Direct and indirect Coombs test: what is it and how is it performed

The Coombs test is a diagnostic test that seeks to identify the presence of antibodies fixed in the red blood cells and causing their premature destruction. Red blood cells are the most abundant component in the blood and are responsible for transporting oxygen to all cells in the body, so their prompt destruction would pose a risk to the health of the person.

There are two types of Coombs test:

• Indirect Coombs test. It is also known as an indirect antiglobulin test. This test is used to detect those antibodies that act freely against certain red blood cells.
• Direct Coombs test. Detects antibodies attached to the surface of red blood cells and causing their destruction.

• The indirect Coombs test is performed whenever a blood transfusion is to be performed. If the test is positive and antibodies are detected, it will be necessary to identify by another test which antibody it is.
• If after performing a blood transfusion the person has any type of allergy or unfavorable reaction, it is likely that doctors will perform an indirect and a direct Coombs test to determine the cause of the reaction. Similarly, once the condition is controlled, an indirect Coombs test can be performed again to check the amount of antibodies.
• This test is also used during pregnancy , since it allows to determine if there are antibodies that can cross the placenta and attack the cells of the fetus, causing a condition known as hemolytic disease of the newborn.
• The indirect Coombs test allows to identify in pregnant women whose blood type is Rh negative, if there is the presence of Rh antibody . An Rh negative mother can develop antibodies if she is in contact with the red blood cells of a baby whose blood type is Rh positive. In these cases, the indirect Coombs test is usually performed at 28 weeks of gestation and before delivery as a preventive method.
• The diagnosis of autoimmune hemolytic anemia is another function of the indirect Coombs test. This condition occurs when the body produces antibodies against its own antigens, causing not only anemia, but also causing lupus, chronic lymphocytic leukemia , mycoplasma pneumonia, and mononucleosis.

• The main function of the direct Coombs test is to determine if a person with hemolytic anemia (destruction of red blood cells is greater than production) is caused by antibodies that react negatively to red blood cells. This occurs in the case of autoimmune hemolytic anemia, where the body self-destroys red blood cells.
• Diagnosing hemolytic disease of the newborn is another function of the direct Coombs test. This condition, which we explained above, is caused by blood incompatibility between mother and baby, is not usually frequent since it can be prevented with the indirect Coombs test.
• Like the indirect Coombs test, the direct test is also used to find out the cause of an adverse reaction after a blood transfusion, indicating whether the person has generated antibodies against the red blood cells contained in the donor’s blood.

Normal values

When the Coombs test, direct or indirect, gives a result within the range, it can be ensured that there is no clumping of cells and, therefore, there are no antibodies that attack the red blood cells.

Positive direct Coombs test

This result determines the existence of antibodies that act against red blood cells. Once the results are received, the medical specialist must look for the cause of said disorder, among which are:

• Hemolytic anemia.
• Chronic lymphocytic leukemia.
• Hemolytic disease of the newborn.
• Infectious mononucleosis.
• Mycoplasma infection.
• Adverse transfusion reactions.
• Lupus.
• Syphilis.
• Old age.

Positive indirect Coombs test

It means that there are antibodies in the body that act against red blood cells because they are interpreted as foreign bodies. This can be caused by:

• Autoimmune hemolytic anemia caused by drugs.
• Blood incompatibility.
• Hemolytic disease of the newborn.

Both tests are carried out in a chemical laboratory with the extraction of a blood sample from the person.

This article is merely informative, at FastlyHeal .com we do not have the power to prescribe medical treatments or make any type of diagnosis. We invite you to see a doctor in the case of presenting any type of condition or discomfort.

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