Type and Screen Health Article

Definition

Blood typing is a laboratory test that identifies blood group antigens (substances that stimulate an immune response) belonging to the ABO blood group system. The test classifies blood into four groups designated A, B, AB, and O. Antibody screening is a test to detect atypical antibodies in the serum that have been formed as a result of transfusion or pregnancy. An antibody is a protein produced by lymphocytes that binds to an antigen, facilitating its removal by phagocytosis or lysis. The type and screen (T&S) is performed on persons who may need a transfusion of blood products. These tests are followed by the compatibility test (cross-match). This test insures that no antibodies are detected in the recipient's serum that will react with the donor's red blood cells.

Purpose

Blood typing and screening are most commonly performed to ensure that a person who needs a transfusion will receive blood that matches (is compatible with) his or her own; and that clinically significant antibodies are identified if present. People must receive blood of the same blood type; otherwise, a severe transfusion reaction may result.

Prenatal care

Parents who are expecting a baby have their blood typed to diagnose and prevent hemolytic disease of the newborn (HDN), a type of anemia also known as erythroblastosis fetalis. Babies who have a blood type different from their mothers are at risk for developing this disease.

Determination of paternity

A child inherits factors or genes from each parent that determine his blood type. This fact makes blood typing useful in paternity testing. The blood types of the child, mother, and alleged father are compared to determine paternity.

Forensic investigations

Legal investigations may require typing of blood or such other body fluids as semen or saliva to identify criminal suspects.

Description

Blood typing and screening tests are performed in a blood bank laboratory by technologists trained in blood bank and transfusion services. The tests are performed on blood after it has been separated into cells and serum (the yellow liquid left after the blood cells are removed). Costs for both tests are covered by insurance when the tests are determined to be medically necessary.

Blood bank laboratories are usually located in blood center facilities, such as those operated by the American Red Cross, that collect, process, and supply blood that is donated, as well as in facilities, such as most hospitals, that prepare blood for transfusion. These laboratories are regulated by the United States Food and Drug Administration (FDA) and are often inspected and accredited by a professional association such as the American Association of Blood Banks (AABB).

Blood typing and screening tests are based on the reaction between antigens and antibodies. An antigen can be anything that triggers the body's immune response. The body produces a special protein called an antibody that has a uniquely shaped site that combines with the antigen to neutralize it. A person's body normally does not produce antibodies against its own antigens.

The antigens found on the surface of red blood cells are important because they determine a person's blood type. When red blood cells having a certain blood type antigen are mixed with serum containing antibodies against that antigen, the antibodies combine with and stick to the antigen. In a test tube, this reaction is visible as clumping or aggregating.

Although there are over 600 known red blood cell antigens organized into 22 blood group systems, routine blood typing is usually concerned with only two systems: the ABO and Rh blood group systems. Antibody screening helps to identify antibodies against several other groups of red blood cell antigens.

Blood typing

THE ABO BLOOD GROUP SYSTEM. In 1901, Karl Landsteiner, an Austrian pathologist, randomly combined the serum and red blood cells of his colleagues. From the reactions he observed in test tubes, he discovered the ABO blood group system. This discovery earned him the 1930 Nobel Prize in Medicine. A person's ABO blood type—A, B, AB, or O—is based on the presence or absence of the A and B antigens on his red blood cells. The A blood type has only the A antigen and the B blood type has only the B antigen. The AB blood type has both A and B antigens, and the O blood type has neither the A nor the B antigen.

By the time a person is six months old, he or she will have developed antibodies against the antigens that his or her red blood cells lack. That is, a person with A blood type will have anti-B antibodies, and a person with B blood type will have anti-A antibodies. A person with AB blood type will have neither antibody, but a person with O blood type will have both anti-A and anti-B antibodies. Although the distribution of each of the four ABO blood types varies among racial groups, O is the most common and AB is the least common in all groups.

FORWARD AND REVERSE TYPING. ABO typing is the first test done on blood when it is tested for transfusion. A person must receive ABO-matched blood because ABO incompatibilities are the major cause of fatal transfusion reactions. To guard against these incompatibilities, typing is done in two steps. In the first step, called forward typing, the patient's blood is mixed with serum that contains antibodies against type A blood, then with serum that contains antibodies against type B blood. A determination of the blood type is based on whether or not the blood clots in the presence of these sera.

In reverse typing, the patient's blood serum is mixed with blood that is known to be type A and type B. Again, the presence of clotting is used to determine the type.

An ABO incompatibility between a pregnant woman and her baby is a common cause of HDN but seldom requires treatment. This is because the majority of ABO antibodies are IgM, which are too large to cross the placenta. It is the IgG component that may cause HDN, and this is most often present in the plasma of group O mothers.

Paternity testing compares the ABO blood types of the child, mother, and alleged father. The alleged father cannot be the biological father if the child's blood type requires a gene that neither he nor the mother have. For example, a child with blood type B whose mother has blood type O requires a father with either AB or B blood type; a man with blood type O cannot be the biological father.

In some people, ABO antigens can be detected in body fluids other than blood, such as saliva, sweat, or semen. ABO typing of these fluids provides clues in legal investigations.

THE RH BLOOD GROUP SYSTEM. The Rh, or Rhesus, system was first detected in 1940 by Landsteiner and Wiener when they injected blood from rhesus monkeys into guinea pigs and rabbits. More than 50 antigens have since been discovered that belong to this system, making it the most complex red blood cell antigen system.

In routine blood typing and cross-matching tests, only one of these 50 antigens, the D antigen, also known as the Rh factor or Rh_o[D], is tested for. If the D antigen is present, that person is Rh-positive; if the D antigen is absent, that person is Rh-negative.

Other important antigens in the Rh system are C, c, E, and e. These antigens are not usually tested for in routine blood typing tests. Testing for the presence of these antigens, however, is useful in paternity testing, and when a technologist screens blood to identify unexpected Rh antibodies or find matching blood for a person with antibodies to one or more of these antigens.

Unlike the ABO system, antibodies to Rh antigens don't develop naturally. They develop only as an immune response after a transfusion or during pregnancy. The incidence of the Rh blood types varies between racial groups, but not as widely as the ABO blood types: 85% of whites and 90% of blacks are Rh-positive; 15% of whites and 10% of blacks are Rh-negative.

The distribution of ABO and Rh blood groups in the overall United States population is as follows:

• O Rh-positive, 38%
• O Rh-negative, 7%
• A Rh-positive, 34%
• A Rh-negative, 6%
• B Rh-positive, 9%
• B Rh-negative, 2%
• AB Rh-positive, 3%

• AB Rh-negative, 1%

In transfusions, the Rh system is next in importance after the ABO system. Most Rh-negative people who receive Rh-positive blood will develop anti-D antibodies. A later transfusion of Rh-positive blood may result in a severe or fatal transfusion reaction.

Rh incompatibility is the most common and severe cause of HDN. This incompatibility may occur when an Rh-negative mother and an Rh-positive father have an Rh-positive baby. Cells from the baby can cross the placenta and enter the mother's bloodstream, causing the mother to make anti-D antibodies. Unlike ABO antibodies, the structure of anti-D antibodies makes it likely that they will cross the placenta and enter the baby's bloodstream. There, they can destroy the baby's red blood cells, causing a severe or fatal anemia.

The first step in preventing HDN is to find out the Rh types of the expectant parents. If the mother is Rhnegative and the father is Rh-positive, the baby is at risk for developing HDN. The next step is performing an antibody screen of the mother's serum to make sure she doesn't already have anti-D antibodies from a previous pregnancy or transfusion. Finally, the Rh-negative mother is given an injection of Rh immunoglobulin (RhIg) at 28 weeks of gestation and again after delivery, if the baby is Rh positive. The RhIg attaches to any Rh-positive cells from the baby in the mother's bloodstream, preventing them from triggering anti-D antibody production in the mother. An Rh-negative woman should also receive RhIg following a miscarriage, abortion, or ectopic pregnancy.

OTHER BLOOD GROUP SYSTEMS. Several other blood group systems may be involved in HDN and transfusion reactions, although they are much less frequent than ABO and Rh. Some of the other groups are the Duffy, Kell, Kidd, MNS, and P systems. Tests for antigens from these systems are not included in routine blood typing, but they are commonly used in paternity testing.

Like Rh antibodies, antibodies in these systems do not develop naturally, but as an immune response after transfusion or during pregnancy. An antibody screening test is done before a cross-match to check for unexpected antibodies to antigens in these systems. A person's serum is mixed in a test tube with commercially-prepared cells containing antigens from these systems. If hemagglutination, or clumping, occurs, the antibody is identified.