Neonatal Hyperbilirubinemia: Unconjugated Bilirubin Exceeding Total Bilirubin Post-Phototherapy

Clinical laboratories frequently encounter discordant bilirubin results, where unconjugated bilirubin (Bu) exceeds total bilirubin (TBil), particularly in neonates undergoing phototherapy. This discrepancy complicates clinical decision-making for neonatal jaundice management.

A 5-day-old female neonate with hyperbilirubinemia and cephalohematoma was analyzed. Bilirubin levels were measured using dry chemistry (Ortho 5600) and validated via wet chemistry (vanadic acid oxidation).

Initial dry chemistry results showed TBil: 352.4 µmol/L, Bu: 388.1 µmol/L, and conjugated bilirubin (Bc): 13.5 µmol/L. Phototherapy-generated bilirubin photoisomers (structural and configurational isomers) interfered with assay specificity. Structural isomers were misclassified as Bc in dry chemistry but excluded from TBil assays, leading to Bu + Bc exceeding TBil. Wet chemistry confirmed TBil (355.4 µmol/L) consistency but highlighted method-dependent variability in isomer detection.

Methodological differences in detecting phototherapy-induced bilirubin isomers contribute to paradoxical results. Clinicians should use consistent methods for serial monitoring, and laboratories must analyze preanalytical factors, such as phototherapy timing, to ensure accurate reporting.