The Coombs' test detects antibodies, or proteins that react against other molecules, against red blood cells in an individual's serum or attached to an individual's red blood cells. The Coombs' test is commonly performed before a blood transfusion to make sure that antibodies in an individual's blood will not cross-react, or be attacked by antibodies in blood obtained from a donor. Transfused blood that does not match the blood of the individual who receives the blood could cause complications in the person who receives the blood.

Who is a candidate for the test?

A person about to undergo a blood transfusion should have this test done.

How is the test performed?

A blood sample is taken from a vein on the forearm or hand. To take a blood sample, the skin over the vein is cleaned with an antiseptic and a strong rubber tube, or "tourniquet," is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle and is collected in a syringe or vial for testing in the laboratory. After the needle is taken out of the vein, the puncture site is covered with a bandaid for a short time to prevent bleeding. When the sample is taken to the laboratory, a simple test is performed to see if the red blood cells clump together.