Coombs test – an antiglobulin test aimed at detecting incomplete anti-erythrocyte antibodies to the Rh factor in Rh-negative blood – a specific protein that is located on the surface of red blood cells of Rh-positive blood. There are two types of this test: direct – detection of antibodies on the surface of red blood cells, indirect – detection of antibodies in blood serum. A direct test is carried out during the diagnosis and monitoring of the treatment of blood diseases: hemolytic anemia, hemolytic disease of newborns and others. An indirect test is performed to assess the compatibility of donor and recipient blood during transfusion, as well as to determine the presence and risk of Rh conflict during pregnancy planning and management. The material for the Coombs test is venous blood, the study is carried out by methods based on the agglutination reaction. Normally, both samples give a negative result. The analysis is performed within one day.
The Coombs test is a clinical study of Rh-negative blood aimed at detecting antibodies to the Rh factor. The sample is used to identify the risk of Rh conflict and hemolytic reactions. In each person, the surface of red blood cells contains a certain set of antigens or agglutinogens – compounds of various nature, by the presence or absence of which the blood group and Rh factor are judged. There are many types of antigens, in medical practice, agglutinogens A and B, which determine the blood group, and agglutinogen D, the Rh factor, are of the greatest practical importance. With a positive Rh factor, D antigens are detected on the outer membrane of erythrocytes, with a negative one – no.
The Coombs test, which is also called an antiglobulin test, is aimed at detecting incomplete anti-erythrocyte antibodies to the Rh factor system in the blood. Antibodies to the Rh factor are specific immunoglobulins that are produced in Rh-negative blood when erythrocytes with agglutinogens D enter it. This can happen when fetal and pregnant blood is mixed, with blood transfusions performed without prior blood typing. The Coombs test exists in two versions – direct and indirect. When performing a direct Coombs test, antibodies fixed on the surface of red blood cells are detected. The study is used to determine the cause of the hemolytic reaction. The indirect Coombs test is aimed at detecting anti-erythrocyte antibodies in blood plasma. It is necessary to determine the compatibility of the blood of the donor and recipient or mother and fetus, allows you to prevent the development of Rh conflict and subsequent hemolysis of red blood cells.
Blood for both versions of the Coombs test is taken from a vein. The analysis is performed by agglutination using antiglobulin serum. The results of the study are used in hematology in identifying the causes of hemolytic reactions, in surgery and resuscitation during hemotransfusions, in obstetrics and gynecology in monitoring pregnancies in women with Rh-negative blood.
The direct Coombs test, which detects antibodies fixed on the surface of red blood cells, is prescribed for hemolytic reactions (destruction of red blood cells) of various origins. The study is indicated for primary autoimmune hemolytic anemia, posttransfusion hemolytic anemia, hemolytic disease of the newborn, hemolysis of erythrocytes caused by autoimmune, tumor or infectious diseases, as well as taking medications, for example, quinidine, methyldopa, procainamide. The indirect Coombs test, which detects antibodies in blood plasma, is used to prevent the development of rhesus conflict. It is indicated for patients in preparation for blood transfusions, as well as pregnant women with a negative Rh factor, provided that the future father of the child has a positive Rh factor.
In order to determine Rh compatibility, the Coombs test is not prescribed to patients with Rh-positive blood. In these cases, antigens are already present on the surface of red blood cells, the production of antibodies cannot be provoked by hemotransfusion or fetal blood entering the bloodstream of a pregnant woman. Also, the study is not shown to pregnant women if both parents have a negative Rh factor – an inherited recessive trait. A child in such pairs always has Rh-negative blood, an immunological conflict with the mother is impossible. In hemolytic pathologies, an antiglobulin test is not used to monitor the success of therapy, since the results do not reflect the activity of the process of destruction of red blood cells.
The limitation of the Coombs test is the complexity of the research procedure – in order to obtain reliable results, it is necessary to comply with temperature and time regimes, rules for the preparation of reagents and biomaterial. The advantages of the Coombs test include its high sensitivity. In hemolytic anemia, the results of this analysis remain positive, even if the hemoglobin, bilirubin and reticulocyte counts normalize.
Preparation for analysis and material collection
The material for performing the Coombs test is venous blood. There are no special requirements for the time of the blood collection procedure and for the preparation of the patient. As with any study, it is recommended to take a break after eating for at least 4 hours, and in the last 30 minutes to give up smoking, physical exertion, and avoid emotional stress. It is also worth discussing with your doctor in advance the need to cancel your medications – some medications may distort the results of the Coombs test. Blood is taken with a syringe from the ulnar vein, less often from the vein on the back of the hand. Within a few hours, the material is delivered to the laboratory.
When performing a direct Coombs test, antiglobulin serum is added to the patient’s blood serum. After a while, the mixture is examined for the presence of agglutinates – they are formed if there are antibodies on red blood cells. If the result is positive, the agglutinating titer is determined. The indirect Coombs test consists of more stages. First, the antibodies in the serum are fixed on the injected erythrocytes during incubation. Then antiglobulin serum is added to the sample, after a while the presence and titer of agglutinates are determined. The timing of the analysis is 1 day.
Normally, the result of the direct Coombs test is negative (-). This means that there are no antibodies associated with red blood cells in the blood, and they cannot be the cause of hemolysis. The normal result of the indirect Coombs test is also negative (-), that is, there are no antibodies to the Rh factor in the blood plasma. When preparing for hemotransfusion for the recipient, this means compatibility with the donor’s blood, when monitoring pregnancy – the absence of Rh-sensitization of the mother, low risk of immunological conflict. Physiological factors, such as eating habits or physical activity, cannot affect the result of the test. Therefore, if the result is positive, a doctor’s consultation is necessary.
Diagnostic value of the analysis
The positive result of the Coombs test is expressed qualitatively, from (+) to (++++), or quantitatively, in titers from 1:16 to 1:256. Determination of the concentration of antibodies on erythrocytes and in blood serum is performed in both types of samples. With a positive result of the direct Coombs test, antibodies are detected on the outer membrane of red blood cells, which lead to the destruction of these blood cells. The cause may be blood transfusion without prior typing – posttransfusion hemolytic reaction, as well as erythroblastosis of the newborn, hemolytic reaction due to the use of drugs, primary or secondary autoimmune hemolytic anemia. Secondary destruction of erythrocytes can be caused by systemic lupus erythematosus, Evans syndrome, Waldenstrom’s macroglobulinemia, paroxysmal cold hemoglobinuria, chronic lymphocytic leukemia, lymphoma, infectious mononucleosis, syphilis, mycoplasma pneumonia.
A positive result of the indirect Coombs test indicates the presence of antibodies to the Rh factor in the plasma. In practice, this means that Rh-sensitization has occurred, there is a possibility of developing Rh-conflict after the infusion of donor blood, during pregnancy. To prevent pregnancy complications, women with a positive result of the Coombs test are placed on a special account.
Treatment of abnormalities
The Coombs test refers to isoserological studies. Its results make it possible to identify a hemolytic reaction, as well as to determine the compatibility of the blood of the donor and recipient, mother and fetus, in order to prevent the development of Rh conflict. If the test result is positive, then it is necessary to seek advice from the attending physician – an obstetrician-gynecologist, hematologist, surgeon.