Coombs Test: Procedure, Conditions That Lead To Positive Coombs Test and Risks Associated With It

Coombs test is a blood test done to find out the type of anemia a person is suffering from. It is also known as antiglobulin testing. ⁽¹⁾

If someone is fatigued, has shortness of breath, cold hands and feet, and paleness in the skin, it indicates anemia i.e. insufficiency of red blood cells. If low red blood cells are confirmed, a Coombs test is ordered to confirm the kind of anemia that person is suffering from.

Coombs test checks certain antibodies in the blood. These antibodies are the proteins made by the immune system when something harmful to health is detected. These antibodies protect the body against harmful invaders by destroying them. Sometimes these antibodies destroy the red blood cells and lead to hemolytic anemia. ⁽²⁾

Coombs test is of two types, direct and indirect.

Direct Coombs test or direct antiglobulin test checks for the antibodies that are attached to the surface of the red blood cells. The reagent used causes the red blood cells with attached antibodies to clump together. It is used for testing hemolytic anemia, transfusion reaction, and erythroblastosis fetalis.

Indirect Coombs test or indirect antiglobulin test checks for the antibodies that are floating in the serum i.e. the liquid part of the blood. During the test, the red blood cells are removed from the sample and the foreign red blood cells that attract the antibodies are introduced. The reagent used in the test clump the new blood cells together, which means there are antibodies in the serum. It is done to test prenatal blood testing and pre-transfusion blood typing.

For the Coombs test, a sample of blood is taken by venipuncture and the blood is tested with antibodies that react with the blood.

This test is often done on infants who might have antibodies in blood if their parent has a different blood type. In them a sample is obtained by doing a small puncture on the heel and blood is collected in a small glass tube or a glass slide.

Coombs test is done to detect the agglutination of the red blood cells. In a negative test, no clumps are detected, which means no antibodies were detected, or if detected they were insignificant. The test is positive if agglutination is present, which indicates the presence of antibodies.

There are several reasons that may lead to a positive Coombs test.

Autoimmune conditions that may lead to hemolysis include: ⁽³⁾

• Autoimmune hemolytic anemia in which the immune system reacts with the red blood cells
• Chronic lymphocytic leukemia
• Systemic lupus erythematosus, which is a common type of lupus
• Antiphospholipid syndrome, a condition in which the blood clots easily

In case the blood transfused is not of a proper match, the immune system may attack the donated blood. Coombs test can detect this condition as early as 2 days after transfusion.

An indirect Coombs test done prior to transfusion can help in such a case.

This condition occurs when the birthing parent and fetus have different blood types like Rh factor positive or negative or ABO type differences. In this case, the birthing parent’s blood system may attack the baby’s blood during labor. This is known as hemolysis disease of the fetus and newborn. It may lead to a high bilirubin level and cause jaundice.

This condition should be properly watched as it can lead to the death of the newborn or the birthing parent. Coombs test is often performed in a pregnant person to check for the antibodies before labor.

Sometimes the body can catch infections that cause the antibodies to attach to the red blood cells. These conditions include mononucleosis, malaria, syphilis, HIV, and infection with mycoplasma.

Drug toxicity can lead to antibodies on the red blood cells. These drugs include cephalosporin, levodopa, dapsone, nitrofurantoin, nonsteroidal anti-inflammatory drugs, and quinidine.

Sometimes, especially in older adults, the Coombs test may be abnormal even without any disease or risk factor.

There is very little risk associated with this test.

The veins and the artery size differ from person to person. Taking blood may be difficult in some people. The other risk associated are:

• Fainting feeling or lightheadedness
• Excessive bleeding from the site where the blood is drawn
• Hematoma
• Risk of infection
• Multiple punctures

Coombs test is done to detect the harmful antibodies in the blood that may be damaging the red blood cells.