Physiologic - Conjugation
Physiologic jaundice
Conjugation
As mentioned before, conjugation of bilirubin is severely limited in the newborn. BUG-T levels are only 1% of adult normal values at term and even lower in prematures. While these decreased levels by themselves would not lead to hyperbilirubinemia, they do in combination with the increased load presented to the liver. As soon as BUG-T activity increases sufficiently to handle the increased load, bilirubin levels start to fall (day 4-5).In the premature baby, physiologic jaundice occurs later and the peak bilirubin is higher than in term infants. The peak bilirubin in prematures regularly reaches 10-12 mg/dL (without therapy) and peaks on day 5-6. This is completely due to the extremely low BUG-T activity. After birth, BUG-T levels increase faster than would happen in utero; however normal bilirubin levels are still not reached until 2-3 weeks of age.Other factors can influence the BUG-T activity. Neonates born to mothers treated during pregnancy or labor with phenobarbital, a drug known to induce hepatic BUG-T activity, have a lower incidence and severity of physiologic jaundice. The same is true for neonates of heroin-abusing mothers (although this is not the case for methadone), as well as post-term newborns and about half of term small-for-gestational age (SGA) babies. There are also genetic differences in BUG-T activities. Physiologic jaundice is more severe in American Indian and Asian newborns, as well as those of certain geographic populations in Greece and Turkey. These variances have been clearly shown to be related to different levels of BUG-T activity and not to other diseases like G6PD deficiency, which are also higher in incidence in these populations.Return to top of page