Determinants of unbound bilirubin and clinical utility of the total bilirubin/albumin ratio in neonates.

backgroundTo identify determinants of unbound bilirubin (UB) and assess whether the total bilirubin/albumin ratio (TB/Alb) can serve as a surrogate index in neonates.

methodsWe retrospectively analyzed 2248 serum samples from 741 infants in a single neonatal intensive care unit (January 2022-October 2025). UB, total bilirubin (TB), direct bilirubin (DB), and albumin (Alb) were measured. Linear mixed-effects models with infant as a random effect accounted for repeated measurements and assessed determinants of UB and its association with TB/Alb. A mixed-effects logistic model evaluated the prediction of UB ≥ 0.6 µg/dL. Receiver operating characteristic analysis assessed the identification of UB ≥ 0.8 µg/dL using TB/Alb.

resultsUB increased with TB and DB and decreased with Alb. TB/Alb was strongly associated with UB and predicted UB ≥ 0.6 µg/dL. For UB ≥ 0.8 µg/dL, the optimal TB/Alb cutoff was 4.0 (sensitivity 0.94, specificity 0.85; area under the curve 0.95).

conclusionIn routine clinical samples, UB was primarily determined by TB and Alb concentrations. TB/Alb showed a clear dose-response relationship with UB and accurately identified high UB, supporting its use when direct UB measurement is unavailable. IMPACT: Unbound bilirubin (UB), the biologically relevant fraction associated with neurotoxicity, is difficult to measure routinely in many neonatal units. In 2248 clinical samples from 741 infants, the total bilirubin/albumin ratio (TB/Alb) showed a strong linear association with UB (UB = 0.165 × TB/Alb - 0.116). TB/Alb = 4 identified high UB levels (≥0.8 µg/dL) with excellent discrimination (AUC 0.952; optimal cutoff 4.03). TB/Alb can serve as a practical surrogate for UB, enabling rapid bedside risk stratification where UB testing is unavailable.