Prenatal Antibody Testing - Rh Antibody

A. What it is

The RBC antibody screen looks for circulating antibodies in the blood directed against red blood cells (RBCs). The primary reason that a person may have RBC antibodies circulating in the blood is because the person has been exposed, through blood transfusion or through pregnancy, to RBCs other than their own (foreign RBCs). These antibodies have the potential to cause harm if a person is transfused with red blood cells, that the antibodies may target or if a pregnant woman has antibodies that target the red cells of her developing little one.

RBCs normally have structures on their surface called antigens. People have their own individual set of antigens on their RBCs, determined by inheritance from their parents. The major antigens or surface identifiers on human RBCs are the O, A, and B antigens, and a person's blood is grouped into an A, B, AB, or O blood type according to the presence or absence of these antigens.

Another important surface antigen is Rh factor, also called D antigen. If it is present on a person's red blood cells, that person's blood type is Rh+ (positive); if it is absent, the blood is type Rh- (negative). In addition, there are many other types of RBC antigens that make up lesser-known blood groups, such as Kell, Lewis, and Kidd blood groups.

There are a few reasons why someone may produce antibodies against RBC antigens.

Following blood transfusions: Antibodies directed against A and B red cell antigens are naturally-occurring; we produce them without having to be exposed to the antigens. Before receiving a blood transfusion, a person's ABO group and Rh type are matched with that of donor blood to prevent a serious transfusion reaction from occurring. That is, the donor's blood must be compatible with the recipient's so that antibodies do not react with and destroy donor blood cells.

If someone receives a blood transfusion, the person's body may also recognize other RBC antigens from other blood groups (such as Kell or Kidd) that the person does not have as foreign. The recipient may produce antibodies to attack these foreign antigens. People who have many transfusions make antibodies to RBCs because they are exposed to foreign RBC antigens with each transfusion.

During pregnancy, with blood type incompatibility between mother and little one: A little one may inherit antigens from the father that are not on the mother's RBCs. The mother may be exposed during pregnancy or at delivery to the foreign antigens on her baby's RBCs when some of the baby's cells enter the mother's circulation as the placenta separates. The mother may begin to produce antibodies against these foreign RBC antigens. This can cause hemolytic disease of the newborn, usually not affecting the first little one but affecting subsequent little ones when the mother's antibodies cross the placenta, attach to the little one's RBCs, and hemolyze them. An RBC antibody screen can help determine if the mother has produced RBC antibodies outside of the ABO blood group.

The first time a person is exposed to a foreign RBC antigen, by transfusion or pregnancy, the person may begin to produce antibodies but his or her cells do not usually have the time during the first exposure to make enough antibodies to actually destroy the foreign RBCs. When the next transfusion or pregnancy occurs, the immune response may be strong enough for enough antibodies to be produced, attach to, and break apart (hemolyze) the transfused RBCs or the little one's RBCs. Antibodies to the ABO antigens are naturally-occurring so do not require exposure to foreign RBCs.

RBC antibodies that are detected with a screen can be identified with an antibody identification test.

B. When it is done

An RBC antibody screen is performed prior to any anticipated blood transfusion.

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

C. What do the results mean

Transfusion: If an RBC antibody screen is positive, then one or more RBC antibodies are present. Some of these antibodies will be more significant than others. When an RBC antibody screen is used to screen prior to a blood transfusion, a positive test indicates the need for an antibody identification test to identify the antibodies that are present. Once the antibody has been identified, 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.

Pregnancy: If an Rh-negative mother has a negative RBC antibody screen, then an Rh immune globulin injection is given within 72 hours to prevent antibody production. If she has a positive test, then the antibody or antibodies present must be identified. If an antibody to the D antigen has been actively formed by the mother, 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 antibodies to the D antigen.

D. How will the results influence the rest of the pregnancy and birth

The RBC screen will show whether your blood has any antigens that could harm your little one, including whether or not you have Rh incompatibility.

If you have Rh incompatibility, your body may begin to make antibodies against your little one's blood.

These antibodies are not a risk in your first pregnancy, because the baby is usually born before any antibodies are made. But these antibodies could harm your unborn little one's in future pregnancies.

Rh incompatibility can be treated with an injection that prevents your body from making antibodies against your baby's red blood cells.

If you are Rh positive, there is no risk of Rh incompatibility.