Coomb's test (Direct)

Antibodies are an essential component of our immune system. They help protect our bodies from infections, but sometimes they may attack the healthy cells of the body. Since all the red blood cells in the body are not alike, our immune system may produce antibodies against the ones that do not match the others. However, if the immune system cannot detect the infection correctly and starts producing antibodies against one's cells, it can cause several health problems.

The Coombs test detects the presence of antibodies that may attack the red blood cells. The Coombs test may help the doctor determine if the antibodies produced by the immune system are responsible for attacking and destroying one's RBCs. This kind of autoimmune destruction of the red blood cells is referred to as hemolytic anaemia.

Of the two Coombs tests, the Direct Coombs Test helps detect antibodies that are attached to the red blood cells. Otherwise known as the Direct Agglutination Test, the Direct Coombs Test is done more frequently. This is because the reagent used in the Coombs test causes the red blood cells with the attached antibodies to stick to each other.

A Direct Coombs Test can also be done to diagnose transfusion reactions and fatal conditions like erythroblastosis fetalis.

Other Names of Direct Coombs Test

• Direct antiglobulin test
• Coombs test
• Direct anti-human globulin test

What does the test detect/measure and who is this prescribed for?

The Direct Coombs test helps to diagnose hemolytic conditions.

A Direct Coombs test is done to detect:

• Autoimmune diseases

Hemolysis can take place in autoimmune conditions. A positive Direct Coombs test can be expected in:

• Autoimmune hemolytic anaemia: A condition in which the immune system attacks its own red blood cells in the body.
• Malignancies like leukaemia.
• Autoimmune diseases like systemic lupus erythematosus and antiphospholipid syndrome.

• Transfusion reaction

Immediately after transfusion, the immune system may attack the donated blood cells. This usually occurs when the blood donor type is not an appropriate match. This can show up as early as two days after the transfusion. The recipient may experience symptoms like dizziness, back pain, dark urine and shortness of breath.

• Erythroblastosis fetalis

Erythroblastosis fetalis is a fatal condition in which the fetus and the parents have different blood types. The difference may be in the Rh factor or ABO blood type. As a result, the immune system of the mother may attack the red blood cells of the infant, which can result in their death. In addition, infants with erythroblastosis fetalis may develop jaundice due to their high bilirubin levels.

• Infection

Certain infections like syphilis, mononucleosis, malaria and AIDS can cause hemolysis. This can be detected through the Direct Coombs test.

• Adverse reactions to some medicines

Certain medicines like cephalosporins (antibiotics), dapsone (antibacterial) and painkillers like ibuprofen can cause the destruction of red blood cells.

Usually, a blood sample is required to carry out a Direct Coombs test. The blood is generally obtained from a vein through venipuncture.

This test can also be conducted on infants. To perform this test, the skin is pricked with a needle, usually on the foot's heel. The blood is usually collected in a small glass tube, slide or test strip.

Test Preparation

Although no precautions are taken before the Direct Coombs test, your doctor may advise you to stop taking certain medicines. You may drink water before going to the laboratory or the collection site.

Your healthcare provider will use a needle to take a blood sample from your arms. At the time of collection of blood, you may experience slight pain. It lasts for a very short duration and is usually of mild intensity. After the needle is removed, you may feel a throbbing sensation. You may have to apply pressure on the site where the needle was inserted. A bandage is usually put on the site for 10-20 minutes. You should avoid lifting your arms for heavy weight or doing any strenuous exercise.

The sample is collected and stored in a container that does not allow it to coagulate before the test begins. It is then sent to the laboratory for analysis. The test is not risky. Rarely, it can cause lightheadedness and fainting. It can cause infection at the site where the needle was inserted. Bleeding can also occur at the site of injection. Excessive bleeding or bleeding for a longer duration can indicate an underlying bleeding condition and should be reported to the doctor.

Test Inclusions: What Parameters are Included?

IgM and IgG antibodies against the RBC membrane

Hemolytic anaemia, Erythroblastosis fetalis, Transfusion-related hemolysis, Autoimmune diseases, Adverse reactions to some medicines

The Direct Coombs test is used to detect the coating of red blood cells with IgG antibodies and complements like C3d.

Normal findings: Negative, no agglutination.

The Coombs test detects the agglutination or formation of clumps of red blood cells.

The test is said to be negative when no clump is formed. A negative test usually indicates that antibodies are not attached to the red blood cells. This also signifies that the symptoms you are experiencing are probably due to another cause.

A positive Direct Coombs test result means that agglutination occurs and antibodies are attached to the red blood cells. Thus, the immune system plays a significant role in destroying red blood cells.

However, a positive Direct Coombs test does not always mean that hemolysis occurs. If hemolysis is suspected, a Direct Coombs test can be done to confirm the provisional diagnosis.