Which statement best describes the significance of a positive DAT in a newborn?

• A. DAT confirms ABO incompatibility
• B. DAT indicates maternal alloantibody coating infant red cells
• C. DAT rules out HDN
• D. DAT only occurs with anti-P1

Answer: (B)

A positive direct antiglobulin test in a newborn shows that maternal antibodies are attached to the baby’s red cells in vivo. This means maternal IgG (or complement) has crossed the placenta and bound to fetal red cell antigens, which can drive hemolysis and underlie hemolytic disease of the newborn. That’s why the best description is that the DAT indicates maternal alloantibody coating infant red cells. It doesn’t pinpoint a specific incompatibility like ABO, and a positive result supports immune-mediated hemolysis rather than ruling it out. Also, the test isn’t limited to anti-P1; many different antibodies can produce coating detected by the DAT.