Coombs test

What is a Coombs test?

The two types of Coombs test are as follows:

• The direct Coombs test involves looking directly at red blood cells found in a sample of blood. The direct Coombs test is sometimes called the direct antiglobulin test (DAT).

• The indirect Coombs test looks at the liquid part of the blood (the plasma). The indirect Coombs test is sometimes called the indirect antiglobulin test (IAT).

Both types of Coombs test are looking for antibodies which may attack red blood cells and lead to them being destroyed.

What is a Coombs test used for?

The direct Coombs test may be used if doctors suspect a person is affected by a type of anaemia called haemolytic anaemia. Haemolytic anaemia is a condition where there are not enough red blood cells in the body because something in the body is destroying them. The Coombs test is done to see if it is the immune system that could be causing the red blood cells to be destroyed.

The indirect Coombs test is used to make sure that blood that has been donated is compatible with the patient who is going to receive it. It is also used to check that a pregnant mother's blood does not contain antibodies that might cause her baby harm. See the separate leaflet called Blood Tests.

How does a Coombs test work?

Direct Coombs test

In a direct Coombs test a special antibody is added to a sample of blood. This test checks whether there are antibodies that have already attached themselves to the surface of the red blood cell. If the antibodies that are added bind to the antibodies on the surface of the cell the test is positive. 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.

Indirect Coombs test

The indirect Coombs test is done on a sample of the liquid part of the blood. It looks for antibodies in the bloodstream that aren't attached to the red blood cells but could bind to certain red blood cells and destroy them.

Antibody response

Your red blood cells have certain proteins on their surface, called antigens. Also, your plasma contains a special type of protein called antibodies, which will attack certain antigens if they are present.

Antigens are like flags to our immune system. They usually identify a substance that is not meant to be in the body (foreign). They can be found on the surface of germs (bacteria) but they can also be found on substances which don't cause disease. For example, they can be found in pollen, blood, or transplanted organs.

The presence of an antigen which is not made by your body causes the immune system to act. This is called an antibody response. 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. However, in some conditions, known as autoimmune diseases, your own body can destroy your own red blood cells.

Transfusion reaction

Human blood is grouped by the different types of antigens that are on the surface of red blood cells. If you receive a blood transfusion, the transfused blood must be the same blood group (type) as yours. It must have the same antigens as those of your red blood cells.

If you receive a transfusion of blood with antigens that are 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 blood group matching is so important.

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.