Antibodies normally bind to foreign substances, such as bacteria and viruses, and destroy them.

Antibody screening tests are done to detect antibodies against red blood cells. Some conditions can result in the production of antibodies against red blood cells.

• Human blood is typed by the presence of certain markers (antigens) on the surface of red blood cells. Blood received in a transfusion must contain antigens that are the same as the antigens on the recipient's own red blood cells (compatible blood). If a person receives a transfusion of blood containing different antigens (incompatible blood), the person's immune system destroys the transfused blood cells. This destruction is called a transfusion reaction and can cause serious illness or even death.
• If the Rh-positive blood of a fetus mixes with the blood of an Rh-negative woman during pregnancy or delivery, the woman's immune system produces antibodies against her fetus's red blood cells. This antibody response is called Rh sensitization. Depending on when it occurs, it can destroy the fetus's red blood cells. Once sensitization has occurred, the fetus can develop mild to severe problems (called Rh disease, hemolytic disease of the newborn, or erythroblastosis fetalis). If untreated, complications from sensitization can, in rare cases, lead to the death of an Rh-positive fetus.
• A form of hemolytic anemia called autoimmune hemolytic anemia is a rare disease that causes a person to produce antibodies against his or her own red blood cells.

The following two tests detect the presence of antibodies against red blood cells: the indirect Coombs' test and the direct Coombs' test. The indirect Coombs' test detects antibodies that could bind to certain red blood cells, bringing problems if there is blood mixing. The direct Coombs' test detects antibodies that are already attached to red blood cells.

Both tests are done on a blood sample taken from a vein.

Antibody Screening Tests
Results

Normal

Normal values for both tests are negative (it means that no antibodies were detected).

• Direct Coombs' test. A negative reaction means that your blood contains no antibodies that are already attached to your red blood cells.
• Indirect Coombs' test. A negative reaction means that your blood is compatible with the blood you are to receive by transfusion. A negative indirect Coombs' test for Rh factor (Rh antibody titer) in a pregnant woman means that she has not developed antibodies against the blood of her fetus (if her fetus has Rh-positive blood). This means that Rh sensitization has not occurred.

Greater than normal values may mean

• Direct Coombs' test. The detection of antibodies attached to your red blood cells (a positive reaction) indicates that your blood has antibodies against red blood cells in your bloodstream. This can be due to a transfusion of incompatible blood or to conditions such as hemolytic anemia, systemic lupus erythematosus (SLE), hemolytic disease of the newborn (HDN), or tumors of the lymph nodes (lymphomas).
• Indirect Coombs' test. The detection of antibodies against red blood cells from a donor (a positive reaction) may indicate that your blood is incompatible with the donor's blood and you cannot receive blood from that person. If the Rh antibody titer test is positive in a woman who is pregnant or is planning to become pregnant, it means that her immune system has formed antibodies against Rh-positive blood Rh sensitization. She will be tested to determine the blood type of her fetus. If the fetus has Rh-positive blood, close monitoring throughout the pregnancy is needed to prevent serious damage to the fetus's red blood cells caused by the mother's immune system. If sensitization has not occurred, it can usually be prevented by an injection of the Rh immune globulin vaccine.