The Coombs' test looks for antibodies that act against your red blood cells.

Blood will be taken from a vein, usually from the inside of the elbow or the back of the hand. First, the place where the needle will be inserted is cleaned with a germ-killing product (antiseptic). An elastic band (tourniquet) is placed around the upper arm to apply pressure. This causes the vein to swell with blood.

The needle is inserted into the vein, and the blood is collected into a tube. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants and young children, the area is cleansed with antiseptic and punctured with a sharp needle or object (lancet). The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to stop any bleeding.

The direct test is performed using your red blood cells and a series of pre-made antibodies. The indirect Coombs test uses the fluid that forms when blood clots (serum) and a series of standardized red blood cells.

No special preparation is necessary for this test.

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.

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

The direct Coombs' test is used to detect autoantibodies on 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 test looks for anti-RBC antibodies that flow freely in blood serum, which is the clear yellowish fluid left after red blood cells and clotting materials are removed. The indirect Coombs' test is only rarely used to diagnose a medical condition. More frequently, it is used to determine whether a person might have a reaction to a blood transfusion.

No agglutination (the absence of clumping of cells) is normal.

An abnormal (positive) direct Coombs' test means you have antibodies that act against your red blood cells. This may be due to:

• Autoimmune hemolytic anemia without another underlying cause
• Drug-induced hemolytic anemia (many drugs have been associated with this complication)
• Erythroblastosis fetalis (hemolytic disease of the newborn)
• Infectious mononucleosis
• Mycoplasmal infection
• Syphilis
• Chronic lymphocytic leukemia or other lymphoproliferative disorder
• Systemic lupus erythematosus or another rheumatologic condition
• Transfusion reaction, such as one due to improperly matched units of blood

The test is also abnormal in some people without any clear cause, especially among the elderly. Up to 3% of people who are in the hospital without a known blood disorder will have an abnormal direct Coombs' test.

An abnormal (positive) indirect Coombs' test means you have antibodies that the body views as foreign. This may suggest:

• Erythroblastosis fetalis hemolytic disease
• Incompatible blood match (when used in blood banks)
• Autoimmune or drug-induced hemolytic anemia

The risks of having blood taken may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple needle sticks to locate veins

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Rakel P, ed. Conn’s Current Therapy 2005. 57th ed. Philadelphia, Pa: WB Saunders; 2005: 471-473.