The Coombs' Test
The coombs' test is 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.
Direct Coomb's 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 red blood cells (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 who has circulating anti-Rh antibodies from a previous exposure to Rh+ antigens (either through blood transfusion or during a prior pregnancy) gives birth to an infant who is Rh+. During pregnancy, those antibodies will cross the placenta and vigorously attack the baby's Rh+ rbcs by adhering to, and then lysing, the cells. Except in those instances where mother was stimulated to produce anti-Rh through a blood transfusion, first pregnancies are not generally at risk for Rh incompatibility.
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, 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.
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 process do not involve antibodies, the coombs' test will be negative.
Indirect Coomb's Test
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 antibodies 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, maternal antibodies which have entered the baby can attack the baby's rbcs and cause hemolysis, potentially resulting in hyperbilirubinemia and anemia.