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. For information on how this is done, see: Venipuncture
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
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 (including quinidine, methyldopa, and procainamide) 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 were no clumping of cells (agglutination), meaning 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:
- Autoimmune hemolytic anemia
- Chronic lymphocytic leukemia or similar disorder
- Drug-induced hemolytic anemia
- 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)
What the risks are
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:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Powers A, Silberstein LE. Autoimmune hemolytic anemia. In: Hoffman R, Benz EJ Jr., Shattil SS, et al., eds. Hematology: Basic Principles and Practice. 5th ed. Philadelphia, Pa: Churchill Livingston Elsevier; 2008: chap 47.
Schrier SL, Price EA. Extrinsic nonimmune hemolytic anemias. In: Hoffman R, Benz EJ Jr., Shattil SS, et al., eds. Hematology: Basic Principles and Practice. 5th ed. Philadelphia, Pa: Churchill Livingston Elsevier; 2008: chap 48.
Schwartz RS. Autoimmune and intravascular hemolytic anemias In: Goldman L, Schafer AI, eds.Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 163.