The bilirubin can be either unconjugated or conjugated. Hyperbilirubinemia in children is usually unconjugated and most often caused by problems with red blood cell stability and survival or by defects in the bilirubin-conjugating enzyme uridine diphosphoglucuronate glucuronosyltransferase (UGT).

It is apparent in infants when the serum bilirubin value is greater than 4 to 5 mg/dL (68.4 to 85.5 mcmol/L) and in older children at values greater than 2 to 3 mg/dL (34.2 to 51.3 mmol/L). Serum total bilirubin is measured in the laboratory as the sum of two components: unconjugated (“indirect”) and conjugated (“direct”) fractions. The.