The Coombs test looks for antibodies that may stick to your red blood cells and cause red blood cells to die too early.
Direct antiglobulin test; Indirect antiglobulin test
How the Test is Performed
A blood sample is needed.
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. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.
Why the Test is Performed
There are two types of the Coombs test:
The direct Coombs test is used to detect antibodies that are stuck to the surface of red blood cells. Many diseases and drugs can cause this. These antibodies sometimes destroy red blood cells and cause anemia. Your doctor may order this test if you have signs or symptoms of anemia or jaundice.
The indirect Coombs test looks for free-flowing antibodies against certain red blood cells. It is is most often done to determine if you may have a reaction to a blood transfusion.
A normal result means there was no clumping of cells (agglutination). This means you have no antibodies to red blood cells.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What Abnormal Results Mean
An abnormal (positive) direct Coombs test means you have antibodies that act against your red blood cells. This may be due to:
- Hemolytic anemia
- Chronic lymphocytic leukemia or similar disorder
- Erythroblastosis fetalis (hemolytic disease of the newborn)
- Infectious mononucleosis
- Mycoplasmal infection
- Systemic lupus erythematosus
- Transfusion reaction, such as one due to improperly matched units of blood
The test may also be abnormal without any clear cause, especially among the elderly.
An abnormal (positive) indirect Coombs test means you have antibodies that will act against red blood cells your body views as foreign. This may suggest:
- Autoimmune or drug-induced hemolytic anemia
- Erythroblastosis fetalis hemolytic disease
- Incompatible blood match (when used in blood banks)
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.
Other risks associated with having blood drawn are slight but may include:
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Elghetany MT, Banki K. Erythrocytic disorders. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, Pa: Elsevier Saunders; 2011:chap 32.