How blood transfusions are done

A transfusion is the infusion of a blood component through tubing connected to a needle or fine tube (catheter) that is in a vein. The amount and part of the blood transfused depends on what the patient needs.

First, lab tests such as a complete blood count (CBC) are done to find out if the patient’s symptoms are likely to be helped by a transfusion. This test measure the levels of components within the blood such as red blood cells, white blood cells, and platelets. Coagulation (clotting) tests may also be done if abnormal bleeding is a problem.

If a transfusion is needed, more blood tests must be done to find a donated blood component that closely matches the patient.

Blood types

Blood types are important when it comes to transfusions. If you get a transfusion that does not work with your blood type, your body’s immune system could mount a defense against the donated blood. This can cause a serious or even life-threatening transfusion reaction (described in the section called “Possible risks of blood transfusion”).

To be sure no mistakes are made, donated blood is carefully “typed” (tested to find out what type it is) both when it is taken from the donor and again once it is received by the hospital lab. The blood bag is labeled with the type of blood it contains. When a person needs a blood transfusion, a blood sample is drawn from him or her and tested the same way.

All blood has the same components, but not all blood is the same. People have different blood types, which are based on substances called antigens on a person’s blood cells. The 2 most important antigen factors in blood typing are called ABO and Rh.

Each person is an ABO blood type — A, B, AB, or O — which means antigen A, antigen B, both antigens, or neither antigen is found on their blood cells. Each person also is either Rh-positive or Rh-negative (you either have it or you don’t). These 2 factors can be combined into 8 possible blood types.

ABO blood types

Two antigens on blood cells (A and B) determine a person’s ABO blood type (A, B, AB, or O). In the United States, the most common blood type is O, followed closely by type A.

• If you have type O blood, you can only get type O red blood cell transfusions. But you can give your red blood cells to people with type A, B, AB, or O blood, which is why you are sometimes called a universal donor. (Universal donor blood is only used in extreme emergencies. For example, if a person is bleeding severely and nearing death, there may no time for testing. In everyday practice, people in the US are always given the exact same type of red blood cells that they have.)
• If you have type A blood, you cannot get either type B or AB red blood cells.
• If you have a type B blood, you cannot get type A or AB red blood cells.
• If you are a type AB blood, you can get transfusions of O, A, B, or AB red blood cells.

Rh factor

Blood is either Rh-positive or Rh-negative, depending on whether the red blood cells have Rh antigens on their surface. A person that has type A, Rh-positive blood is called A positive, whereas a person with type A, Rh-negative blood would be A negative.

If you have Rh-positive blood, you can get Rh-positive or Rh-negative red blood cell transfusions. But people with Rh-negative blood should only get Rh-negative red blood cells except in emergencies. This is because an Rh-positive blood transfusion can cause a person with Rh negative blood to make antibodies against the Rh factor. If an Rh-negative woman makes antibodies like this, it can harm any Rh-positive babies she may have in the future. Her anti-Rh antibodies can attack Rh-positive blood cells in the fetus.

Other antigens

There are other antigens on red blood cells that can lead to transfusion reactions. These are rare because people do not make antibodies against them unless they have had transfusions before. Still, these antigens may become a factor in matching blood for a person who has had many transfusions in the past, as is the case for some people with cancer.

Plasma, platelets, and blood type

Plasma transfusions follow a different set of rules than red blood cells and whole blood (based on the antibodies in the plasma):

• People with type O blood can get any type of plasma.
• People with type A blood can get A or AB plasma.
• People with type B blood can get B or AB plasma.
• People with type AB blood can get only AB plasma.

For platelet and cryoprecipitate transfusions, matching the blood type of the donor to the recipient is usually not critical, but labs still try to match them. This may become important for patients who have already had many transfusions or who have had transfusion reactions in the past.

Antibodies and cross-matching

After blood is typed, a test called an antibody screen is done to see if a patient’s plasma contains other antibodies besides those against A, B, and Rh. If there are extra antibodies, the cross-matching may take longer. This is because some units of donor blood may not fully match the recipient’s, even though they have the same ABO and Rh types.

Before a person can get a transfusion of red blood cells, one more lab test called a cross-match must be done to make sure that the donor blood is compatible with the recipient’s.

A unit of the proper ABO and Rh type is selected, and a drop of donor blood is mixed with a drop of plasma from the patient. The mixture is watched to see if the patient’s plasma causes the donor blood cells to clump. This may happen if the patient has extra antibodies to a protein in the donor unit. If there are no problems (no clumping), a cross-match takes about 30 minutes.

A cross-match is usually not needed for a platelet or plasma transfusion unless the platelets look like they contain some red blood cells.

The transfusion process

Most blood transfusions are given in the hospital or in outpatient clinics. Acetaminophen (Tylenol^®) and diphenhydramine (Benadryl^®) are often given before a transfusion to help reduce the symptoms of minor transfusion reactions.

Red blood cell transfusions are usually started at a slow rate while the patient is watched closely for the signs and symptoms of a transfusion reaction.

Vital signs (such as temperature, heart rate, and blood pressure) are checked often. Each unit of blood is usually transfused over a couple of hours. Other components, like plasma and platelets, go in much faster — smaller volumes take much less time.

A visiting nurse can give transfusions in the home if precautions are taken to be sure the patient is kept safe. Patients who get home transfusions are often very sick, not able to travel to a health care facility, and need frequent transfusions for a long time.

The same standards that apply to hospital transfusions must be followed in the home. A doctor must be sure that a patient’s heart and lung function are stable before they can be transfused at home. Emergency medical care must be available close by in case it is needed. Also, the blood must be kept at the proper temperature while being taken to the home.