Direct coombs test

Introduction
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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.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

What do the test results mean?

Normally, red blood cells do not clump together.
If clumping does occur, the individual's red blood cells have antibodies against those red cells on their surfaces. This may indicate:

autoimmune hemolytic anemia, which is an immune attack against the body's own red blood cells

drug-induced hemolytic anemia, or red blood cell destruction related to various medications

erythroblastosis fetalis, an anemia in the fetus which occurs when the mother's blood type is Rh-negative, and her immune system makes antibodies that destroy the red blood cells of the Rh-positive fetus

mononucleosis, also known as the "kissing disease," an infection caused by the Epstein-Barr virus

infection with Mycoplasma bacteria, which are the germs that cause tuberculosis

systemic lupus erythematosus, which is an immune attack against the body's own tissues, resulting in arthritis and other symptoms

a reaction to a blood transfusion