The Coombs test is a type of blood test that assesses the presence of specific antibodies that attack red blood cells, causing their destruction and potentially leading to the emergence of a type of anemia known as hemolytic.
The Coombs test can be direct, when you want to evaluate the red blood cells, or indirect, when the blood serum is evaluated.
In addition to anemia, this test can also help identify other diseases that affect blood cells such as leukemia, lupus, mononucleosis and erythroblastosis fetalis, also known as hemolytic disease of the newborn, as well as identifying the risk of transfusion reactions.
Types of Coombs test
Depending on what you want to evaluate, the Coombs test can be classified into two types:
- Direct Coombs test: directly evaluates red blood cells, checking whether there are antibodies linked to the red blood cell and whether these antibodies are derived from the person’s own immune system or received through transfusion. This test is typically performed to detect autoimmune hemolytic anemia. Know the symptoms of hemolytic anemia;
- Indirect Coombs test: evaluates the blood serum, identifying the antibodies present there, and is generally requested in transfusion situations, to ensure that the blood that will be donated is compatible with the person receiving it.
Regardless of the type of Coombs test to be performed, no preparation is necessary to perform the exam. However, some medications can interfere with the results, so it is important to inform your doctor about their use so that guidance can be given regarding their discontinuation.
How the exam is carried out
The Coombs test is performed using a blood sample, which must be collected in a clinical analysis laboratory. The collected blood is sent to the laboratory, where the direct or indirect Coombs test will be carried out, depending on the objective.
In the direct Coombs test, the Coombs reagent is added to the patient’s blood, allowing the visualization of antibodies that may be linked to the red blood cells. In the indirect Coombs test, the blood is collected and centrifuged, separating the red blood cells from the serum, which contains the antibodies. Red blood cells ‘pre-marked’ with antibodies are added to the serum with the aim of checking whether there are autoantibodies present in the serum and, consequently, in the patient’s blood.
When is indicated
The Coombs test is done to investigate the following situations:
- Autoimmune hemolytic anemia;
- Drug-induced immune hemolytic anemia;
- Reaction to blood transfusion;
- Hemolytic disease of the newborn;
- Systemic lupus erythematosus, without hemolytic anemia.
The indirect Coombs test may also be requested by the obstetrician during the first trimester of pregnancy to check for antibodies against the baby’s Rh-positive blood in women who are Rh-negative, which can cause erythroblastosis fetalis. Learn more about erythroblastosis fetalis.
What does the result mean
The Coombs test result can be positive or negative:
1. Negative Coombs
The result of the Coombs test is negative when the antibody that causes the destruction of the red globes is not present, and is therefore considered a normal result.
2. Coombs positive
When the result is positive it means that there is an antibody in the blood and, therefore, if the result is positive in the direct Coombs test it means that the person may have a disease such as:
- Autoimmune hemolytic anemia;
- Infection by Mycoplasma sp.;
- Syphilis;
- Leukemia;
- Lupus erythematosus;
- Mononucleosis.
In the case of an indirect Coombs test, a positive result means that the person has an antibody that can cause clots when receiving another type of blood and, therefore, care must be taken when carrying out a blood transfusion.
In any case, it is always important that the result is evaluated by the doctor who ordered it, as some of the person’s history may alter the result.
