Coombs tests are done to find certain antibodies that attack red blood cells. Antibodies are proteins made by the immune system. Normally, antibodies bind to foreign substances, such as bacteria and viruses, and cause them to be destroyed.
The following conditions cause antibodies to be made.
Transfusion reaction
Human blood is typed by certain markers (called antigens) on the surface of red blood cells. If you get a blood transfusion, the transfused blood must match your type. That means the transfused blood must have the same antigens as your red blood cells. If you get a transfusion of blood with antigens different from yours (incompatible blood), your immune system destroys the transfused blood cells. This is called a transfusion reaction and can cause serious illness or even death. This is why matching blood type is so important.
Rh sensitization
Rh is an antigen. The full name for this antigen is Rhesus factor.
If you have Rh-negative blood and are pregnant with a baby (fetus) that has Rh-positive blood, Rh sensitization may occur. Your baby may have Rh-positive blood if the other birth parent has Rh-positive blood. Rh sensitization happens when the baby's blood mixes with your Rh-negative blood during pregnancy or delivery. This causes your immune system to make antibodies against a baby's red blood cells in future pregnancies. This antibody response is called Rh sensitization and, depending on when it happens, can destroy the red blood cells of the baby before or after it is born. If sensitization happens, a fetus or newborn can develop mild to severe problems (called Rh disease or erythroblastosis fetalis). In rare cases, if Rh disease is not treated, the fetus or newborn may die.
A person with Rh-negative blood can get a shot of Rh immunoglobulin (such as RhoGAM) that almost always stops sensitization from occurring. Problems from Rh sensitization have become very rare since Rh immunoglobulin was developed.
Autoimmune hemolytic anemia
A type of hemolytic anemia called autoimmune hemolytic anemia is a rare disease that causes antibodies to be made against a person's own red blood cells.
Two blood tests can check for antibodies that attack red blood cells: the direct Coombs test and the indirect Coombs test. The direct Coombs test is done on a sample of red blood cells from the body. It detects antibodies that are already attached to red blood cells. The indirect Coombs test is done on a sample of the liquid part of the blood (serum). It detects antibodies that are present in the bloodstream and could bind to certain red blood cells, leading to problems if blood mixing occurs.
Why It Is Done
Direct Coombs test
The direct Coombs test finds antibodies attached to your red blood cells. The antibodies may be those your body made because of disease or those you get in a blood transfusion.
The direct Coombs test also may be done on a newborn baby with Rh-positive blood if the person who gave birth has Rh-negative blood. The test shows whether the person made antibodies and if the antibodies moved through the placenta to the baby.
Indirect Coombs test
The indirect Coombs test finds certain antibodies that are in the liquid part of your blood (serum). These antibodies can attack red blood cells but are not attached to your red blood cells. The indirect Coombs test is commonly done to find antibodies in a recipient's or donor's blood before a transfusion.
A test to determine whether a person who is pregnant has Rh-positive or Rh-negative blood (Rh antibody titer) is done early in pregnancy. If they are Rh-negative, steps can be taken to protect the baby.
How To Prepare
In general, there's nothing you have to do before this test, unless your doctor tells you to.
How It Is Done
A health professional uses a needle to take a blood sample, usually from the arm.
How long the test takes
The test will take a few minutes.
Watch
How It Feels
When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.
Risks
There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.
Results
Normal
No antibodies are found. This is called a negative test result.
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Direct Coombs test. A negative test result means that your blood does not have antibodies attached to your red blood cells.
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Indirect Coombs test. A negative test result means that your blood is compatible with the blood you are to receive by transfusion. A negative indirect Coombs test for Rh factor (Rh antibody titer) in someone who is pregnant means that they have not developed antibodies against the Rh-positive blood of the baby. This means that Rh sensitization has not occurred.
Abnormal
Direct Coombs test. A positive result means that your blood has antibodies that fight against red blood cells. This can be caused by a transfusion of incompatible blood. Or it may be related to conditions such as autoimmune hemolytic anemia or hemolytic disease of the newborn (HDN).
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Indirect Coombs test. A positive test result means that your blood is incompatible with the donor's blood and you can't receive blood from that person. If the Rh antibody titer test is positive in someone who is pregnant or is planning to become pregnant, it means that they have antibodies against Rh-positive blood (Rh sensitization). The person will be tested early in pregnancy to check the blood type of the baby. If the baby has Rh-positive blood, the person will be watched closely throughout the pregnancy to prevent problems to the baby's red blood cells. If sensitization has not occurred, it can be prevented by a shot of Rh immunoglobulin.
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Current as of: September 25, 2025