Coombs’ test - direct

Definition

The direct Coombs' test measures the presence of antibodies on the surface of red blood cells.

How the test is performed

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. 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 or young children:

The area is cleansed with antiseptic and punctured with a sharp needle or a 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 the puncture site if there is any bleeding.

How to prepare for the test

No special preparation is necessary for this test.

How the test will feel

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.

Why the test is performed

The direct Coombs' test is used to detect autoantibodies against your own red blood cells (RBCs). 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, jaundice, or abnormalities in the appearance of RBCs under the microscope.

Normal Values

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

What abnormal results mean

A positive direct Coombs' test indicates antibodies against the red blood cells, which may indicate one of the following conditions:

• 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.

What the risks are

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

Special considerations

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.