The Coombs Test

What is a Coombs test?

The Coombs test is used to detect antibodies against red blood cells (erythrocytes). A so-called Coombs serum is used to determine the antibodies. It is obtained from the serum of rabbits and sensitized to human antibodies.
The test is used if haemolytic anemia, rhesus incompatibility or shortly before a blood transfusion is suspected. Hemolytic anemia is a form of anemia and rhesus incompatibility describes a blood group incompatibility.
There are two different types of Coombs test, the direct and indirect test differ in terms of indication and execution.

The reasons for a Coombs test

The direct Coombs test is used if haemolytic anemia is suspected. Hemolytic anemia describes anemia in which the blood cells break down prematurely due to damage.
In autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis or chronic lymphatic leukemia, antibodies are formed that are directed against the body's own blood cells (so-called erythrocytes). The binding of the antibodies leads to the early breakdown of the blood cells or to the clumping of the blood cells in the vessels. Both of these lead to a drop in the hemoglobin value.

Another indication for the direct Coombs test is haemolyticus neonatorum disease, here antibodies against the rhesus factor are formed in the body of the rhesus-negative mother. If the unborn child is rhesus-positive, the antibodies from the maternal one can enter the fetus' circulation and initiate an increased breakdown of the blood cells there. Newborns stand out with severe anemia and jaundice.

The Coombs test is used regularly in transfusion medicine. It is used to determine blood groups before a blood transfusion (bedside test), in which a small blood sample is mixed with various sera from blood groups A, B, AB and 0. If the blood remains fluid, the patient's blood is compatible with the corresponding blood group. The transfusion can be carried out.

The indirect Coombs test can also be used to search for free antibodies in the patient's body (antibody search test) e.g. during maternity check-ups, in preparation for a transfusion for oncological patients or people who have already received a blood transfusion.

Find out all about the topic here: Rh intolerance.

The preparation

In preparation for the Coombs test, sera with antibodies that are to be tested for are first prepared. They are filled into different test tubes or applied to test cards.

In everyday clinical practice e.g. with the bedside test there are already prepared test cards that can be used immediately. Next, blood is drawn from the patient and prepared for the test. Depending on which type of Coombs test is carried out, one needs the blood cells or the serum (liquid part of the blood) of the patient.

The sequence

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 hemolytic anemia or blood group incompatibility in the body. The Coombs serum contains antibodies against human IgG antibodies. The mixture is heated (incubated) for a long time. If there are antibodies on the red blood cells, they clump and the test becomes positive.

The indirect Coombs test consists of two parts. The aim is to determine antibodies that are free in the blood and are not bound to blood cells.
This time the liquid portion (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, they will bind to the red blood cells. This reaction is not visible. Only in the second step, as in the direct Coombs test, is Coombs serum with antibodies against human antibodies added and warmed up. Here, too, the solution clumps if the result is positive.

The evaluation

If antibodies are combined with a suitable Cooms serum, antibodies in the serum (liquid part of the blood) bind with the patient's antibodies. Antibodies are Y-shaped, which means that they can always bind two binding partners and thus cross-link red blood cells with one another. A large accumulation of blood cells is created. These clumps become visible - the blood agglutinates (clumps) and the test is positive.
If the patient does not have the appropriate antibodies, the serum remains liquid. You can't see any change - the test is negative.

What are the alternatives?

An alternative to the Coombs test is the immunological ELISA method (enzyme-linked immunosorbent assay), in which certain antibodies are marked with an enzyme. This antibody is added to the test solution. If the labeled antibodies react with the antibodies in the test solution, the bound enzyme is activated and produces a dye. Corresponding test tubes change color - 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 a positive ELISA.