Sign inWhat is a Coombs' test?
A Coombs' test is a type of blood test. It is named after the person who invented it, Dr Robin Coombs. There are two types of Coombs' test. The direct Coombs' test involves looking directly at red blood cells found in a sample of blood. The indirect Coombs' test looks at the plasma (the liquid part of our blood).
Both types of Coombs' test look for antibodies which may attack red blood cells and cause them to be destroyed.
How does a Coombs' test work?
Red blood cells normally have certain proteins on their surface, called antigens. In some conditions, known as autoimmune diseases, your own body can destroy your red blood cells. Also, your plasma contains antibodies which will attack certain antigens if they are present.
Antigens are like flags to our immune system. They usually identify a molecule as being foreign. They can be found on the surface of bacteria, but they can also be found on substances which don't cause disease. For example, in pollen, blood, or transplanted organs. The presence of an antigen which is not made by your body generates an antibody response. Antibodies are a special type of protein which attack antigens. This is one of the ways our body protects us from illness. It recognises bacteria and viruses by their antigens and destroys them using antibodies.
Human blood is typed by the antigens found on the surface of red blood cells. If you get a blood transfusion, the transfused blood must match your type. It 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.
In a direct Coombs' test a special antibody is added to a sample of blood. This test checks whether there are antibodies already bound to the surface of the red blood cell. If the test is positive the antibodies that are added bind to the antibodies on the surface of the cell. This causes the red blood cells to clump together in the test tube. A positive test shows that part of the immune system is causing red cells to be destroyed.
The indirect Coombs' test is done on a sample of the liquid part of the blood. It detects antibodies that are present in the bloodstream and could bind to certain red blood cells and destroy them.
What is a Coombs' test used for?
The direct Coombs' test may be used if doctors suspect a person is affected by a haemolytic anaemia. Haemolytic anaemia is a condition where there are not enough red blood cells because they are being destroyed by something within the body. The Coomb's test is often done to see if the immune system could be causing red blood cells to be destroyed.
The indirect Coombs' test is used to make sure donor blood is compatible with the patient who might receive it. It is also used to check that a pregnant mother's blood does not contain antibodies that might cause her baby harm. For more information see separate leaflet called 'Blood Test - Blood Grouping' .
What happens during a Coombs' test?
A Coombs' test involves taking a sample of blood. The blood sample is then sent to the laboratory where the Coombs' test is carried out.