Diagnosis of ABO HDFN may be difficult because the direct antiglobulin test (DAT) on a newborn's red cells is unreliable. In fact, many labs do not routinely perform a DAT on infants born to Rh-positive females. Cord blood is often retained for a specified time should the infant develop signs of HDFN and required testing.
If ABO HDFN is possible (based on incompatible ABO blood groups and a positive DAT) and the mother's antibody screen is negative, many laboratories do not investigate the positive DAT as would be done for unexpected antibodies like anti-D or anti-K. Instead, the infant's plasma is tested against group A1 (or B cells) and group O screen cells using the indirect antiglobulin test (IAT). A positive reaction with A1 or B cells, but not group O cells, would suffice to report a case of possible ABO HDFN.
NOTE: Before ABO HDFN is considered as a possible cause of jaundice and anemia in the newborn, other causes should be considered, such as erythrocyte membrane defects or red cell enzyme deficiencies.