Bilirubin Metabolism

Bilirubin is a flow problem, not a diagnosis by itself. Heme breakdown produces lipid-soluble unconjugated bilirubin; albumin carries it to the liver, where UGT1A1 adds glucuronic acid and makes it water-soluble. The conjugated product can then leave in bile. When the indirect fraction accumulates, look upstream for increased production or impaired conjugation. When the direct fraction rises, look for hepatocyte injury, impaired bile transport, or obstruction. That distinction turns a yellow patient into a localization problem. In adults, total bilirubin is approximately 5–21 µmol/L, and jaundice usually becomes visible at about 34–43 µmol/L. These adult reference points do not determine treatment in a newborn: neonatal decisions depend on the bilirubin value, the infant’s age in hours, gestational age, and neurotoxicity risk factors. A direct bilirubin elevation is never physiologic in a newborn, and jaundice persisting beyond 14 days requires evaluation for cholestasis rather than reassurance alone.