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.
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 is an antigen. The full name for this antigen is Rhesus factor.
If a pregnant woman with Rh-negative blood is pregnant with a baby (fetus) with Rh-positive blood, Rh sensitization may occur. The baby may have Rh-positive blood if the father has Rh-positive blood. Rh sensitization happens when the baby's blood mixes with the mother's blood during pregnancy or delivery. This causes the mother's immune system to make antibodies against the 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 woman with Rh-negative blood can get a shot of Rh immunoglobulin (such as WinRho) 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 whose mother has Rh-negative blood. The test shows whether the mother has made antibodies and if the antibodies have moved through the placenta to her 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 woman has Rh-positive or Rh-negative blood (Rh antibody titre) is done early in pregnancy. If she is Rh-negative, steps can be taken to protect the baby.
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 on the site and then put on 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.
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.
Antibody tests (Coombs tests) are done to find antibodies that attack red blood cells.
No antibodies are found. This is called a negative test result.
- Direct Coombs test. A negative test result means that your blood does not have antibodies attached to your red blood cells.
- 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 titre) in a pregnant woman means that she has not developed antibodies against the Rh-positive blood of her baby. This means that Rh sensitization has not occurred.
- Direct Coombs test. A positive result means your blood has antibodies that fight against red blood cells. This can be caused by a transfusion of incompatible blood or may be related to conditions such as hemolytic anemia or hemolytic disease of the newborn (HDN).
- 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 titre test is positive in a woman who is pregnant or is planning to become pregnant, it means that she has antibodies against Rh-positive blood (Rh sensitization). She will be tested early in pregnancy to check the blood type of her baby. If the baby has Rh-positive blood, the mother 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.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Having a blood transfusion in the past.
- Being pregnant within the past 3 months.
- Taking some medicines, such as cephalosporins, sulfa medicines, tuberculosis medicines, insulin, and tetracyclines.
What To Think About
A newborn baby (whose mother has Rh-negative blood) may have a direct Coombs test to check for antibodies against the baby's red blood cells. If the test is positive, the baby may need a transfusion with compatible blood to prevent anemia.
Current as of: September 23, 2020
Author: Healthwise Staff
E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine
Martin J. Gabica MD - Family Medicine