A Coombs test is a blood test to diagnose hemolytic disease of the fetus and newborn (HDFN).
As part of standard prenatal testing, it detects antibodies against Rh-positive red blood cells. If maternal antibodies are detected, this informs the level of care required during pregnancy and allows for early intervention if HDFN develops. Following delivery, a Coombs test may be used to diagnose HDFN in the newborn and ensure immediate treatment of associated jaundice and anemia.
What is HDFN?
Hemolytic disease of the fetus and newborn (HDFN) is an immune-mediated red blood cell disorder that occurs when a baby’s RBCs break down quickly, which is called hemolysis. HDFN is caused by a mismatch between a mother’s and her baby’s blood type (A, B, AB, or O) or Rhesus (Rh) factor (Rh-positive or Rh-negative) during pregnancy.
How is a Coombs test used in HDFN?
In pregnancy, a Coombs test is used in identifying the risk of HDFN developing in the fetus. As part of prenatal testing, it detects whether the Rh-negative mother has been sensitized to the Rh-positive blood type of her baby. This occurs when maternal blood and fetal blood come into contact, due to the incompatibility of the two blood factors.
Once sensitization takes place, the maternal antibodies can cross the placenta and attack the fetal red blood cells, leading to HDFN. Post-delivery, it diagnoses HDFN by detecting the antibodies that are directly attached to the baby’s red blood cells.
Learn more about HDFN testing and diagnosis
Direct vs Indirect Coombs test
There are two types of Coombs tests:
Indirect Coombs test
An indirect Coombs test or indirect antiglobulin test checks for antibodies in the bloodstream that could attack red blood cells. It is given during pregnancy to determine whether or not the Rh-negative mother has been sensitized.
Direct Coombs test
A direct Coombs test or direct antiglobulin test checks for antibodies already attached to red blood cells. It is given when a newborn presents with jaundice or has been identified as at-risk of HDFN. After birth, this detects the presence or absence of HDFN in the newborn.
What to expect from a Coombs test
During pregnancy: A small sample of blood is taken from a vein in the arm or hand of the pregnant mother and sent for analysis at a laboratory. Test results are usually available in 24 hours. If the results are positive for Rh antibodies, the level of antibodies in the blood will be tested using an antibody titer test to better understand the strength of the risk.
You will then need to be closely monitored throughout your pregnancy in case your baby becomes Rh sensitized. Regular blood tests and ultrasounds will monitor the evolution of the Rh antibodies in the blood. If antibody levels become critical, the fetus will undergo an MCA Doppler test to measure the blood flow in the baby’s brain.
Post-delivery: A heel prick blood test is performed on the newborn, with a small blood sample sent to the laboratory for testing. If it is positive, HDFN is confirmed and treatments such as phototherapy for jaundice and blood transfusions for anemia will be considered.
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