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. 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 fight against type B blood. In the United States, about 40% of the white population, 27% of African Americans, 28% of Asians, and 16% of Native Americans are type A.
The B antigen, you have type B blood. Your plasma has antibodies that fight against type A blood. In the U.S., about 11% of the white population, 20% of African Americans, 27% of Asians, and 4% of Native Americans are type B.
Neither the A nor B antigen, you have type O blood. Your plasma has antibodies that fight against both type A and type B blood. In the U.S., about 45% of the white population, 49% of African Americans, 40% of Asians, and 79% of Native Americans are type O.
Both the A and B antigens, you have type AB blood. Your plasma does not have antibodies against type A or type B blood. In the U.S., about 4% of the white population, 4% of African Americans, 5% of Asians, and less than 1% of Native Americans are type AB.
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.
Why It Is Done
A blood type test is done:
Before a person gets a blood transfusion.
Before a person donates blood.
Before a person donates an organ for transplantation.
Before surgery.
When a woman is planning to become pregnant or first becomes pregnant.
To show whether two people could be blood relatives.
To check the identify of a person suspected of committing a crime.
How To Prepare
You do not need to do anything before you have this test.
How It Is Done
The health professional drawing blood will:
Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
Clean the needle site with alcohol.
Put the needle into the vein. If the needle is not placed correctly or if the vein collapses, more than one needle stick may be needed.
Hook a tube to the needle to fill it with blood.
Remove the band from your arm when enough blood is collected.
Put a gauze pad or cotton ball over the needle site as the needle is removed.
Put pressure to the site and then a bandage.
How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
Risks
There is very little chance of a problem from having blood sample taken from a vein.
You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
Results
Blood type tests are done before a person gets a blood transfusion and to check a pregnant woman's blood type. The following table shows the compatibility of blood types between blood donors and recipients.
Read the table as follows: A person who has A-negative blood can receive A-negative or O-negative blood.
Blood types that match A person who has: Can receive:
A- blood ----------- A-, O- blood
A+ blood -----------A-, A+, O-, O+ blood
B- blood -----------B-, O- blood
B+ blood -----------B-, B+, O-, O+ blood
AB- blood -----------AB-, O- blood
AB+ blood -----------AB-, AB+, A-, A+, B-, B+, O-, O+ blood
O- blood -----------O- blood
O+ blood -----------O-, O+ blood
Minor antigens (other than A, B, and Rh) on the red blood cells are also checked for a match before a blood transfusion.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
Taking methyldopa, levodopa, or some antibiotics, such as cephalexin. These medicines may cause a false-positive result.
Having a blood transfusion in the 3 months before the blood type test.
Having a bone marrow transplant.
What To Think About
Severe transfusion reactions are rare today because blood type is always done before a person receives a blood transfusion.
Blood type may be done to check the identity of birth parents. For example, in a paternity case, if the blood types of a mother and her child are checked against blood types of the possible fathers, the real father can sometimes be found by the blood type match. But blood type is more useful in proving that a man is not the father than it is in proving that he is the father. Other tests, such as a human leukocyte antigen (HLA) type, may be done. For more information, see the topic Human Leukocyte Antigen (HLA).
Additional blood tests are done on donated blood before using it. These include hepatitis, syphilis, and human immunodeficiency (HIV). For more information, see the topics Hepatitis B Virus Test, Hepatitis C Virus Test, Syphilis Tests, and Human Immunodeficiency Virus (HIV) Test.
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