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 conjugates it with glucuronic acid and makes it water-soluble. Conjugated bilirubin can then leave the liver in bile and enter the intestine. When the indirect fraction accumulates, look upstream for increased production, reduced hepatic uptake, or impaired conjugation. When the direct fraction rises, look for hepatocyte injury, impaired bile transport, or obstruction. Always interpret the result with the laboratory's reference interval and the clinical setting rather than applying a single universal adult range. In newborns, treatment decisions depend on the bilirubin value, age in hours, gestational age, and neurotoxicity risk factors. Persistent, clinically significant jaundice beyond 14 days requires measurement of the direct fraction and further investigation rather than reassurance alone.