Associated Private Epitopes Approximate Frequency 1C A1, 36, 10, 11, 19 79% 2C A2, 28, 9 66% 28C A28, 33, 34, 26 20% 5C B5, 15, 18, 35, 53, 70 50% 7C B7, 22, 27, 42, 40, 13, 47, 48 51% 8C B8, 14, 18, 39, 51 42% 12C B12, 21, 13, 40, 41 44% Bw4/4C (See Table 15-1) 79% Bw6/6C (See Table 15-1) 87% Example: Recipient phenotype: A1,3;B8,27 Public phenotype (see table): 1C;7C;8C;4C;6C Potential public mismatches: 2C;5C;12C;28C Recipient antibody screen: anti-2c (anti-a2,28,9) If no grade A or BU match is available, select donors lacking potential public mismatches. Failing this, select any donor lacking the 2C public epitope. Otherwise, select donors based on platelet crossmatch. (Adapted with permission from Rodey. 23 ) Other Problems Platelet transfusion may fail to produce a satisfactory increment in circulating platelets for many reasons. Alloimmunization to antigens unique to the platelet membrane may contribute in some cases and attempts have been made, with varying success, to characterize antibodies and/or to circumvent their effects by crossmatching techniques. There is also intense interest in, but no consensus about, the importance of leukocyte reduction in the prevention or amelioration of alloimmunization, of febrile reactions, and of adverse immunomodulatory effects of transfusion. Contraindications to Platelet Transfusion There are several conditions for which platelet transfusions may be requested but are contraindicated. Relative contraindications include conditions in which the likelihood of benefit is remote, thus serving only to waste a valuable component. An example would be prophylactic platelet transfusions in a stable patient with platelet refractoriness of known cause. Platelet transfusion should be avoided, except in life- or organ-threatening hemorrhage, for patients with thrombotic thrombocytopenic purpura (TTP) or heparin-induced thrombocytopenia. These
11 Chapter 19: Blood Transfusion Practice 423 conditions are associated with platelet thrombi and major thrombotic complications may follow platelet transfusions. 24 The use of platelet transfusions in immune thrombocytopenia is controversial. For a patient with ITP, prophylactic transfusion of platelets prior to splenectomy is usually unnecessary. 25 Granulocyte Transfusion The use of granulocyte transfusions for adult recipients has decreased. New antibiotics, adverse effects attributable to granulocyte transfusions, and the advent of recombinant growth factors have all contributed to this decline. Nevertheless, in selected patients transfused granulocytes may produce clinical benefits. 26,27 Even if the prevention of cytomegalovirus (CMV) transmission is an issue, granulocytes should not be administered through a leukocyte-reduction filter. The preparation, storage, pretransfusion testing, and quality control of granulocytes are discussed in Chapter 7. The use of granulocyte transfusions in neonates is discussed in Chapter 22. Indications and Contraindications The goals of granulocyte transfusion should be clearly defined before a course of therapy is initiated. In general, the patient should meet the following conditions: 1. Neutropenia (less than 500/µL). 2. Fever for hours, unresponsive to appropriate antibiotic therapy, or bacterial sepsis unresponsive to antibiotics or other modes of therapy. 3. Myeloid hypoplasia. 4. A reasonable chance for recovery of bone marrow function. Patients with documented granulocyte dysfunction, such as those with profound reversible neutropenia or those with chronic granulomatous disease, may also be candidates to receive granulocyte transfusions. Prophylactic granulocyte transfusion is inappropriate. 1 Other Considerations Granulocyte transfusions have not been proven effective in patients with localized infections or for the treatment of infections due to agents other than bacteria. It may be prudent to irradiate granulocytes to avoid the risk of graftvs-host disease (GVHD). CMV transmission can best be prevented by the use of a CMV-seronegative donor. Special Cellular Blood Components Leukocyte Reduction The use of leukocyte-reduced cellular blood components has been advocated as a means of lowering the risk of nonhemolytic febrile reactions, HLA alloimmunization, and disease transmission. The approximate leukocyte content of common blood components is summarized in Table Prevention of Febrile Nonhemolytic Transfusion Reactions Multitransfused and/or multiparous patients may become alloimmunized to HLA or leukocyte antigens and may experience febrile nonhemolytic reactions when transfused with blood components containing white blood cells. The use of blood components with a residual leukocyte content below often, but not always, prevents such reactions. Patients with antiplatelet antibodies may develop fever because of platelet incompatibility, and accumulated cytokines in the supernatant plasma of platelet concentrates may trigger febrile reactions in the absence of alloimmunization.
12 424 AABB Technical Manual Table Approximate Leukocyte Content of Blood Components (per Unit) 7 Whole Blood 10 9 RBCs 10 8 Washed RBCs 10 7 RBCs, Deglycerolized RBCs, leukocyte-reduced by filtration * <10 Platelets, apheresis Platelets 10 7 Platelets, Pheresis, leukocyte-reduced * <10 * Leukocyte reduction with third-generation leukocyte adsorption filter. Prevention of HLA Alloimmunization Prevention of HLA alloimmunization may be desirable for patients who require long-term platelet support or who may require eventual transplantation. Leukocyte reduction has been proposed as a way to achieve this, but its effectiveness has not been conclusively demonstrated. 28,29 Leukocyte reduction of all cellular components can be achieved with specialized filters. Certain apheresis devices provide a platelet concentrate with significantly reduced white cell numbers; the need for additional leukocyte reduction should be assessed by review of white cell counts performed for quality control. Prevention of Transfusion-Transmitted Viruses and Bacteria CMV transmission can be prevented either by administration of components from seronegative donors or by rigorous leukocyte reduction of blood components from random donors. 30,31 The transmission of Epstein-Barr virus (EBV) and human T-cell lymphotropic virus (HTLV-I/II) may also be reduced with the use of leukocyte-reduced components. For certain indications prestorage leukocyte reduction may decrease multiplication of Yersinia and other bacteria during refrigerated storage of red cells. 32 Irradiation The only indication for irradiation of a cellular blood product is to prevent GVHD. Although leukocyte-reduced components may be less likely to cause posttransfusion GVHD, irradiation remains the only acceptable method for preventing this adverse effect of transfusion. 21,33 For more detail on GVHD, see Chapter 25. Fresh Frozen Plasma Transfusion Physiologic Principles In normal coagulation, the fibrin clot forms on the platelet plug. Coagulation results from a complex but ordered enzyme cascade. See Fig 19-2 and the reviews by Boon 35 and Colman et al. 36 The central procoagulant enzyme is thrombin. The coagulation cascade is often divided into the intrinsic and the extrinsic pathways, the in-vitro activity of which can be measured by the activated partial thromboplastin time (aptt) and prothrombin time (PT), respectively, but in vivo, the cascades are interdependent. Fresh Frozen Plasma (FFP) contains all the clotting factors, including the labile Factors V and VIII. Minimal levels of coagulation factors (see Table 19-6) are required for normal formation of fibrin and hemostasis. Normal plasma contains coagulation factors in excess, a reserve that usually allows patients to receive up to a full volume replacement of red cells and crystalloid/colloid without needing FFP. Pa-
13 Chapter 19: Blood Transfusion Practice 425 Figure The coagulation cascade. The production of thrombin from prothrombin via the action of activated Factor X (Xa), Factor V, Ca ++ and platelet phospholipid is the key step in the formation of a fibrin clot. Activation of Factor X can occur by either the intrinsic or extrinsic systems, but multiple areas of overlap between the two occur. (Adapted with permission from Thompson et al. 34 ) Table Biologic Data for Selected Coagulation Factors Approximate Plasma Concentration (mg/ml) Biological t 1/2 (Hours) Levels to Achieve for Hemostasis Factor V % Factor X % Factor VII % Factor VIII ± % Factor IX ± % Factor XI % Factor XIII % Fibrinogen ±3 1 mg/ml Prothrombin ± % von Willebrand factor %
14 426 AABB Technical Manual tients with liver disease have less physiologic reserve and are more susceptible to dilutional coagulopathy. Rapid replacement of more than one blood volume sometimes results in dilutional coagulopathy, so close monitoring of hemostasis and coagulation tests is important. In the treatment of massive hemorrhage, the use of Whole Blood, if available, halves the number of donor exposures that occurs with administration of FFP and RBCs. Monitoring Hemostasis The PT, aptt, and measurement of fibrinogen level are commonly used to monitor coagulation. Results should be interpreted with three considerations in mind: 1) mild prolongations of the PT or aptt occur before the residual factor concentration falls below the level normally needed for hemostasis; 2) significant deficiencies of coagulation factors (or the presence of coagulation factor inhibitors) cause clearly prolonged values for the PT or aptt; and 3) an infusion of FFP that increases the concentration of factors by 20% will have a far greater impact on a greatly prolonged PT or aptt than on a mildly prolonged PT or aptt. The infusion of two units of FFP inapatientwithaptof14.5seconds (normal seconds) is unlikely to provide any clinical benefit and is also unlikely to correct the PT to the normal range. Indications for FFP Guidelines exist for the appropriate use of FFP. 16 FFP is a valuable therapeutic component for clinically significant Factor XI deficiency and for other congenital deficiencies for which no suitable clotting factor concentrate is available. FFP is most likely to be of clinical benefit in patients with multiple factor deficiencies, and of limited clinical benefit in patients with inhibitors to any coagulation factor. It is the preferred replacement component for plasma exchange treatment of thrombotic thrombocytopenic purpura (TTP) or hemolyticuremic syndrome (HUS). In patients with TTP or HUS unresponsive to standard plasma exchange, use of FFP, cryoprecipitate removed has been suggested, although no controlled trials document its therapeutic superiority over FFP. 24 Vitamin K Deficiency The most common cause of multiple coagulation abnormalities among hospitalized patients is deficiency of the vitamin K-dependent factors (Factors II, VII, IX, and X). Vitamin K deficiency results in an elevated PT, with or without elevation of aptt. Before patients are treated with FFP to correct a coagulopathy with an abnormal PT or aptt, unrecognized vitamin K deficiency should be considered. See later section on Pharmacologic Alternatives to Transfusion. Although most patients with vitamin K deficiency do not require FFP and are better treated with parenteral vitamin K, transfusion of plasma is occasionally needed to treat active bleeding. Patients with serious hemorrhage should receive FFP or liquid plasma, since parenteral vitamin K will require several hours to reverse the deficiency. Because Factors II, VII, IX, and X are stable during storage, plasma provides effective replacement. One unit of coagulation factor activity is defined as the amount of that factor in 1 ml of normal plasma. Transfusion of ml of FFP into the average sized adult (3000 ml plasma volume) will generally provide sufficient coagulation factors to achieve hemostatic levels in patients whose vitamin K deficiency results from excess treatment with the antagonist agent coumadin. Concurrent vitamin K supplementation should also
15 Chapter 19: Blood Transfusion Practice 427 be given. Although the PT can provide useful information about response to therapy, the need for additional treatment should be guided by the clinical response and not by the results of laboratory tests. It is rarely necessary to correct the PT or aptt to normal to achieve adequate hemostasis. Liver Disease Patients with liver disease have multiple derangements, each of which contributes to an increased bleeding tendency. Abnormalities include: portal hypertension and engorgement of systemic collateral shunts, splenomegaly with secondary thrombocytopenia, decreased synthesis of all coagulation factors except Factor VIII, dysfibrinogenemia, decreased clearance of fibrin(ogen) degradation products, decreased clearance of fibrinolytic activators, and decreased synthesis of inhibitors of the fibrinolytic system. Because Factor VII has the shortest in-vivo biologic half-life and thus requires the highest synthetic rate, decreased hepatic synthesis prolongs the PT more than the aptt. Because the defect in hepatocellular disease is in primary protein synthesis, supplemental vitamin K will not correct the abnormality. FFP is an appropriate replacement therapy for the multiple deficiencies found in severe liver disease, but is often used inappropriately. Coagulopathy. The most common error is to attribute all bleeding to coagulopathy and give systemic treatment when the actual cause is localized bleeding. For example, bleeding esophageal varices usually respond better to local hemostatic measures than to intravenous infusion of FFP. A second common error in treating liver-associated coagulopathy is overdependence on the result of the PT. A normal PT is rarely, if ever, required for cessation of serious bleeding. Active bleeding in a patient with a near-normal PT often reflects an additional problem, such as a mucosal lesion or cut vessel. Because Factor VII has such a short half-life (approximately 5 hours), the posttransfusion concentration of Factor VII will rapidly decline in cirrhotic patients with inadequate endogenous Factor VII synthesis. The goal of FFP therapy in severe liver disease should be to correct or prevent bleeding complications, not to achieve a normal prothrombin time. Other Problems. Patients with liver disease may also have abnormalities of platelet plug formation and fibrinolysis. Severe splenomegaly may impair the response to platelet transfusions. Platelet function in some patients with liver disease can be enhanced by administration of 1-deamino-8-D-arginine vasopressin (desmopressin, DDAVP). 37 Cryoprecipitate can be given if there is severe hypofibrinogenemia, but FFP contains enough fibrinogen to treat the hypofibrinogenemia of most patients with severe liver disease. Severe hepatic disease results in an increase in systemic fibrinolysis, which may not respond to FFP alone. Antifibrinolytic agents in combination with plasma therapy can be useful in these patients. See section on Pharmacologic Alternatives to Transfusion. Dilutional Coagulopathy Massive blood loss and replacement with crystalloid and/or colloid solutions may produce a dilutional coagulopathy, 38 but most patients can tolerate loss and replacement of at least one blood volume without developing impaired hemostasis. Thrombocytopenia generally develops before plasma clotting factors are diluted to the point of causing impaired hemostasis. FFP is unlikely to be beneficial if the PT is less than 1.5 times the midpoint of the normal range and the aptt is less than 1.5 times the upper limit of the
16 428 AABB Technical Manual normal range. If surgical hemostasis has not been achieved and significant continued bleeding is expected, FFP may be indicated. 16 Massive bleeding that requires replacement of more than one blood volume may cause hypotension with tissue ischemia that additionally prolongs the PT or aptt. Some patients undergoing intensive daily plasmapheresis may develop significant dilutional coagulopathy; most patients with adequate liver function can tolerate daily one plasma-volume apheresis without requiring supplemental FFP. Disseminated Intravascular Coagulation DIC occurs if there is circulating thrombin that induces widespread fibrin formation in the microcirculation and consumption of platelets and coagulation factors, particularly fibrinogen, Factor V, and Factor VIII. Fibrin strands in the microcirculation may cause mechanical damage to red cells, a condition called microangiopathic hemolysis, with deformed red cells (schistocytes) in the circulation. Widespread microvascular thrombi promote tissue ischemia and release of tissue factor, which further activates thrombin. Fibrinolysis of microvascular fibrin causes increased quantities of fibrin degradation products to enter the bloodstream. A number of clinical conditions can initiate DIC, including shock, tissue ischemia, sepsis, disseminated cancer, and obstetric complications such as abruptio placentae or amniotic fluid embolism. The common precipitating event is a procoagulant signal for thrombin production that exceeds the normal physiologic defenses against disseminated thrombin activity. Treatment of DIC depends on correcting the underlying problem and preventing further hypotension and tissue ischemia. If fibrinogen level is below 100 mg/dl, FFP or Cryoprecipitated AHF may be indicated. Deficiency of Protein C, Protein S, or Antithrombin Protein C and protein S are vitamin K- dependent proteins with anticoagulant effects. Protein S converts inactive protein C to an active state, in which it inactivates Factor V and Factor VIII:C and increases vascular release of the fibrinolytic protein, tissue plasminogen activator. Patients with deficiencies of protein C or protein S have a predisposition to thrombotic complications and are often treated with anticoagulants. Coumadin treatment, however, can cause these vitamin K-dependent proteins to fall to dangerously low levels, leading to skin necrosis and further thrombosis. Transfusion of FFP can serve as an immediate source of supplemental protein C or protein S for patients with severe deficiencies, although a human Protein C concentrate is available. Antithrombin, is a circulating protein with anticoagulant properties, is stable in FFP and in refrigerated liquid plasma, and is available as a concentrate. Administration of antithrombin is discussed in the section on Plasma Derivatives and Plasma Substitutes. Other Conditions Hereditary angioneurotic edema results from a congenital deficiency of C1- esterase inhibitor, an inhibitory protein that regulates complement activation. Patients with this condition may experience life-threatening subglottic edema following complement activation. FFP or liquid plasma contains normal levels of C1- esterase inhibitor and can be therapeutic.
17 Chapter 19: Blood Transfusion Practice 429 Misuse of Fresh Frozen Plasma Plasma should not be used as a volume expander, as a nutritional source, or to enhance wound healing. 16 Transfusing plasma for volume expansion carries a risk of transmitting disease that can be avoided by using crystalloid or colloid solutions. Plasma is also not a suitable source of immunoglobulins for patients with severe hypogammaglobulinemia, since an intravenous preparation of immunoglobulin is available. FFP is often given prophylactically to patients with mild to moderate prolongation of the PT or aptt prior to invasive procedures, but there is little or no evidence that this prevents bleeding complications. One study demonstrated that the preprocedure PT or aptt did not correlate with the likelihood of bleeding following paracentesis or thoracentesis, 39 and two studies have documented that bleeding at the time of liver biopsy could not be predicted by the preprocedure PT, aptt, or platelet count. 40,41 Because these tests do not accurately predict the risk of bleeding, there is little logic to transfusion intended to improve the results. Cryoprecipitate Transfusion Cryoprecipitated AHF (CRYO) is a concentrate of high-molecular-weight plasma proteins that precipitate in the cold, including von Willebrand factor (vwf), Factor VIII, fibrinogen, Factor XIII, and fibronectin. The primary clinical use of CRYO is for intravenous supplementation of Factor XIII and fibrinogen, although it is also used topically as a fibrin sealant. CRYO is seldom used for patients with hemophilia because Factor VIII concentrates (some of which contain vwf activity) are available commercially and have been processed to reduce or eliminate the risk of blood-borne viral infection. Because CRYO contains ABO antibodies, consideration should be given to ABO compatibility when the infused volume will be large relative to the recipient s red cell mass. von Willebrand Syndromes von Willebrand syndromes are the most common major inherited coagulation abnormalities. 42 The conditions are usually autosomal dominant and represent a collection of quantitative and qualitative abnormalities of vwf, the major protein mediating platelet adhesion to damaged endothelium. The protein also transports Factor VIII. As a result, patients with von Willebrand syndromes have varying degrees of abnormal platelet plug formation (prolonged bleeding time) and partial deficiency of Factor VIII (prolonged aptt). vwf exists in the plasma as a family of multimeric molecules with a wide range of molecular weights. The high-molecular-weight species of vwf are the most hemostatically effective. Laboratory evaluation demonstrates a specific deficiency in the level of vwf. vwf is often measured as ristocetin cofactor activity because vwf is required for the platelet-aggregating effect of ristocetin in vitro. Mild cases of von Willebrand syndrome can often be treated with DDAVP, which causes a release of endogenous stores of Factor VIII and vwf. See section on Pharmacologic Alternatives to Transfusion. However, DDAVP is contraindicated in one type (Type IIb) of the syndrome. Many Factor VIII concentrates do not contain therapeutic levels of vwf, but at least two with satisfactory levels are commercially available. In the absence of a suitably therapeutic virus-inactivated concentrate, severe von Willebrand syndrome can be treated with FFP
18 430 AABB Technical Manual or with CRYO. CRYO provides a much higher concentration of high-molecular-weight vwf than FFP. The quantity of CRYO required to treat bleeding episodes or to prepare for major surgery varies greatly among patients with von Willebrand syndromes. In addition to the clinical response of the patient, the template bleeding time, the level of Factor VIII, or the ristocetin cofactor activity may help to guide therapy. Fibrinogen Abnormalities Hypofibrinogenemia may occur as a rare isolated congenital deficiency or may be acquired as part of the DIC syndrome. Dysfibrinogenemias may be congenital or acquired and represent conditions in which fibrinogen is immunologically present but functionally defective. Patients with severe liver disease frequently exhibit a dysfibrinogenemia. CRYO is the only concentrated fibrinogen product currently available. On average, one unit of CRYO contains approximately 250 mg of fibrinogen 1 ;the minimum required by Standards is 150 mg. 21 Calculating CRYO Dose for Fibrinogen Content The amount of transfused CRYO required to raise the fibrinogen level depends upon the nature of the bleeding episode and the severity of the initial deficiency. The amount of CRYO required to raise the fibrinogen level can be calculated as follows: 1. Weight (kg) 70 ml/kg = blood volume (ml). 2. Blood volume (ml) (1.0 hematocrit) = plasma volume (ml). 3. Mg of fibrinogen required = (Desired fibrinogen level in mg/dl initial fibrinogen level in mg/dl) plasma volume (ml). 4. Bags of CRYO required = mg of fibrinogen required 250mgfibrinogen/bag of CRYO. Topical Use The fibrinogen in CRYO has been used during surgery as a topical hemostatic preparation (fibrin sealant or fibrin glue). 43 One to two units of CRYO are thawed and drawn into a syringe. Topical thrombin (usually of bovine origin) is drawn into a second syringe. The contents of the two syringes are simultaneously applied to the bleeding surface, where fibrinogen is converted to fibrin by the action of thrombin. Some patients who have been exposed to fibrin sealant have developed antibodies that inhibit bovine thrombin and human Factor V. 44 Factor VIII Deficiency Each unit of CRYO prepared from a single blood donation should contain a minimum of 80 international units (IU) of Factor VIII. 21 Although no longer the component of choice, CRYO can serve as replacement therapy for patients with hemophilia A if virus-inactivated Factor VIII concentrates are unavailable. 16 If CRYO is used, the amount required to provide a therapeutic dose of Factor VIII is based on calculations similar to those used for AHF. See section on Plasma Derivatives and Plasma Substitutes. Fibronectin Fibronectin is an opsonic glycoprotein thought to play a role in the clearance of blood-borne particulate matter such as bacteria and protein aggregates. Uncontrolled observations suggested that infused fibronectin might be of value in treating patients with sepsis, burns, or trauma, but studies did not support the clinical efficacy of fibronectin therapy. 45
19 Chapter 19: Blood Transfusion Practice 431 Plasma Derivatives and Plasma Substitutes Plasma derivatives are concentrates of specific plasma proteins prepared from pools of plasma. Cohn fractionation, which relies on the precipitation of various plasma proteins in cold ethanolwater mixtures, was developed during World War II and is still used with some modifications. 46 After fractionation, derivatives undergo further processing to purify and concentrate the proteins, and inactivate any contaminating enveloped viruses. Virus-inactivation procedures include heat treatment, the use of chemical solvents and detergents, or affinity column purification. Some plasma proteins are also produced by recombinant DNA technology. These products appear to be efficacious, well-tolerated, and carry no known disease risk. Factor VIII Concentrates Hemophilia A, a congenital deficiency of Factor VIII, results from abnormality of a gene on the X chromosome and, therefore, is fully expressed in males and transmitted by female carriers. Factor VIII is critically important in the reactions leading to fibrin formation. The severity of hemophilia A depends upon the patient s level of Factor VIII. Measurement of Factor VIII antigen sometimes gives normal results despite deficient Factor VIII coagulant activity, suggesting a functional defect of the Factor VIII molecule. Characteristic laboratory findings include prolonged aptt, normal PT and template bleeding time, and a severe deficiency of Factor VIII activity. Clinical Observations One unit of Factor VIII activity is defined as the Factor VIII content of 1 ml of fresh, citrated, pooled, normal plasma. The measured level of Factor VIII can be expressed as a concentration, a decimal fraction, or as percent activity. For example, a hemophiliac with one-tenth the normal activity of Factor VIII can be said to have a Factor VIII level of 10 units/dl or 0.1 units/ml or 10% activity. Severe hemophiliacs have Factor VIII levels below 1%, while moderate hemophiliacs typically have 1-5% activity, and mild hemophiliacs have 6-30%. Patients with mild to moderate hemophilia can often be managed without replacement therapy. Careful attention to local hemostasis and the use of topical antifibrinolytics can often prevent the need for transfusions. Systemic levels of Factor VIII can be raised in mild hemophilia with the use of DDAVP, 47 which stimulates the release of endogenous Factor VIII from storage sites. DDAVP is inappropriate therapy for patients with severe hemophilia A, who require Factor VIII replacement. Product Preparation Antihemophilic Factor (AHF, Factor VIII) is prepared from large volumes of pooled normal plasma or cryoprecipitate by various separation methods. Recombinant Factor VIII concentrates are also available. Plasma-derived AHF concentrates are treated for virus inactivation either by heating in solution (pasteurization) or by exposure to a solvent/detergent combination. The specific activity (Factor VIII units/mg protein) of presently available concentrates has been dramatically increased in concentrates prepared with affinity columns or by recombinant technology. As a result, the safety and potency of AHF has greatly increased and, in hemophiliacs treated exclusively with new preparations, the incidence of posttransfusion viral hepatitis has dramatically declined. 48 The cost of AHF has also increased, due to the increased complexity of manufactur-