Bilirubin conjugates in the serum of newborn human infants were investigated using the alkaline methanolysis-high-performance liquid chromatography method, a specific and sensitive method for measurement of unconjugated
bilirubin and
bilirubin mono- and diester conjugates. Serum samples were analyzed from 13 premature infants, 11 full term newborns, 22 healthy adults, seven pregnant women at term and their corresponding infants cord blood at delivery, 46 cord blood specimens obtained at unselected deliveries, three cord bloods from infants with maternal-fetal blood group incompatibility, and two cord bloods from infants with intrauterine
hypoxia.
Bilirubin conjugates were not detectable in the healthy adults, maternal blood, or in the cord blood specimens except from infants with blood group incompatibility or intrauterine
hypoxia. The two isomeric monoconjugates of
bilirubin appeared in serum during the first 24 to 48 postnatal h in both premature and full term infants, followed by the diconjugate on the 3rd day. Conjugated
esters accounted for 2 to 5% of the total
bilirubin, with the diconjugate constituting 21% of total conjugated pigment (day 3). In all instances, the unconjugated serum
bilirubin concentration had increased to at least 2 mg/dl in the course of physiologic
neonatal hyperbilirubinemia before
bilirubin conjugates became detectable. Both premature and full term human infants displayed the identical pattern of
bilirubin conjugation in serum.