Which test is used to determine whether the newborn's red blood cells have been coated by maternal antibodies after birth?

• A. Indirect Coombs test
• B. Direct Coombs test on newborn RBCs
• C. ABO typing
• D. RBC count

Answer: (B)

The test used to determine if the newborn’s red blood cells have been coated by maternal antibodies after birth is the direct antiglobulin test, performed on the newborn’s washed RBCs. This assay directly looks for antibodies already attached to the surface of the infant's red cells. If the test is positive, it means maternal IgG antibodies are bound to the fetal RBCs, which can lead to hemolysis and is seen in conditions like hemolytic disease of the newborn.

Understanding the distinction helps: the direct Coombs test detects in vivo sensitization of the infant’s RBCs by antibodies, while the indirect Coombs test screens the mother’s serum for antibodies that could react with RBCs. ABO typing and a simple RBC count don’t assess whether antibodies are coating the cells.