Coombs Test, Direct

Coombs Test, Direct

Definition

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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.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 is involved in preparation for the test?

An individual scheduled to undergo the direct Coombs' test should request specific instructions from his or her healthcare professional.

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 an unborn infant 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 infant
  • mononucleosis, also known as the "kissing disease," an infection caused by the Epstein-Barr virus
  • infection with Mycoplasma bacteria, a cause of pneumonia
  • 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

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