This test is routinely performed on pregnant women who are Rh-negative. It is not unusual for a small number of fetal cells to pass into the mother’s blood stream, which can then cause her to produce an antibody. Although this can occur in all mothers, it is more likely to occur in an Rh-negative mother who has an Rh-positive baby.

At this point in the pregnancy (approximately 28 weeks gestation), a second antibody screen may be requested to see if the mother has developed an antibody that was not detected during the first prenatal visit. If the first antibody screen was negative and the Rh-negative mother has not received a blood transfusion during the pregnancy, then it is most likely that an antibody detected in the second antibody screen is due to exposure to an Rh-positive baby’s red blood cells. This cannot be taken for granted though. If the screen is positive, it must be followed by an antibody identification test to determine which antibody(s) is present.

If the antibody screen is negative for Rh antibodies, then an Rh immune globulin injection (also referred to as RhoGAM), may be given to the Rh-negative mother to prevent her from reacting to the baby’s cells. Additional injections may be administered following invasive procedures, such as amniocentesis or CVS, and following any event where there is the possibility of the baby’s blood transferring to the mother (for example, trauma to the mother’s abdomen). If the mother has already developed the antibody to the Rh factor/antigen, then another injection will not be useful.

Limitations of antibody screen:
An antibody screen will be positive if an Rh immune globulin injection was given within the prior six months. An accurate history of prior Rh immune globulin injections is important in deciding if the positive test is due to the injection or the mother having produced the antibody to the Rh factor.

Related Pages
Tests: Indirect Antiglobulin Test