Of the Coombs test detects antibodies against red blood cells in a patient's serum and is used, for example, in a standardized way in the context of maternity care and blood group determination. The test procedure works with rabbit serum and exists in a direct and an indirect form, which are used for various questions.
What is the Coombs Test?
The so-called Coombs test is used to detect antibodies against erythrocytes. The test detects antibodies of the IgG class. These antibodies are considered "incomplete" antibodies and cannot cause the blood cells to stick together. However, because of the pentamer structure, IgM antibodies can induce such a bonding and are therefore called "complete" antibodies.
In the Coombs test, so-called Coombs serum, which is also known as antihuman globulin, is used to detect antibodies. Coombs serum consists of the blood serum of rabbits which have undergone immunization against human antibodies of the IgG class. The test takes place either in the test tube or as part of a microcolumn agglutination. The Coombs test goes back to the Cambridge pathologist Coombs and is mainly used in hematology to diagnose haemolytic anemia. These anemias can affect, for example, newborns with Rh incompatibility.
In transfusion medicine, the test is also used for serological tolerance tests. The term Coombs test basically only refers to the examination technique and thus the use of anti-human globulin. In the text procedure, a direct and indirect form is distinguished.
Function, effect & goals
The direct Coombs test detects IgG that stick to the erythrocytes. As part of the test, the erythrocytes are taken from the patient's blood and freed from plasma. The examiner then adds them to the Coombs serum and incubates them in this way.
If the blood carries antibodies against erythrocytes and these antibodies are bound to erythrocytes, the Coombs serum and its antibodies bind to the human IgG of the test sample. With the addition of a reaction enhancer, agglutination occurs and the test is evaluated as positive. The Coombs indirect test works in a slightly different way. This test consists of two steps and detects antibodies against foreign red blood cells. These antibodies circulate freely in the blood sample and are not bound to erythrocytes. The first step of the indirect test procedure is an incubation of the blood plasma sample with test erythrocytes.
If antibodies are present in the test serum, they bind to the erythrocytes, although no adhesion occurs. In the second step, the Coombs serum is mixed with the test erythrocytes and they stick together. With a positive indirect Coombs test, for example, the rhesus incompatibility can be demonstrated by documenting incomplete antibodies in the mother's blood.
The direct Coombs test only exists in the variant described above and is therefore always aimed at detecting or excluding the antibody load on patient erythrocytes.
The indirect Coombs test is associated with various forms of use, which usually correspond to an antibody test or a serological tolerance test. The indirect test can, however, also be used in the context of further examinations and is then used, for example, to determine various antibody specificities. The test method of the indirect test remains the same, but its name may vary in individual cases with the question of the test. For this reason, an indirect Coombs test cannot be requested from the laboratory, but must specify the purpose or goal of the test.
Risks, side effects & dangers
There are generally few risks or side effects associated with the Coombs test. Taking the blood may be uncomfortable for the patient. Bruising is also possible. However, these times regress within a few days.
Some people feel tired, feel sick, or have a headache when they have blood drawn. As a rule, these symptoms do not last any longer, but rather remit on the same day. In any case, relatively little blood is taken from the patient for the test, so that side effects only occur in extremely rare cases. The Coombs test does not require inpatient care, but can be performed on an outpatient basis. How long it takes the laboratory to perform depends on the type of test method and the objective of the test.
The test is of particular clinical relevance in the case of autoimmune hemolytic anemia, in which the body's own produced antibodies of the immune system cause hemolysis of the erythrocytes and thus cause anemia. The direct Coombs test is usually positive for such diseases. This allows the doctor to make a relatively reliable diagnosis to the patient after a positive test. The situation is different if the test is negative. A negative direct Coombs test does not necessarily mean that the disease is excluded. There is also a Coombs-negative variant of autoimmune hemolytic anemia. In this case, a patient with a negative test must endure further diagnostic procedures.
In the case of the disease described, for example, the determination of autoantibodies or environmental diagnostics are among the further diagnostics after a negative test. In connection with other diseases, a negative Coombs test can definitely be rated as an exclusion. Under certain circumstances, for certain questions, a positive Coombs test may be followed by further examinations, which allow a more precise classification of the phenomenon present or additionally confirm the positive results of the test. Indications for the Coombs test include blood grouping, blood transfusions, maternity care or suspected Rh incompatibility.
.jpg)
