RH Disease & Incompatibility

Coombs' Test

frequently used in the evaluation of a jaundiced infant. Understanding how the test is done and what it means is critical to the correct interpretation of a positive result.

RhoGAM

(a Rh immunoglobulin) prevents the mother from forming antibodies: all Rh neg women receive this around 28 weeks of pregnancy and within 72 hours of birth

Rh Isoimmunization or Rh Disease

A condition that occurs when a woman with Rh negative blood is exposed to Rh + blood cells, leading to development of Rh antibodies: these antibodies can cross the placenta and destroy baby's RBCs: which can cause hemolytic disease of the newborn

Indirect Coombs' Test

Conversely, active hemolysis may be present with a negative coombs' test. Conditions that cause the RBC to be inherently defective in some way (hereditary spherocytosis, G6PD deficiency, etc) can also result in severe hyperbilirubinemia, but because these processes do not involve antibodies, the coombs' test will be negative. This is the test that is done on the mother's blood sample as part of her prenatal labs. Frequently referred to as the "antibody screen", this test identifies a long list of minor antigens that could either cause problems in the newborns or cause problems in the mother if transfusion is necessary. Not all antibodies detected by this screen are clinically significant with regard to the baby, so it is helpful to have the lab identify which antibody is present. If the antibody identified is clinically significant, then the pathophysiology is the same as for Rh or ABO incompatibility. If mixing of maternal and fetal blood occurs during pregnancy or the birth process, these antibodies can also attack the baby's RBCs and cause hemolysis, potentially resulting in hyperbilirubinemia and anemia.

ABO Incompatibility

Significant problems with ABO incompatibility occur mostly with babies whose mothers have O blood type and where the baby is either A or B blood type. If different blood types mix, an immune response occurs, and the person will produce antibodies to attack the foreign blood antigen. During pregnancy the mother's and baby's blood generally do not mix. The mothers and babies circulation is kept separate by the placental membrane. However, some circumstances can cause the two blood types to mix, such as miscarriage, trauma and birth, and sometimes they may mix for reasons unknown. Antibodies against the foreign blood types A and B may be formed. These antibodies could then pass across the placental membrane into the baby's circulation and may result in the destruction of some of the baby's red blood cells. This destruction of red cells causes an increase in the production of bilirubin - a waste product. If too much bilirubin is produced, it can overwhelm the baby's normal waste elimination processes and lead to jaundice.

Direct Coombs' Test

This is the test that is done on the newborn's blood sample, usually in the setting of a newborn with jaundice. The test is looking for "foreign" antibodies that are already adhered to the infant's RBCs, a potential cause of hemolysis. This is referred to as "antibody-mediated hemolysis". The two most commonly recognized forms of antibody-mediated hemolysis in newborns are Rh incompatibility and ABO incompatibility. Rh incompatibility occurs when a mother who is type Rh - (and has naturally occurring anti-Rh antibodies in her serum) gives birth to an infant who is Rh+. If any mixing of maternal and fetal blood occurs during pregnancy or the birth process, the mother's anti-Rh antibodies will vigorously attack the baby's Rh+ RBCs by adhering to, and then lysing the cells. ABO incompatibility occurs by the same general mechanism. Type O mothers are most commonly impacted, since they carry both anti-A and anti-B antibodies. If the infant is type A, type B, or type AB, the risk for incompatibility exists. This is frequently referred to as a "set-up". If mixing of maternal and fetal blood occurs during pregnancy or the birth process, these antibodies can also attack the baby's RBCs and cause hemolysis. In general, this reaction is less serious than Rh incompatibility (which can be fatal if severe and untreated), and usually only results in jaundice and mild anemia. An important thing to remember is that the presence of a positive coombs' test in the lab does not necessarily result in hyperbilirubinemia in the infant. The risk of needing phototherapy is certainly greater, but there are many factors impacting bilirubin levels, and assessment of all of these elements is critical to making an appropriate decision about treatment.

Sensitization

can occur during any event that may cause maternal/fetal blood to mix: childbirth miscarriage abortion amniocentesis ectopic pregnancy abdominal trauma external cephalic version