What Is a Coombs Test?

Antibodies are a part of your immune system. They fight germs, but sometimes they make a mistake and target your body's healthy cells instead. The Coombs test checks your blood for antibodies that attack red blood cells. You might also hear it called an antiglobulin test or red blood cell antibody screening.

Not everyone's red blood cells are alike. Your immune system will make antibodies if it finds ones that don't match yours. They're keyed to specific areas on the outside of the cell. Some of these antibodies are related to your blood type.

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.

Why You Get an Indirect Coombs Test

Doctors use the indirect Coombs test, also called IAT, to prevent problems.

They'll check your blood before you get 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 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 her unborn baby.

Why You Get a Direct Coombs Test

A direct Coombs test, or DAT, may help explain why you're not feeling great or have symptoms that suggest trouble related to your blood.

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.

A blood disease called autoimmune hemolytic anemia happens when antibodies destroy your own red blood cells faster than your body can make them. You can get it because of:

Babies with yellowish skin and eyes may have hemolytic disease of the newborn (HDN). Some antibodies from their mother could be attacking their red blood cells. This happens most often when the part of the baby's blood type inherited from the father doesn't mix well with the mother's.

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How It's Done

A technician uses a needle to take a small sample of blood from a vein in your hand or arm. You may feel a small skin prick and have a little bleeding or bruising where the needle goes in. Then they'll send your blood to a lab.

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.

Cross-matching is a special kind of IAT that may be done before a blood transfusion. The lab mixes your serum (where the antibodies are) with red blood cells from the donor.

What the Indirect Results Mean

A negative indirect Coombs test is good news. It usually means you don't have antibodies in your serum, so you:

  • Can safely get blood from that 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.

What 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.

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

WebMD Medical Reference Reviewed by Neha Pathak, MD on November 24, 2017

Sources

SOURCES:

MSD Manual Professional Version: "Overview of Hemolytic Anemia."

LabTestsOnline.org: "RBC Antibody Screen," "Direct Antiglobulin Test."

Institute for Transfusion Medicine, Transfusion Medicine Updates: "Indirect and Direct Antiglobulin (Coombs) Testing and the Crossmatch."

Deane, L. Blood Groups and Red Cell Antigens, National Center for Biotechnology Information, 2005.

Medscape: "Direct Antiglobulin Testing," "Transfusion Reactions Workup."

American Journal of Hematology: "The direct antiglobulin test: A critical step in the evaluation of hemolysis."

Stanford Medicine, Newborn Nursery: "The Coombs' Test."

Johns Hopkins Lupus Center: "Coombs' Test."

University of Rochester Medical Center: "Blood Type and Crossmatch."

Archives of Pathology & Laboratory Medicine Online: "The Direct Antiglobulin Test: Indications, Interpretations, and Pitfalls."

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