An Indirect Antiglobulin Test (IAT) is used to screen your blood for antibodies directed against red blood cell (RBC) antigens other than the A and B antigens. It is performed as part of a “type and screen” whenever a blood transfusion is anticipated. If an antibody is detected, then an antibody identification test must be done to determine which antibodies are present. During a crossmatch, a variation of the IAT is performed. Donor’s RBCs and your plasma are mixed and processed to see if there is any clumping of RBCs (agglutination) in the test tube that might indicate an incompatibility that would affect you if the blood was transfused into you. In the case of blood transfusions, RBC antibodies that are present must be taken into account and donor blood must be found that does not contain the antigen(s) to which you have produced antibodies.

If you have an immediate or delayed reaction to a blood transfusion, your doctor will order both an IAT and Direct Antiglobulin Test (DAT) to help investigate the cause of the reaction. (The DAT detects RBC antibodies attached to red cells.) Another IAT may be run after the acute situation has resolved to see if the patient has developed any new antibodies.

During pregnancy, the IAT is used to screen for antibodies in the blood of the mother that might cross the placenta and attack the baby’s red cells, causing hemolytic disease of the newborn (HDN). The most serious cause is an antibody produced in response to the RBC antigen called the “D antigen” in the Rh blood group system. A person is considered to be Rh-positive if the D antigen is present on their RBCs and Rh-negative if the D antigen is not present. An Rh-negative mother may develop an antibody when she is exposed to blood cells from an Rh-positive fetus. To prevent this, an Rh-negative mother may have an IAT performed early in her pregnancy, at 28 weeks, and again at the time of delivery. If there are no Rh antibodies present at 28 weeks, then the woman is given an injection of Rh immune globulin (RhIg) to clear any Rh-positive fetal RBCs that may be present in her bloodstream to prevent the production of Rh antibodies by the mother.

At birth, the baby’s Rh status is determined. If the baby is Rh-negative, then the mother does not require another RhIg injection; if the baby is Rh-positive, then another IAT test will be performed on the mother. If the test is negative, the mother is given additional RhIG.

This test may rarely be used to help diagnose autoimmune-related hemolytic anemia. This condition may be caused when a person produces antibodies against their own RBC antigens. This can happen with some autoimmune disorders, such as systemic lupus erythematosus, with diseases such as lymphoma or chronic lymphocytic leukemia, and with infections such as mycoplasma pneumonia and mononucleosis. It can also occur in some people with the use of certain medications, such as penicillin.

An IAT is performed prior to any anticipated blood transfusion and as a follow-up to a transfusion reaction.

Signs and symptoms of a blood transfusion reaction may include:

• Fever, chills
• Rash
• Back pain
• Bloody urine
• Feeling faint or dizzy

An IAT is performed as part of every woman’s pregnancy workup. In Rh-negative women, it is also done at 28 weeks, prior to giving an RhIg injection, and after delivery if the baby is found to be Rh-positive. In Rh-negative pregnant women with known Rh antibodies, the IAT is sometimes ordered as a monitoring tool to roughly track the amount of antibody present.

If an IAT is positive, then one or more RBC antibodies are present. Some of these antibodies will be more significant than others. When an IAT is used to screen prior to a blood transfusion, a positive IAT indicates the need for an antibody identification test to identify the antibodies that are present. Once the antibody has been identified, then donor blood must be found that does not contain the corresponding antigen(s) so that the antibody will not react with and destroy donor RBC antigens following a blood transfusion.

If an Rh-negative mother has a negative IAT, then it is safe for a short window of time (72 hours) to give an RhIg injection to prevent antibody production. If she has a positive IAT, then the antibody or antibodies present must be identified. If there is an Rh antibody present, then the RhIg injection is not useful. If she has a different antibody, then the RhIg injection can still be given to prevent her from producing Rh antibodies.

A circulating RBC antibody, once present, will never truly go away. If it has been many years since antigen exposure, circulating antibody levels may drop to undetectable levels. However, if the patient is exposed to the antigen again, production will kick quickly into gear and attack the RBCs.

Each blood transfusion that a person has exposes them to the combination of antigens on the donor’s RBCs. Whenever the transfused RBCs contain antigens foreign to the recipient’s RBCs, there is the potential to produce an antibody. If someone has many blood transfusions over a period of time, they may produce antibodies against many different antigens. This can make finding compatible blood increasingly difficult.