The procedure
If a direct Coombs test is performed, the red blood cells are filtered out of the patient’s blood. It is to be tested whether there are antibodies of the IgG type on them, which cause haemolytic anaemia or blood group incompatibility in the body. The Coombs serum contains antibodies against human IgG antibodies.
The mixture is heated (incubated) for a longer time. If there are antibodies on the red blood cells, they will clump together and the test will be positive. The indirect Coombs test consists of two parts.
The aim is to determine antibodies that are freely present in the blood and are not bound to blood cells. This time the liquid part (serum) is filtered from the patient’s blood. First, red blood cells are added to the serum for the test.
If there are free antibodies in the serum, these bind to the red blood cells. This reaction is not visible. Only in the second step, as in the direct Coombs test, Coombs serum with antibodies against human antibodies is added and heated. If the result is positive, the solution also agglutinates.
The evaluation
When antibodies are combined with matching Cooms serum, antibodies of the serum (liquid part of the blood) bind with antibodies of the patient. Antibodies have a Y-shaped structure, which means that they can always bind two binding partners and thus cross-link red blood cells with each other. This results in a large accumulation of blood cells.
These clumps become visible – the blood agglutinates (clumps) and the test is positive. If the patient has no antibodies, the serum remains liquid. No change can be seen – the test is negative.
What are the alternatives?
An alternative to the Coombs test is the immunological ELISA procedure (Enzyme-linked Immunosorbent Assay), in which certain antibodies are marked with an enzyme. This antibody is added to the test solution. If the labelled antibodies react with the antibodies of the test solution, the bound enzyme is activated and produces a dye.
Corresponding test tubes change colour – the test is positive. In addition to the ELISA procedure, there is also the WESTERN-Blot, which is often used as a confirmatory test for positive ELISA.
