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 for the determination of the antibodies. It is obtained from the serum of rabbits and is sensitised to human antibodies.
The test is used in suspected cases of haemolytic anaemia, Rhesus incompatibility or shortly before a blood transfusion. Haemolytic anaemia is a form of anaemia and Rhesus incompatibility describes a blood group incompatibility. There are two different types of the Coombs test, the direct and indirect test differ in indication and performance.
The reasons for a Coombs test
The direct Coombs test is used when haemolytic anaemia is suspected. Haemolytic anaemia describes an anaemia in which the blood cells break down prematurely due to damage. In autoimmune diseases such as systemic lupus erxthematosus, rheumatoid arthritis or chronic lymphatic leukaemia, antibodies are formed which are directed against the body’s own blood cells (so-called erythrocytes).
The binding of the antibodies leads to the premature breakdown of the blood cells or to the clumping of the blood cells in the vessels. Both lead to a drop in haemoglobin levels. A further indication for the direct Coombs test is Morbus haemolyticus neonatorum, where the body of the rhesus-negative mother produces antibodies against the rhesus factor.
If the unborn child is rhesus positive, the antibodies from the mother’s body can enter the circulation of the fetus and initiate an increased breakdown of blood cells. Newborns are conspicuous by their severe anaemia and jaundice. The Coombs test is regularly used in transfusion medicine.
It is used to determine the blood group prior to a blood transfusion (bedside test), in which a small blood sample is mixed with various sera of blood groups A, B, AB and 0. If the blood remains liquid, 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 screening test), e.g. during maternity check-ups, in preparation for a transfusion in oncological patients or people who have already received a blood transfusion.
To prepare for the Coombs test, sera containing antibodies to be tested are first prepared. They are filled into different test tubes or applied to test cards. In everyday clinical practice, e.g. for the bedside test, there are already prepared test cards that can be used immediately. Next, blood is taken from the patient and prepared for the test. Depending on which type of Coombs test is carried out, the patient’s blood cells or serum (liquid part of the blood) is required.