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Transfusion Primer

Blood Transfusions

A unit of blood is 1 pint (450 milliliters) and is mixed with anti-clotting agents to prevent clotting. Each year, approximately 12 million to 14 million units of blood are donated in the United States. Generally, a blood donor must be at least 17 years old, healthy, and weigh more than 110 pounds.

Before donating blood, the donor is given information to read and a health history is taken to ensure that the donor has not been exposed to diseases that can be transmitted by blood. (It is also important to determine if donating blood is safe for that person's own health.)

The donor's temperature, pulse, blood pressure and weight are obtained along with a few drops of blood to make sure the donor is not anemic. Once the needle has been placed, tt usually takes less than 10 minutes for the unit of blood to be removed . Sterile, single-use equipment is used so there is no danger of infection to the donor. Donors should drink extra fluids and avoid exercise that day.

Autologous blood donation is the donation of blood for one's own use, usually prior to surgery. Apheresis is the procedure in which only a specific component of a donor's blood is removed (usually platelets, plasma or white cells). Using this technique more of that specific component can be removed than can be derived from one unit of blood.

Each unit of blood can be separated into several components so that each component can be given to someone who needs it. Thus, a single unit of blood can help many people. Components include:

  • Packed RBCs
  • Fresh frozen plasma
  • Platelets
  • WBCs
  • Albumin
  • Immunoglobulins (antibodies)
  • Cryoprecipitate anti-hemolytic factor
  • Factor VIII concentrate
  • Factor IX concentrate

Red blood cells ('packed cells') are transfused to increase oxygen-carrying capacity in patients who are bleeding or very anemic. One unit of blood increases the hemoglobin by 1 g/dl and the hematocrit by 2 to 3 percent.

Plasma (stored as 'fresh frozen plasma' and thawed before use), is transfused to treat bleeding disorders when many clotting factors are missing. This occurs in liver failure, when too much of a blood thinner has been given, or when severe bleeding and massive transfusions result in low levels of clotting factors.

Platelets are transfused in people with low platelet count (thrombocytopenia) or poorly functioning platelets. Each unit of platelets raises the platelet count by approximately
5,000 platelets per microliter of blood.

Albumin makes up 60 percent of the protein in plasma. It is produced in the liver and is used when blood volume needs to be increased and fluids alone have not worked. Immunoglobulins are given to persons who have been exposed to a certain disease such as rabies, tetanus or hepatitis to help prevent that disease.

Factor VIII concentrate and cryoprecipitate are used in hemophilia A (classic hemophilia) since this is caused by a factor VIII deficiency.

Factor IX concentrate is used in hemophilia B ('Christmas disease'), which is caused by a deficiency of clotting factor IX.

The Donation Process


When prospective donors enter a blood bank, they are asked to read educational materials. These materials contain information on the risks of infectious diseases transmitted by blood transfusion, including the signs and symptoms of AIDS. Prospective donors are asked to acknowledge in writing that they have read and understood these materials, that they have had the chance to ask questions, and that they have provided accurate information. The prospective donors can elect to leave at this point without donating. (Self-deferral can occur at any point in the donation process when a donor voluntarily chooses not to complete the process.)

Health History

The next step is the giving a detailed health history. The history is designed to ask questions that protect the health of both the donor and the recipient. To ensure that every donor is asked the same questions, the Blood Donor Center uses a uniform donor history questionnaire. In addition to questions about transfusion-transmissible diseases, donors are asked questions to determine whether donating blood might endanger their health. If a prospective donor responds positively to any of these questions, he or she will be deferred or asked not to donate blood.

Physical Examination

The next step in the donation process is a short physical examination that includes checking the blood pressure, pulse, and temperature. A few drops of blood are taken and tested to ensure that anemia is not present. An abnormality found in any part of the physical examination may be a cause for deferral. Donors must also meet the weight requirement of 110 pounds.

The Donation Itself

A donor who passes successfully through these steps proceeds to the actual whole blood donation process. The donor lies down or sits in a reclining chair and the skin covering the inner part of the elbow joint is cleansed. A new, sterile needle connected to plastic tubing and a blood bag is inserted into an arm vein. The donor is asked to squeeze his hand repeatedly to help blood flow from the vein into the blood bag. Ordinarily, one unit of blood, roughly equivalent to a pint, is collected. After the blood is collected, it is sent to the laboratory for testing and component preparation. The donor is taken to an observation area for light refreshments and a brief rest.


People disqualified from donating blood are known as 'deferred' donors. A prospective donor may be deferred at any point during the collection and testing process. Whether or not a person is deferred temporarily or permanently will depend on the specific reason for disqualification. If a person is to be deferred, his or her name is entered into a list of deferred donors maintained by the blood center, often known as the 'deferral registry.' If a deferred donor attempts to give blood before the end of the deferral period, the donor will not be accepted. Once the reason for the deferral no longer exists, the donor may return and be re-entered into the system.

Testing Of Donor Blood For Infectious Disease

An important step in ensuring safety is the screening of donated blood for infectious diseases. Today, nine tests for infectious diseases are conducted on each unit of donated blood. The long-used tests for hepatitis B and syphilis have been supplemented with tests for human immunodeficiency virus (HIV-1 and HIV-2), human T-lymphotropic virus (HTLV-I and -II) and the hepatitis C virus (HCV).
These tests are performed on each unit of blood:

Hepatitis B Surface Antigen (HBsAg)

The hepatitis B virus, which mainly infects the liver, has an inner core and an outer envelope (the surface). The HBsAg test detects the outer envelope, identifying an individual infected with the hepatitis B virus.

Antibodies to the Hepatitis B Core (Anti-HBc)

The anti-HBc test detects an antibody to the hepatitis B virus that is produced during and after infection. If an person has a positive anti-HBc test, but the HBsAg test is negative, it may be that the person once had hepatitis B, but has recovered. Some with a positive test for anti-HBc have not been exposed to the hepatitis B virus (called a 'false positive'), but the person may still be permanently deferred from donating blood.

Antibodies to the Hepatitis C Virus (Anti-HCV)

This test is used to screen donors for the hepatitis C virus (HCV). It works by detecting antibodies manufactured in reaction to portions of the virus called antigens. HCV causes inflammation of the liver, and up to 80 percent of those exposed to the virus develop chronic infection. As in other forms of hepatitis, individuals may be infected with the virus, but not
realize they are carriers.

Antibodies to the Human Immunodeficiency Virus, Types 1 and 2 (Anti-HIV-1, -2)

This test is designed to detect antibodies directed against antigens of the HIV-1 or HIV-2 viruses. HIV-1 is much more common in the United States, while HIV-2 is prevalent in Western Africa. Donors are tested for both viruses because both are transmitted by infected blood, and a few cases of HIV-2 have been identified in the US. Both can cause acquired immunodeficiency syndrome, (AIDS).

Antibodies to Human T-Lymphotropic Virus, Types I and II (Anti-HTLV-I, -II)

This test screens for antibodies directed against portions of the HTLV-I and HTLV-II viruses. Both of these viruses are relatively uncommon in the United States, but do occur more frequently in certain populations. HTLV-I is more common in Japan and the Caribbean. HTLV-II infections are usually associated with intravenous drug usage, especially among people who share needles or syringes.


This test is done to detect infection with the spirochete that causes syphilis. Blood centers began testing for this in the late 1940s when syphilis rates were much higher. The risk of transmitting syphilis through a blood transfusion is exceedingly small because the spirochete is fragile and unlikely to survive blood storage.

Nucleic Acid Amplification Testing (NAT)

NAT employs testing technology that directly detects the genetic material of viruses. Because NAT detects a virus's genetic material it offers the opportunity to reduce the window period during which an infecting agent is undetectable by traditional tests. NAT is being used to detect HIV-1 and HCV, and this technology is under investigation for detecting other infectious agents.

Confirmatory Testing

All the listed tests are referred to as screening tests. Because these tests are so sensitive, some donors may have a false positive result even when he was never exposed. In order to sort out true infections screening tests may be followed with more specific tests called confirmatory tests. These tests help determine whether a donor is truly infected. If a test result from a donated unit of blood is abnormal, the unit is discarded and the donor is notified.

Autologous Blood Donation

Preoperative Donation

The most common autologous donation is the preoperative donation of blood for possible use by the donor during elective surgery. For example, a person might give one unit of blood each week for up to six weeks before surgery,(because blood can be stored in its liquid form for up to 42 days). Patients cannot make autologous donations after 72 hours prior to their surgery so that the body has enough time to replenish its blood volume before the surgical procedure.

A significant amount of iron is removed with each autologous donation. As a result, iron supplements are often prescribed for patients making autologous donations.

Autologous donation is most often employed in surgery on bones, blood vessels, the urinary tract, and the heart, when the likely need for transfusion is high. Autologous blood accounts for nearly 5 percent of all blood donated. Autologous blood donors must be medically stable patients who are free of infection. There is no age limitation and many children and elderly patients have successfully completed autologous donations. The process of donating blood stimulates the bone marrow to produce new blood cells. If there is enough time for recovery, the collected cells may be wholly or partially replaced before surgery.

When blood loss during surgery is less than anticipated, transfusion of autologous blood may not be medically necessary. Although the risk of a complication from autologous blood is low, some residual risk persists. Forty-four percent of autologous donations are unused by the autologous donor. Other stored units are generally are discarded because current standards do not allow transfusion of these units to another patient for safety reasons. In emergencies, however, these units may be used for another patient provided the unit has been fully screened.

Blood Dilution (Hemodilution)

Blood dilution is the removal of one or more units of blood just before surgery for transfusion to the patient during the operation. Hemodilution is used to decrease the loss of red blood cells during surgery. In this procedure, after blood is drawn the patient is given intravenous fluids to compensate for the amount of blood removed. Since the number of red blood cells in the person's circulatory system will have been diluted, fewer red blood cells will be lost from bleeding during the operation. After surgery, the patient's own blood is replaced.

Perioperative Blood Collection

In perioperative blood collection, blood lost by the patient during surgery is recovered and recycled. Most perioperative blood collection programs use machines in which shed blood is collected and the red blood cells are concentrated and washed prior to transfusion. This procedure is widely used for surgical procedures, such as cardiac, vascular, orthopaedic, and trauma or transplant cases. This technique is generally not used in cancer surgery or surgery of the lower gastrointestinal tract.

Postoperative Blood Collection

In postoperative blood collection, blood that is lost in the early postoperative period is collected from a drainage tube at the surgical site and transfused back to the patient, either washed or unwashed. This is used primarily in cardiac and orthopaedic surgery.

Blood Types

There are four major blood types: A, B, AB, and 0. The blood types are determined by proteins called 'antigens' that are found on the surface of the red cells. The percentages of different blood types in the U.S. population are:

  • A+ - 34 percent
  • A- - 6 percent
  • B+ - 9 percent
  • B- - 2 percent
  • AB+ - 3 percent
  • AB- - 1 percent
  • O+ - 38 percent
  • O- - 7 percent

There are two main antigens called A and B. If the A antigen is on the RBC, then it is type A blood. When B antigen is present, it is type B blood. When both A and B antigens are present, it is type AB blood and when neither is present, the blood is type O.

When an antigen is present on the red cell, antibody (also called agglutinin) to the opposite antigen is present in the plasma. For instance, type A blood has anti-type-B antibodies and Type B blood has anti-type-A antibodies. Type AB blood has no antibodies in the plasma, and type O blood has both. These antibodies are absent at birth but are formed during infancy.

Along with the ABO blood group system, there is an Rh blood group system. Though there are many Rh antigens that can be present on the surface of the RBC, the D antigen is the most common. If the D antigen is present, then that blood is Rh+. If not, then the blood is Rh-. In the United States, 85 percent of the population is Rh+ and 15 percent is Rh-. Unlike in the ABO system, the corresponding antibody to the Rh antigen does not develop spontaneously but only when the Rh- person is exposed to Rh antigen by blood transfusion or during pregnancy. For example, when an Rh- mother is pregnant with an Rh+ fetus, then the mother forms antibodies that can travel through the placenta and cause a disease of the newborn called hemolytic disease of the newborn, or 'erythroblastosis fetalis.'

Transfusion Reactions

Before blood is transfused into a patient, the blood type must be determined so that a transfusion reaction does not occur.

Reactions occur when the antigens on the RBCs of the donor blood react with the antibodies present in the recipient's plasma. For example, if donor blood of type A (contains A antigens) is given to someone with type B blood (who has anti-type A antibodies in the plasma), then a transfusion reaction will occur.

The opposite does not occur. It is unusual for the antibodies in the plasma of the donated blood to react to the antigens on the recipients RBCs because very little plasma is transfused and it gets diluted to a level too low to cause a reaction.

If a transfusion reaction occurs, an antibody attaches to antigens on several RBCs causing them to clump together and plug up blood vessels. The body then destroys them through a process called ‘hemolysis,' releasing hemoglobin from the RBCs into the blood. Hemoglobin is broken down into bilirubin, which can cause jaundice. Severe reactions between poorly matched donor and recipient can even be lethal.

When an emergency blood transfusion is necessary and the recipient's blood type is unknown, he can be given type O- blood because type O- blood has no antigen on its surface that could react with antibodies in the recipient's plasma. For this reason people with type O- blood are called a 'universal donors.' People with type AB blood are called a 'universal recipients' because they have no antibodies that could react with donated blood or any type.

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