The Coombs test is a blood test that enables to check for those antibodies that attack red blood cells. The test is also called an antiglobulin test or red blood cell antibody screening.

This is an immunology laboratory procedure that is used to detect the presence of antibodies against circulating red blood cells (RBCs) in the body.

There are two types of Coombs tests. The direct test looks for antibodies that are stuck to red blood cells. The indirect test looks for antibodies floating in the liquid part of your blood, called serum.

Indirect Coombs Test

Indirect antiglobulin testing (IAT) also known as Indirect Coombs Test, is used to detect unbound antibodies to RBCs, which may be present in the patient’s serum.

The indirect Coombs test looks for antibodies that are floating in the blood, which could potentially act against certain red blood cells. This test is most often done to determine if one shows a reaction to a blood transfusion.

The person’s blood will be checked before doing a transfusion to make sure it doesn't have antibodies that would react badly to the donated blood. It's part of the "type and screen" process.

Pregnant women will get a prenatal antibody screening with an indirect Coombs test. It checks the mother's blood to see if there are antibodies that could pass to and harm the unborn baby.

Direct Coombs Test

A direct Coombs test, or direct antiglobulin testing DAT, helps to detect why a person has symptoms that suggest trouble related to their blood.

The direct Coombs test can detect antibodies that are stuck to the surface of red blood cells due to certain diseases and/or the usage of certain drugs. In such cases, the antibodies sometimes destroy the red blood cells causing anaemia. 

In such cases, a direct coombs test will be recommended to test for signs or symptoms of anaemia or jaundice.

You might get sick after a blood transfusion if the donor's blood wasn't a good match. Your body may recognize those other blood cells as foreign and make antibodies to get rid of them, even though they're meant to help.

Test Interpretations

Both the direct and indirect tests can look for simply the presence of antibodies in general or for a specific antibody. Before a blood transfusion, each package of donated blood also needs to be tested.

What do the Indirect Results Mean

A negative indirect Coombs test also known as a normal result, usually means there was no clumping of cells and you don't have antibodies in your serum, so you:

• Can safely get blood from a donor
• Don't need to worry about trouble with your unborn baby

A positive result before a blood transfusion is a warning that the doctor will have to be careful when choosing donor blood. People who need a lot of blood transfusions may develop a lot of different antibodies and have a harder time finding blood that will work.

A positive indirect Coombs test during pregnancy means you may need to take steps to protect your baby. Not all antibodies the test finds are harmful, so depending on what the test was looking for, you may need more tests to narrow down which ones you have so your doctor will know what to do next.

An abnormal (positive) in the indirect Coombs test will mean the body has antibodies that will act against red blood cells that the body views as foreign, which may be due to:

• Erythroblastosis fetalis
• Incompatible blood match (when used in blood banks)
• Diseases like lupus and leukaemia
• Infections such as mononucleosis
• Medicines, including penicillin

What do the Direct Results Mean?

A positive direct Coombs test shows you have antibodies attached to your red blood cells, but it doesn't necessarily tell you which ones or why. This is also known as ‘abnormal results’ which may be due to:

• Autoimmune haemolytic anaemia
• Chronic lymphocytic leukaemia or similar disorder
• Blood disease in new-borns called erythroblastosis fetalis (also called haemolytic disease of the new-born)
• Infectious mononucleosis
• Mycoplasma infection
• Syphilis
• Systemic lupus erythematosus
• Transfusion reaction, such as one due to improperly matched units of blood 

Regardless of the result of a direct Coombs test, you may need other tests to find the right diagnosis and treatment.

Conclusion

The Coombs test is effective in checking your blood for antibodies that attack red blood cells. This test may is usually used to screen your blood before a procedure, such as a blood transfusion or to find out if you have certain conditions, such as autoimmune haemolytic anaemia.