The Kleihauer-Betke (KB) test is a blood test used during pregnancy to screen maternal blood for the presence of fetal red blood cells. It is mainly used to assess the severity of a fetomaternal hemorrhage (FMH), a condition in which a disruption in the placental barrier allows fetal blood to enter the maternal circulation. Fetal blood may enter the pregnant individual’s blood circulation naturally throughout pregnancy and childbirth, when there are traumatic or obstetric complications to pregnancy, or due to invasive prenatal procedures (e.g., amniocentesis).
Pregnant individuals are considered “Rh negative” when they do not have the Rhesus (Rh) factor on the surface of their red blood cells. For these individuals, the introduction of Rh-positive blood into their blood circulation may induce the formation of anti-Rh antibodies. This process can occur when fetal blood enters the maternal circulation, thereby inducing the formation of maternal anti-Rh antibodies against fetal red blood cells. These antibodies are not able to cross the placenta during the first pregnancy, so they cannot harm the first baby. However, in future pregnancies, pre-formed anti-Rh antibodies may cross the placenta and destroy the infant’s Rh-positive red blood cells, resulting in fetal anemia (i.e., low levels of red blood cells).
If an Rh-negative individual is pregnant with an Rh-positive fetus, anti-Rh immune globulin (e.g., RhoGAM) is given at 28 weeks gestation and again after childbirth to prevent the pregnant individual from forming anti-Rh antibodies. Individuals may be given an additional dose of anti-RH immune globulin in situations where a fetomaternal hemorrhage may have occurred. The anti-Rh immune globulin binds to any Rh-positive red blood cells from the fetus that sneak into the pregnant individual’s circulation, preventing the formation of maternal antibodies. A standard dose of anti-Rh immune globulin is enough to prevent the formation of anti-Rh antibodies in response to 15 milliliters (about 0.3 ounces) or less of Rh-positive blood. The results of the KB test can help quantify the size of the fetomaternal hemorrhage, thereby assisting in calculating the needed dose of anti-Rh immune globulin.