Skin Residual Bilirubin Volume (SRBV): A Physiologically Informed Framework for Transcutaneous Bilirubin Interpretation in Neonates

BackgroundNeonatal jaundice management increasingly relies on transcutaneous bilirubinometry (TcB), yet discrepancies with serum bilirubin (TSB) have limited its clinical reliability. This study introduces Skin Residual Bilirubin Volume (SRBV) as a physiologically grounded framework to enhance TcB interpretation. ObjectiveTo evaluate SRBV as an explanation for TcB-TSB discordance and assess whether incorporating SRBV improves the interpretability and reliability of TcB measurements during diagnosis, phototherapy, and recovery. MethodsTcB readings (MBj20) were calibrated against laboratory TSB in non-jaundiced neonates (TSB <3 mg/dL). Neonates undergoing phototherapy were monitored using paired TcB measurements before and after treatment breaks (TBL-out and TBL-return). TSB was measured before treatment, at mid-treatment, and prior to discharge. Patterns of TcB-TSB disparity and an observed reproducible Recovery Value Flip (RVP) phenomenon were analysed. ResultsAcross 102 neonates, TBL consistently equalled or exceeded TSB, supporting the additive SRBV model. Early in phototherapy, TBL-return > TBL-out, indicating persistent cutaneous bilirubin. A reproducible RVP occurred mid-treatment, after which TBL-return < TBL-out coincided with sustained bilirubin decline. Fractional SRBV contribution increased with baseline bilirubin and persisted into recovery, demonstrating dynamic, patient-specific cutaneous bilirubin retention. ConclusionSRBV provides a biologically plausible explanation for TcB-TSB discordance and dynamic TcB behaviour. Incorporating SRBV into TcB interpretation enables physiologically informed monitoring, improving safety and reliability in laboratory-limited neonatal settings. Significance StatementTranscutaneous bilirubinometry is widely used but limited by disagreement with serum bilirubin. This study introduces SRBV as a physiological explanation for TcB variability and proposes an SRBV-adjusted framework that transforms TcB measurements into actionable, non-invasive clinical guidance.