Diagnosis of ABO HDFN is supported by these findings:
- ABO incompatibility between mother and child, with mother typically group O
- Negative maternal antibody screen
- Weakly positive or negative cord DAT
- Newborn hyperbilirubinemia with jaundice occurring in the first 24 hours
- Increased spherocytes and reticulocytosis in the newborn
- Presence of IgG anti-A or anti-B in cord plasma/serum
In rare cases, a negative maternal antibody screen and positive DAT on the cord blood could also be due to a maternal antibody to a low-frequency antigen that the newborn inherited from the father and that is not present on the screen cells.
However, if ABO incompatibility exists, this rare possibility is typically ignored. Crossmatching RBC for transfusion to the newborn with maternal plasma would prevent incompatible blood from being transfused.