Coombs' test Health Article

Definition

The Coombs' test looks for antibodies that may bind to your red blood cells and cause premature red blood cell destruction (hemolysis).

Alternative Names

Direct antiglobulin test; Indirect antiglobulin test

How the test is performed

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm. 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, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to prepare for the test

No special preparation is necessary for this test.

How the test will feel

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.

Why the test is performed

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

The direct Coombs' test is used to detect antibodies that are already bound to 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 Coombs' test looks for unbound circulating antibodies against a series of standardized red blood cells. The indirect Coombs' test is only rarely used to diagnose a medical condition. More often, it is used to determine whether a person might have a reaction to a blood transfusion.