Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.
Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm. 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, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.
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 antibodies that are already bound to 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 Coombs' test looks for unbound circulating antibodies against a series of standardized red blood cells. The indirect Coombs' test is only rarely used to diagnose a medical condition. More often, it is used to determine whether a person might have a reaction to a blood transfusion.
No clumping of cells (agglutination), indicating that there are no antibodies to red blood cells, is normal.
Normal value ranges may vary slightly among laboratories. Talk to your doctor about the meaning of your specific test results.
An abnormal (positive) direct Coombs' test means you have antibodies that act against your red blood cells. This may be due to:
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 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:
- 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, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 164.