Author : Dr Herbert A. Perkins Blood Centers of the Pacific; Clinical Professor of Medicine, University of California San Francisco, CA
2008-07-28
2008-07-28
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| Red blood cells. Source : CDC |
What makes blood so important?
Our
bodies are made up of living cells which will die unless fed by
substances from the outside world and unless waste products are
eliminated. Blood circulates throughout our tissues, bringing oxygen
from the lungs and food from the intestines. It carries waste products
to the kidneys and lungs. It also carries chemical signals from one
type of tissue to another. Blood circulates because it is pumped by the
heart. Blood pressure has to be maintained to push blood to all parts
of the body. A very severe sudden drop in blood pressure means that
blood will not flow through the brain. Loss of consciousness results
and interruption of flow for more than four minutes is likely to cause
permanent brain damage or death. Lesser, but more prolonged drops in
blood pressure can damage the kidneys. What is blood?
Blood is a living tissue consisting of cells suspended in a liquid called plasma. Approximately 45% of blood’s volume is composed of red blood cells. These cells contain a substance called hemoglobin, which gives blood its red color. Hemoglobin carries oxygen from the lungs to the tissues where it is used. Hemoglobin also carries carbon dioxide (a waste product) from the tissues back to the lungs, where it is excreted. A low level of hemoglobin is called anemia. Depending on how severe it is, anemia can cause weakness and shortness of breath, while the heart rate speeds up in an effort to compensate.There are normally approximately 5,000,000 red blood cells in a microliter of blood. (There are a million microliters in a liter, and a liter is a bit more than a quart.) Each microliter of blood also contains about 5,000 to 10,000 white blood cells. White blood cells are the soldiers in our fights against infections. They may attack foreign organisms directly or by making antibodies to do the work.
There are 150,000 to 400,000 blood platelets in a microliter of blood. These are fragments of cells and they take up very little space. Platelets are responsible for plugging leaks in blood vessels and they thus keep us from bleeding.
The plasma portion of blood is mostly water, but it contains important proteins like albumin. Without albumin the water in the blood would leak out into the surrounding tissues. Plasma also contains globulins, which include antibodies. It carries hormones and other messengers from one cell to another. And it contains salts of various kinds.
When should blood be transfused?
The most obvious and well recognized situation is hemorrhage – loss of blood from the circulation. This may occur through an accident, especially serious when a high pressure artery is cut. Or it may occur on an elective basis when surgery is performed. Certain types of surgery are inevitably associated with relatively high loss of blood (liver transplantation, open heart surgery, some types of back surgery). With sudden hemorrhage the most important priority is to keep the circulation going – to keep the pressure up – to prevent shock. Thus, the initial treatment may consist of infusing a salt solution or albumin. Eventually, it may be necessary to transfuse red blood cells, but experience has shown that replacing red blood cells only when the hemoglobin level falls to 70 grams per liter (from a normal level of 150) is better for most patients1.Anemia results not only from hemorrhage, but from excessive destruction of red blood cells in certain disease states and from decreased production of red blood cells in the bone marrow. In fact, one of the most common indications for transfusion of red blood cells is marrow depression caused by chemotherapy given to treat cancer. Red blood cells will be transfused in these situations until the hemoglobin is high enough to allow the patient to function appropriately. There are inherited conditions, such as thalassemia and sickle cell disease, in which the patient does not synthesize red blood cells appropriately, resulting in decreased production and increased loss. These patients may require repeated transfusions as long as they live. Blood can also be life-saving for the newborn infant. Babies with Rh hemolytic disease may have to be treated with exchange transfusions, replacing their own blood with its high bilirubin and Rh-positive red blood cells with normal blood containing Rh-negative red blood cells.
Transfusions of platelets are required in situations where the platelet count becomes so low that the possibility of serious bleeding is present. Primary concern centers on the possibility of bleeding into the brain, resulting in a stroke or death. The most common indication for platelet transfusion is cancer therapy. Platelets can also be very low in untreated leukemia and in aplastic anemia, a condition in which the marrow fails to produce red blood cells, white blood cells, and platelets.
Transfusions of white blood cells are rarely required. A type of white blood cell called granulocytes is the first line of defense in bacterial and fungal infections. If the granulocyte count becomes very low and the patient is not responding to antibiotics, it may be necessary to transfuse them. Normal circulating blood also contains a low frequency of hematopoietic stem cells. These are the cells which, in the marrow, are able to develop into mature blood cells of all types – red blood cells, white blood cells, and platelets. These are also the cells which result in successful marrow transplants. In recent years transplantation of hematopoietic stem cells has more often used peripheral blood as a source than marrow. Another type of white blood cells, called lymphocytes, is transfused only in very special occasions. They may be required after a hematopoietic stem cell transplant when surviving host cells that are trying to destroy the graft (transplanted cells) need to be killed by donor lymphocytes. In other circumstances, lymphocytes pre-immunized to respond to certain viruses (e.g., Epstein-Barr Virus, Cytomegalovirus) may be infused to control infection.
Plasma can be stored frozen and used to transfuse patients lacking certain plasma proteins. More commonly plasma is sent for commercial fractionation resulting in more purified products used to treat hemophiliacs and other patients lacking specific plasma factors.
Component therapy
Because the indications for transfusion are based on the specific component of blood required by the patient, donated blood is separated into components. It became possible to do this under aseptic conditions when plastic bags were developed. A series of bags can be purchased which are connected by tubing in a single closed system. Blood can be collected into the primary bag which is then spun in a centrifuge. Centrifugal force results in the heavier red blood cells falling to the bottom of the bag, the white blood cells and platelets layer on top of the red blood cells, and the plasma is at the top. If the spin is sufficiently gentle, the platelets remain in the plasma, which can then be squeezed off into an attached bag, leaving red blood cells in the original bag which can be sealed off and disconnected. The platelet-rich plasma is then spun harder to settle the platelets, and plasma is squeezed off into a third bag. Thus, each donation of whole blood results in three components: red blood cells, platelet concentrate, and plasma.A fourth component may also be obtained. If frozen plasma is thawed under appropriate conditions a precipitate of proteins important to prevent bleeding is obtained (cryoprecipitate). This is a concentrate of factor VIII (antihemophiliac factor), fibrinogen, and von Willebrand’s factor.
Table 1
Preparation of Blood Components
Whole blood
↓
Red blood cells + Platelet-rich plasma
↓
Platelet concentrate + Plasma
↓
Cryoprecipitate + Cryo-poor plasma
Component therapy has advanced further by development of a process to separate the components at donation – a process known as apheresis. The blood donor is bled directly into an automated device which separates components and returns to the donor what is not needed. The donor’s blood is in contact only with a disposable plastic system. Using this approach, it is possible to get a platelet concentrate at a single donation sufficient to treat one or two patients, whereas it would take six platelet concentrates from a whole blood donation for one patient. Apheresis can also be used to collect plasma in large volumes. More recently, blood banks are using apheresis to collect two red blood cells units at a single donation. Other combinations of components are also possible.
Storage of components
Red blood cells are stored at very closely controlled refrigerated temperatures. They must not freeze and they may not rise above 4° C (39° F). Red blood cells are collected in a solution which keeps blood from clotting and which contains nutrients important for their survival, in a closed container where they have no access to further nutrients and no way to get rid of waste products. Since most of these nutrients are removed with the platelets and plasma, blood banks usually add them back to the separated red blood cells. With this additive approach it is possible to store red blood cells in the refrigerator for up to 42 days. (Red blood cells live an average of 120 days in the circulation.)Platelets, unfortunately, clump and are damaged in the cold. It is thus necessary to store them at a temperature from 20° -24° C (68°- 75° F). They must also be kept in constant slow motion to distribute the acid produced by their metabolism. Under these conditions platelet concentrates can be stored only 5 to 7 days. Tests of the donor’s blood take up to 48 hours, so maintaining an adequate inventory of platelets without unacceptable outdating is a challenge.
Plasma and cryoprecipitate are easily stored using freezers.
Adverse effects of blood transfusion
Most patients worried about possible harm from a blood transfusion focus their concern on viral infections – especially AIDS and hepatitis. This is understandable. In the 1970’s it was well documented that 10% of blood recipients were infected with a hepatitis virus2. And in the early 1980’s HIV contaminated the blood supply at a rate very much higher than was appreciated at that time3. Today, however, these infections have been largely eliminated by careful donor histories and by laboratory tests which include exquisitely sensitive nucleic acid testing. (Nucleic acids are the primary components of viruses, bacteria and protozoa. Small traces of nucleic acids can be amplified until they are easy to detect.)Today in the United States, the risk of HIV infection from blood donations is less than 1 in 2 million. This residual risk is caused by donors so recently infected that no viruses can be detected in the blood sample taken for testing. The risk of hepatitis C is equally low. Hepatitis B risk is somewhat greater, but is only 1 in 100,000. A more recent virus risk is West Nile Virus, which has swept across the US in the last half dozen years, causing 23 transfusion-transmitted cases a few years ago. In a remarkably rapid response, the blood bank community and its commercial suppliers developed and implemented a nucleic acid test for West Nile Virus which brought the threat under control. Dengue fever is beginning to be a cause for concern in the southern US.
The risks of transmissible infections can vary immensely in different regions of the world, in part because of differences in the prevalence of each disease and in part from the ability to perform the sophisticated and expensive tests which are routine in most developed countries.
Table 2
Laboratory Tests for Detection of transmissible Agents in Blood Donors
Tests for: Prior Exposure Infectious Agents
HIV Anti-HIV NAT
HBV Anti-HBc HBsAg
HCV Anti-HCV NAT
ALT
HTLV Anti-HTLV
WNV NAT
Syphilis Antibody
Bacteria Culture
Chagas disease Antibody
Abbreviations: HIV =- Human Immunodeficiency Virus. HBV = Hepatitis B Virus. HCV = Hepatitis C Virus. HLTV = Human T Cell Lymphotropic Virus. WNV = West Nile Virus. ALT = alanine aminotransferase (a non-specific liver function test). NAT = Nucleic acid amplificiation test. HBsAg = Hepatitis B Surface Antigen. Today bacterial infections should be a greater cause of concern than viral infections. Despite all efforts at disinfecting the donor’s skin, bacteria can be picked up by the needle from deeper layers (e.g., sweat glands) which are inaccessible to the disinfecting process. Moreover, completely healthy donors have periods of transient, low-level bacteremia. It is platelet concentrates, stored at room temperature, which are at most risk (1 in 3,000 to 1 in 5,000). It is fortunate that the donor’s white blood cells and antibodies kill most bacteria; also most patients, many already receiving antibiotics, destroy the bacteria on receipt. In recent years, blood banks have cultured platelet concentrates for bacteria. This has eliminated only about one half of the bacterial transmissions, however4.
Protozoa (which are single cell organisms) can also be transmitted by blood transfusion. Malaria, a serious problem in much of the world, causes few transfusion-transmission cases in the US. Blood banks prevent its transmission by deferring donors who have been in a malaria area. Unfortunately, this results in a large number of deferred donors, most of them without risk of transmitting malaria. Babesiosis has resulted in a few transmissions. This is an infection of red blood cells caused by a bite from an infected tick, which can be symptomatic and even lethal in a subject with poor immune responses. It has been a problem in the northeast and midwest sections of the USA. Chagas disease is caused by Trypanosoma cruzi and is transmitted by the “kissing bug” . This is a major transmission problem in Central and South America primarily from donors brought up in poor conditions in rural areas. It can result in serious heart problems many years after the original infection. Chagas disease has been transmitted by transfusion in only a few cases in the US and Canada; but, because of the large number of immigrants from endemic areas, blood banks in the US are now testing for Chagas infection.
“Mad cow disease” is a degenerative neurological disease caused by prions, which act like infectious agents even though they appear to be pure protein. The human form of this disease, called variant CJD Disease, is caused by eating beef from infected cows. Infected humans can transmit variant CJD by blood transfusion. Four transfusion transmissions have been reported from the United Kingdom. Although not a problem in the United States, blood banks have had to defer donors with prolonged stays in the United Kingdom and elsewhere in Europe.
Table 3
Adverse Effects of Blood Transfusion
Transmission of infectious diseases Relative risk (USA)
Viral
Human Immunodeficiency Viruses (HIV) 1 in 2,000,000
Hepatitis B Virus (HBV) 1 in 100,000
Hepatitis C Virus (HCV) 1 in 2,000,000
Human T Cell Lymphotropic Viruses (HTLV) 1 in 600,000
West Nile Virus rare
Bacterial 1 in 5,000
Protozoal
Malaria rare
Chagas Disease rare
Babesiosis rare
Variant Creutzfeld-Jacob Disease (vCJD) none recognized
Non-infectious complications
Hemolytic
ABO mismatch 1 in 6,000
Rh(D) rare
Other antigens rare
Febrile
White blood cells (WBC) 1 in 100
Cytokines (chemicals released from WBC) 1 in 100
Allergic
Hives 1 in 50
Anaphylaxis rare
Transfusion-Related Acute Lung Injury (TRALI) 1 in 5,000
Graft-versus-host disease rare
Immunomodulation ?
Non-infectious adverse effects should be of even greater concern. Hemolytic reactions occur when red blood cells are destroyed – usually by antibodies. The most important red blood cell antigens are A and B of the ABO blood group. These are important because anti-A and anti-B occur naturally in the plasma of patients who lack A or B on the red blood cells. There are four ABO types:
Table 4
Types
|
A
|
B
|
AB
|
O
|
RBC antigens
|
A
|
B
|
AB
|
Neither
|
Antibodies
|
Anti-B
|
Anti-A
|
None
|
Anti-A, Anti-B
|
These antibodies are important because, in addition to their universal prevalence, they tend to be very strong and they destroy red cells directly in the circulation with the help of a complex of plasma proteins known as complement. The effect of ABO-incompatible red cells can vary. In some cases there is no obvious clinical effect. Chills, fever, and backache are common. More severe cases go into shock, and death can occur. The worst aspect of the problem is that transfusion of ABO-mismatched blood is almost always the result of human error. Either the tube used to type and crossmatch the patient’s blood had the wrong name on it or the wrong bag of blood was grabbed in an emergency. Hospitals are working hard on additional methods to avoid these mishaps. To avoid human error, some of the approaches use bar codes or radio frequency signals which give a computer control over the matching process.
Antibodies to other red cell antigens are more likely to destroy red blood cells slowly, primarily in the tissues. One other antigen, however, is routinely typed: the Rh(D) antigen. This is routine because it is a very potent antibody former, and because it has been the frequent cause of hemolytic disease of the newborn.
There are hundreds of other red cell antigens, but blood banks do not usually type for them unless the recipient has been shown to have an antibody against them.
Chills and fever have been common complications of blood transfusions. A major cause was discovered with the demonstration that recipient antibodies to donor white blood cells were responsible. The majority of these reactions could be prevented or alleviated by removal of most of the white blood cells from the transfused component. The antibodies were directed against human leukocyte antigen (HLA) factors, which proved to play a major role in the rejection of organ transplants. Most blood banks now remove most of the white cells from all of their components. Some chill and fever reactions still occur, either because recipients are very sensitive to the small amounts of remaining white blood cells or because of chemicals secreted by the white blood cells before their removal.
Allergic reactions also occur in a small percentage of transfusions. Usually only a few hives occur. Occasionally more general swelling may occur. Very rarely severe anaphylaxis or even death may result. Most reactions are assumed to be caused by something in the donor blood to which the recipient is allergic or something in the recipient with which the donor blood reacts. The severe anaphylactic reactions are caused by patient antibodies to a donor plasma protein the patient lacks.
The reaction responsible for the most transfusion-related deaths reported to the US Food and Drug Administration in recent years is also the least understood. Called Transfusion-Related Acute Lung Injury or TRALI, it lacks a clear definition or specific test. It manifests as shortness of breath, low blood oxygen levels, and diffuse opacities on x-ray of the lungs. It is difficult to distinguish from fluid overload, which is another equally common complication of blood transfusion. The majority of cases have been attributed to donor antibodies reacting with recipient white blood cells. Most patient recover spontaneously in a day or two, but approximately 5% are reported to have died.
Graft-versus-host disease occurs when donor lymphocytes engraft, recognize host cells as foreign, and attempt to reject the host. This syndrome is a common complication of hematopoietic stem cell transplantation. It is most likely to occur after transfusion if the recipient is immunosuppressed (i.e., unable to make an adequate immune response to the presence of foreign antigens). This syndrome can also occur if the donor’s HLA antigens are all shared with the recipient but the recipient has additional HLA antigens not shared with the donor. In the latter situation, the recipient cannot reject the donor cells, but the donor cells can recognize the recipient as foreign. Graft-versus-host disease is more likely to occur if the donor and recipient are related and if the transfused cells are relatively fresh. Mortality is very high, but prevention is possible by irradiating the blood component to be transfused. Irradiation is indicated when the recipient is known to be immunosuppressed or when the donor is related.
There is evidence that blood transfusions have an immunomodulating effect; that is, they interfere with immune responses. Evidence for this lies in the superior survival of whole organ grafts in recipients who have had a blood transfusion. This evidence led to hypotheses that blood transfusion, by impairing immune responses, could result in more post-operative infections and more cancer recurrences. Although some animal work and human experiences support these hypotheses, there are too many conflicting reports to provide unquestioning acceptance of these hypotheses. Less clear in the mechanism of the effects are a few reports which claim greater mortality among trauma patients the more they are transfused. The problem is that more transfusions go to the patients with the worst prognosis, and it is difficult to be sure that the control series is appropriately matched for all other factors.
Blood donation
Transfusions save lives, but blood will not be available without donors. Every year the demand for blood goes up as patient age increases and new procedures (e.g., organ transplantation) require large volumes of blood components. Every year the reasons to defer blood donors increase as questions in the history expand and tests are added. Most of the test deferrals are caused by false positive results, but blood banks are still required to defer the donors. Ninety five per cent of eligible individuals do not give blood regularly, either because they won’t take the time or because they are afraid of needles. In general the blood supply has been adequate to meet most patient needs. In the USA in 2001 (the latest year for which figures are available), 15,320,000 units of red blood cells were collected and 13,898,00 were transfused. 12,898,000 platelet units were collected and 10,196,000 were transfused5. However, these figures ignore components removed from inventory because of positive tests, the requirement of patients for type-specific cells and fluctuating need for blood components. Some patients with special needs go short, elective surgery is occasionally canceled in some areas, and some hospitals find themselves trying to decide which patient is most in need of the last available platelet concentrate. Blood banks continue to strive to find new ways to stimulate donation. The biggest help in recent years has come from the new ability to collect two red cell units at a time by apheresis.Considerable effort has been made to use the patient’s own blood for transfusion to avoid infection transmission and immune reactions. There are situations in surgery in which it is possible to collect blood as the patient loses it and return it to the patient’s circulation. And it is possible for patients to donate blood prior to elective surgery for subsequent retransfusion. Although appropriate use of autologous blood should be encouraged, autologous blood is frequently not used, or is transfused in a situation where it is not indicated. Transfusion of autologous blood is not without risk. Bacterial contamination is always a possibility.
An even less useful source of blood is the directed donor, one selected by the patient. This is always done on the assumption that the patient or the patient’s family can identify safer donors than can the blood bank, and this is not true. Persons asked to donate for a specific patient may be hesitant to confess a fact which makes them ineligible as a donor when they have been trying to keep that fact secret from family and friends. And if the directed donor is eligible, there is much more likelihood that the donation will end up wasted, as it is saved in case the patient needs it later.
Artificial blood
Unfortunately, artificial blood does not exist. Blood is a complex living tissue. Investigators are trying to develop an oxygen carrier which can substitute for red blood cells, at least briefly. Two general approaches have been attempted. One uses fluorocarbons with emulsifiers; the other uses hemoglobin from outdated human blood or bovine hemoglobin. For almost 30 years it has appeared likely that an oxygen carrier based on hemoglobin might be shortly available, but each time hope was denied. Although increasing knowledge about the causes of the side effects of these products raised hopes that success may eventually occur, a 2008 meta-analysis of all available data was very discouraging, since it concluded that all of the hemoglobin-based red cell substitutes increased mortality 30% and the risk of myocardial infarction 2.7 fold.Even if such substitutes are evetually successful, they will not solve the problem of blood donor shortages. These oxygen carriers are not expected to remain in the circulation very long, so they will be useful only to help in a temporary emergency. Moreover, if they are based on human hemoglobin, blood donation will be necessary to provide that product. Investigators are starting to develop ways to culture red blood cell stem cells with the hope of growing mature red blood cells in quantities sufficient for transfusion. Such hopes will take decades to fulfill, if at all.
Universal red blood cells
It would help the blood supply and minimize reactions if we did not have to match blood types. Two approaches have been tried here. One coats red blood cells with polyethylene glycol with the intent of masking antigens without interfering significantly with the survival and function of red blood cells. Unfortunately, many patients appear to make antibodies to polyethylene glycol. The other approach has identified enzymes with the capability of removing the terminal sugar from type A or B red blood cells, converting them to type O. It has been particularly difficult to remove A1 antigens and patients make antibodies against the A or B antigens even when those antigens cannot be detected in the laboratory. The work continues.Pathogen inactivation
As indicated above, neither our donor histories nor our laboratory tests completely prevent transmission of infections by transfusion. Moreover, this reactive approach will not control new agents from entering the blood supply until they have been recognized and tests have been developed. Another approach is to add an agent which will kill infectious agents without harming any blood component. Agents do exist which can prevent transmission of viruses, bacteria, or protozoa with acceptable effects on blood components. Most widely used has been the solvent-detergent method developed at the New York Blood Center. This destroys lipid-enveloped viruses, which include HIV, Hepatitis C Virus, and Hepatitis B Virus, but does not destroy viruses without a lipid envelope (Hepatitis A, Parvovirus). It is extensively used to excellent effect by plasma derivative manufacturers along with heat and physical separation methods. The solvent-detergent method has also been used with fresh-frozen plasma in Europe, but it is not available in the US because the product tested here appeared to increase the risk of thrombosis. Cerus has developed a method to treat platelets using amotosalen and ultra violet light, which seems effective on all types of infectious agents. It requires additional steps to prepare platelet concentrates and results in a slightly less effective product. This approach is used in some European countries, but has not been approved for use in the US. A similar approach is under investigation for plasma. Some European countries also use methylene blue for plasma. There is concern that these compounds work by damaging the nucleic acids of the infectious agents and theoretically could damage recipient nucleic acids with resulting increased risk of cancer. The difficult component has been the red blood cells. Hemoglobin blocks the ultraviolet light ranges needed. Several other approaches are under investigation. One problem has been the production of red cell antibodies by recipients after exposure to the treated red blood cells.Regulation of Blood Banks
US blood banks have been closely regulated by the federal government for many years, originally by the Division of Biologic Standards of the National Institutes of Health, and subsequently by the Food and Drug Administration. Following recognition of the AIDS transfusion crisis, FDA requirements became more detailed and strict. Good manufacturing practices are now thoroughly indoctrinated into all blood bank employees. Changes in donor questions, specific tests employed, and devices used all require prior approval by the FDA. Similar scrutiny of blood banks is in place in Canada and in Europe. In addition, the Standards of the AABB provide more detailed requirements for every procedure utilized by blood banks and by hospital transfusion services. Very detailed records are kept. These regulations and standards have played a significant role in achieving the current remarkable safety record of blood transfusions. Blood banks and hospital transfusion services throughout the world have played a major role in developing and evaluating the new procedures and tests which have done so much to make blood transfusion safe. Credit must also go to the US Centers for Disease Control and Prevention for its important role in controlling the spread of infectious diseases through transfusion.Conclusion
Although a large number of adverse effects from transfusion still occur, the aggregate number of serious effects is very low; and the remarkable decrease in risk of infectious disease transmission is noteworthy. The adverse effects are barely worth noting as compared with the number of lives saved by transfusion of blood components. Nonetheless, as in any medical decision, the decision to transfuse an individual patient must be made by comparing the anticipated benefits with the risks. Blood components should not be transfused unless they are necessary.REFERENCES
- Klein HG, Spahn DR, Carson JL, et al.: Red blood cell transfusion in clinical practice. Lancet 2007;370:415-26.
- Aach RD, Szmuness W, Mosley JW, et al.: Serum alanine aminotransferase of donors in relation to the risk of non-A, non-B hepatitis in recipients. New Engl J Med 1981;304:989-94.
- Busch MP, Young MJ, Samson SM, et al.: Risk of Human Immunodeficiency Virus (HIV) transmission by blood transfusions before the implementation of HIV-1 antibody screening. Transfusion 1991;31:4-11.
- Eder AE, Kennedy JM, Dy BA, et al.: Bacterial screening of apheresis platelets and the residual risk of septic transfusion reactions: the American Red Cross experience (2004-2006). Transfusion 2007;47:1134-42.
- Sullivan MT, Cotton R, Read EJ, Wallace EL: Blood collection and transfusion in the United States in 2001. Transfusion 2007;47:385-94.