A newborn with a positive DAT and clinical suspicion of HDN most likely indicates Rh incompatibility (Rh HDN).

• A. ABO HDN
• B. Kell HDN
• C. Rh HDN
• D. No HDN

Answer: (C)

A positive direct antiglobulin test means maternal antibodies are bound to the newborn’s red cells. In newborn hemolytic disease of the newborn (HDN), this happens when maternal IgG antibodies cross the placenta and attach to fetal red-cell antigens, tagging them for destruction. The Rh system is classically involved because anti-D IgG crosses readily and binds the fetal Rh(D) antigen, coating red cells and driving hemolysis. This leads to anemia and increased bilirubin after birth, with the DAT remaining positive due to ongoing antibody coating. ABO incompatibility can also cause HDN, but it is usually milder and not as strongly associated with this pattern. Kell HDN is less common but can present similarly. Therefore, the positive DAT in a suspected HDN newborn most strongly points to Rh incompatibility.