Blood Type Test

Test Overview

Blood type tests are done before a person gets a blood transfusion and to check a pregnant woman's blood type. Human blood is typed by certain markers (called antigens) on the surface of red blood cells. Blood type may also be done to see if two people are likely to be blood relatives.

The most important antigens are blood group antigens (ABO) and the Rh antigen, which is either present (positive, +) or absent (negative, -). So the two most common blood type tests are the ABO and Rh tests.

ABO test

The ABO test shows that people have one of four blood types: A, B, AB, or O. If your red blood cells have:

• The A antigen, you have type A blood. The liquid portion of your blood (plasma) has antibodies that attack type B blood. About 42% of people (42 in 100) in the United States have type A blood, with 6% having A-negative (A-) blood and 36% having A-positive (A+) blood.^{footnote 1}
• The B antigen, you have type B blood. Your plasma has antibodies that attack type A blood. About 10% of people (10 in 100) in the U.S. have type B blood, with 2% having B-negative (B-) blood and 8% having B-positive (B+) blood.^{footnote 1}
• Neither the A nor B antigen, you have type O blood. Your plasma has antibodies that attack both type A and type B blood. About 44% of people (44 in 100) in the U.S. have type O blood, with 7% having O-negative (O-) blood and 37% having O-positive (O+) blood.^{footnote 1}
• Both the A and B antigens, you have type AB blood. Your plasma does not have antibodies against type A or type B blood. About 4% of people (4 in 100) in the U.S. have type AB blood, with 1% having AB-negative (AB-) blood and 3% having AB-positive (AB+) blood.^{footnote 1}

Blood received in a transfusion must have the same antigens as yours (compatible blood). If you get a transfusion that has different antigens (incompatible blood), the antibodies in your plasma will destroy the donor blood cells. This is called a transfusion reaction, and it occurs immediately when incompatible blood is transfused. A transfusion reaction can be mild or cause a serious illness and even death.

Type O-negative blood does not have any antigens. It is called the "universal donor" type because it is compatible with any blood type. Type AB-positive blood is called the "universal recipient" type because a person who has it can receive blood of any type. Although "universal donor" and "universal recipient" types may be used to classify blood in an emergency, blood type tests are always done to prevent transfusion reactions.

Minor antigens (other than A, B, and Rh) that occur on red blood cells can sometimes also cause problems and so are also checked for a match before giving a blood transfusion.

Serious transfusion reactions are rare today because of blood type tests.

Rh test

Rh blood type checks for the Rh antigen (also called the Rh factor) on red blood cells. If your red blood cells:

• Have the Rh antigen, your blood is Rh-positive.
• Do not have the Rh antigen, your blood is Rh-negative.

For example, if you have the A and Rh antigens, your blood type is A-positive (A+). If your blood has the B antigen but not the Rh antigen, your blood type is B-negative (B-).

Rh blood type is especially important for pregnant women. A problem can occur when a woman who has Rh-negative blood becomes pregnant with a baby (fetus) that has Rh-positive blood. This is called Rh incompatibility. If the blood of an Rh-positive baby mixes with the blood of an Rh-negative mother during pregnancy or delivery, the mother's immune system makes antibodies. This antibody response is called Rh sensitization and, depending on when it occurs, can destroy the baby's red blood cells.

Rh sensitization does not generally affect the health of the baby during the pregnancy in which the sensitization occurs. But the health of a baby with Rh-positive blood during a future pregnancy is more likely to be affected. After sensitization has occurred, the baby can develop mild to severe problems (called Rh disease or erythroblastosis fetalis). In rare cases, if Rh disease is not treated, the baby may die.

An Rh test is done in early pregnancy to check a woman's blood type. If she is Rh-negative, she can get a shot of Rh immunoglobulin that almost always prevents sensitization from occurring. Problems from Rh sensitization have become very rare since Rh immunoglobulin was developed.