What is a Coombs' Test?
29 September 2014
Patients with insufficient amount of red blood cells in their body may experience fatigue, tiredness, cold hands and feet, pale skin and shortness of breath. Low red blood cells lead to the condition of anaemia, which may require a blood transfusion.
To find out what kind of anaemia you have, a Coombs’ test is carried out.
What Does a Coombs’ Test Do?
The test will determine if the body contains certain antibodies in the blood stream. When your immune system detects that something is or could be harmful to your body, it produces the protein antibodies.
However, the immune system could be wrong and create antibodies by incorrect detection, which can cause other health problems, and not destroy what is harmful.
The Coombs’ test will determine if your antibodies are attacking and destroying your own red blood cells.
Types of Coombs’ Tests
There are two types of Coombs’ tests, the direct and indirect tests.
• The direct test is the most common; it is done from taking a blood sample from the patient via an injection. This test will check if any antibodies are attached to the surface of your red blood cells.
The red blood cells are washed (removed of patient serum), and then separated by antihuman globulin. If there is immunoglobulin in the red blood cell surface in-vivo, the direct Coombs test will show positive, as the antihuman globulin will have agglutinated.
When red blood cells are being destroyed, a condition called haemolytic anaemia is created; it is lead by haemolysis and is when the bone marrow activity cannot compensate for the increase in loss of red blood cells.
• The indirect test is used to detect in-vitro antibody-antigen reactions, and to check for unattached antibodies that are floating in the bloodstream as apposed to being attached to the red blood cells.
Prior to blood transfusions, an indirect Coombs’ test will be carried out to see if there are low concentrations of antibodies in the patient’s serum.
The diagram below by Aria Rad shows how the tests are carried:
At Lorne Laboratory, Coombs’ control cells are made up of a 2-3% suspension of single donor group O red cells that have been removed of all blood group antibodies and resuspended in a preservative solution.
They are used after the Coombs’ tests to check the test was done properly when the test results are negative. They are cells coated with IgG antibody, and if agglutination continues after Coombs’ control cells check, this verifies the negative results.
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