Coombs' Test, Direct
The direct Coombs' test detects antibodies, which are proteins that react against other molecules, on the surface of an individual's red blood cells.
Who is a candidate for the test?
The direct Coombs' test is usually done to see if a person is making antibodies against his or her own red blood cells. These antibodies are called "autoantibodies."
How is the test performed?
To perform the direct Coombs' test, a blood sample is taken from a vein on the forearm or hand. To get a blood sample, the skin over the vein is cleaned with an antiseptic. Next, 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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.
In the laboratory, a simple test is performed to see if the red blood cells agglutinate, or clump together.