GSTM1 and bronchopulmonary dysplasia: However, when we compared the no-or-mild BPD group with the moderate-to-severe BPD group at the same postnatal ages, there were no significant differences in the bilirubin and hematocrit levels, which suggest that the higher CO-Hb levels observed in infants who developed moderate-to-severe BPD may be indicative of ongoing oxidative stress insults rather than physiological hemolysis.