Coombs’ test

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The Coombs' test looks for antibodies that act against your red blood cells.

Direct antiglobulin test; Indirect antiglobulin test

Blood will be taken from a vein, usually from the inside of the elbow or the back of the hand. First, the place where the needle will be inserted is cleaned with a germ-killing product (antiseptic). An elastic band (tourniquet) is placed around the upper arm to apply pressure. This causes the vein to swell with blood.

The needle is inserted into the vein, and the blood is collected into a tube. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children:

After cleaning the area, a sharp needle or object (lancet) is used to cause slight bleeding. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to stop any bleeding.

The direct test is performed using your red blood cells and a series of pre-made antibodies. The indirect Coombs test uses the fluid that forms when blood clots (serum) and a series of standardized red blood cells.

No special preparation is necessary for this test.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing or a bruise may develop at the site where the needle was inserted.

There are two forms of the Coombs' test: direct and indirect.

The direct Coombs' test is used to detect autoantibodies on the surface of red blood cells. Many diseases and drugs (quinidine, methyldopa, and procainamide) can lead to production of these antibodies. These antibodies sometimes destroy red blood cells and cause anemia. This test is sometimes performed to diagnose the cause of anemia or jaundice.

The indirect test looks for anti-RBC antibodies that flow freely in blood serum, which is the clear yellowish fluid left after red blood cells and clotting materials are removed. The indirect Coombs' test is only rarely used to diagnose a medical condition. More frequently, it is used to determine whether a person might have a reaction to a blood transfusion.

Review Date: 09/11/2006
Reviewed By: Corey Cutler, MD, MPH, FRCP(C), Assistant Professor of Medicine, Harvard Medical School; Department of Medical Oncology, Dana-Farber Cancer Institute, Boston, MA. Review provided by VeriMed Healthcare Network.