Antibody Tests

Antibody tests are done to find antibodies that attack red blood cells. Antibodies are proteins made by the immune system. Normally, antibodies bind to foreign substances, such as bacteria and viruses, and destroy them.

The following conditions cause antibodies to be made.

Transfusion reaction

Human blood is typed by certain markers (called antigens) on the surface of red blood cells . If you get a blood transfusion, the transfused blood must match your type; that is, it must have the same antigens as your red blood cells. If you get a transfusion of blood with antigens different from yours (incompatible blood), your immune system destroys the transfused blood cells. This is called a transfusion reaction and can cause serious illness or even death. This is why matching blood type is so important.

Rh sensitization

Rh is an antigen. The full name for this antigen is Rhesus factor.

If a pregnant woman with Rh-negative blood has a baby (fetus) with Rh-positive blood, Rh sensitization may occur. The baby gets Rh-positive blood from the father's genes. Rh sensitization happens when the baby's blood mixes with the mother's blood during delivery. This causes the mother's immune system to make antibodies against the baby's red blood cells in future pregnancies. This antibody response is called Rh sensitization and, depending on when it happens, can destroy the baby's red blood cells. If sensitization happens, the baby can develop mild to severe problems (called Rh disease, hemolytic disease of the newborn, or erythroblastosis fetalis). In rare cases, if Rh sensitization is not treated, the baby may die.

A woman with Rh-negative blood can get a vaccine called Rh immune globulin (such as RhoGAM) that almost always stops sensitization from occurring. Problems from Rh sensitization have become very rare since the Rh immune globulin vaccine was developed.

Autoimmune hemolytic anemia

A type of hemolytic anemia called autoimmune hemolytic anemia is a rare disease that causes antibodies to be made against a person's own red blood cells.

Two blood tests can check for antibodies that fight against red blood cells: the direct Coombs test and the indirect Coombs test. The direct Coombs test finds antibodies that are already attached to red blood cells. The indirect Coombs test finds antibodies that could bind to certain red blood cells, leading to problems if blood mixing occurs.

Direct Coombs test

The direct Coombs test finds antibodies attached to your own red blood cells. The antibodies may be those your body made because of disease or those you get in a blood transfusion.

The direct Coombs test also may be done on a newborn baby whose mother has Rh-negative blood. The test shows whether the mother has made antibodies and if the antibodies have moved through the placenta to her baby.

Indirect Coombs test

The indirect Coombs test finds antibodies that are in your blood but not attached to your red blood cells. The indirect Coombs test is commonly done to find antibodies in a recipient's or donor's blood before a transfusion.

An Rh antibody titer, is done early in pregnancy to check a woman's blood type. If she is Rh-negative, steps can be taken to protect the baby.

You do not need to do anything before you have this test.

The health professional drawing blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. If the needle is not placed correctly or if the vein collapses, more than one needle stick may be needed.
• Hook a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then a bandage.

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

There is very little chance of a problem from having blood sample taken from a vein.

• You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
• In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

Antibody tests are done to find antibodies that fight against red blood cells.

Normal

No antibodies are found. This is called a negative test result.

• Direct Coombs test. A negative test result means that your blood does not have antibodies already attached to your red blood cells.
• Indirect Coombs test. A negative test result 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 baby if her baby has Rh-positive blood. This means that Rh sensitization has not occurred.

Abnormal test results

• Direct Coombs test. A positive result means your blood has antibodies that fight against red blood cells. This can be caused by a transfusion of incompatible blood or may be related to conditions such as hemolytic anemia, systemic lupus erythematosus (SLE), hemolytic disease of the newborn (HDN), lymphoma, mycoplasma infection, advanced stage cancer, or infectious mononucleosis.
• Indirect Coombs test. A positive test result means 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 she has antibodies against Rh-positive blood (Rh sensitization). She will be tested early in pregnancy to check the blood type of her baby. If the baby has Rh-positive blood, the mother will be watched closely throughout the pregnancy to prevent problems to the baby's red blood cells. If sensitization has not occurred, it can be prevented by the Rh immune globulin vaccine.

Reasons you may not be able to have the test or why the results may not be helpful include:

• Having a blood transfusion in the past.
• Having a dextran injection, or a recent X-ray with contrast material given in a vein (intravenously, or IV).
• Being pregnant in the past 3 months.
• Taking some medicines, such as cephalosporins, sulfa medicines, tuberculosis medicines, insulin, and tetracyclines.

A newborn baby (whose mother has Rh-negative blood) may have a direct Coombs test to check for antibodies against the baby's red blood cells. If the test is positive, the baby may need a transfusion with compatible blood to prevent anemia.

Other Works Consulted

• Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.

• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.

• Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.