Coombs’ test - indirect

Definition

The indirect Coombs' test measures the presence of antibodies to red blood cells in the blood (see also Coombs' test - direct).

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.

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 indirect Coombs' test detects circulating antibodies against red blood cells (RBCs). The major use of this test is to determine if the patient has antibodies in the blood capable of attaching to RBCs. These antibodies are other than the major ABO system or the Rh type.

The test is only rarely used to diagnose a medical condition, but is essential for use by laboratories such as blood banks. Blood banks use the indirect Coombs' test to determine whether there is likely to be an adverse reaction to blood to be transfused.

Normal Values

No agglutination (absence of clumping of red blood cells) is normal.

What abnormal results mean

An abnormal indirect Coombs' test may indicate the presence of an antibody against an antigen that the body views as foreign:

• Antibodies to antigens not present on the individual's red cells, usually caused by prior transfusions
• Erythroblastosis fetalis hemolytic disease of the newborn
• Incompatible cross-matched blood (when the test is used in the blood bank)

If you have antibodies against your own RBCs, the indirect Coombs' test results may be abnormal, if there are excess antibodies beyond what your red blood cells can absorb. This may indicate autoimmune hemolytic anemia or drug-induced hemolytic anemia.

What the risks are

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

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.

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Indirect antiglobulin test