Skin residual bilirubin volume: a physiologically informed framework for transcutaneous bilirubin interpretation in neonates.

BACKGROUND

Transcutaneous bilirubinometry (TcB) is increasingly relied upon for neonatal jaundice management in settings without access to laboratory-based total serum bilirubin (TSB) testing. However, inconsistent agreement between transcutaneous bilirubin levels (TBL) and TSB limits its reliability as a standalone clinical tool.

OBJECTIVE

To propose and evaluate skin residual bilirubin volume (SRBV) as a physiological explanation for TcB-TSB discordance and to assess its impact on TcB interpretation during diagnosis, treatment, and recovery.

METHODS

This observational, multicentre study was conducted in resource-limited neonatal care settings. TcB readings were calibrated against laboratory TSB in non-jaundiced neonates (TSB <3 mg/dL). Neonates undergoing phototherapy were monitored using paired TcB measurements obtained immediately after treatment interruption (TBL-out) and prior to treatment resumption (TBL-return). TSB was measured before treatment, during treatment, and prior to discharge where feasible. Patterns of TcB-TSB disparity and the "Recovery Value Flip (RVP)" phenomenon were analysed.

RESULTS

TBL consistently equalled or exceeded TSB in cases with paired measurements. Early in treatment at higher bilirubin levels, TBL-return exceeded TBL-out, indicating persistent cutaneous bilirubin. A reproducible RVP was observed, after which TBL-return became lower than TBL-out, coinciding with sustained bilirubin decline. SRBV contribution varied with bilirubin level and treatment phase.

CONCLUSION

SRBV 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.