## Brief Summary

Neonatal hyperbilirubinemia is one of the most common causes of neonatal hospitalization and phototherapy treatment. Although phototherapy is highly effective, rebound hyperbilirubinemia following discontinuation of phototherapy may occur in some infants and may require repeat treatment. Early identification of neonates at risk for clinically significant rebound hyperbilirubinemia could help optimize discharge timing and reduce unnecessary hospital stay and repeat bilirubin testing.

This prospective observational study aims to evaluate the predictors of rebound hyperbilirubinemia after phototherapy discontinuation in neonates admitted to the NICU. Particular focus will be placed on the role of delta total serum bilirubin (ΔTSB), defined as the difference between the phototherapy threshold and the measured bilirubin level at the time of phototherapy discontinuation. Clinical, demographic, laboratory, hemolytic, feeding-related, and phototherapy-related variables will also be analyzed.

The primary outcome will be clinically significant rebound hyperbilirubinemia requiring repeat phototherapy within 24-48 hours after discontinuation of the initial phototherapy treatment. Secondary outcomes include rebound bilirubin levels, duration of hospitalization, and factors associated with repeat phototherapy.

## Detailed Description

Neonatal hyperbilirubinemia remains one of the leading causes of neonatal hospitalization worldwide. Phototherapy is the standard treatment for significant unconjugated hyperbilirubinemia and is highly effective in reducing serum bilirubin levels. However, a subset of neonates may develop rebound hyperbilirubinemia after discontinuation of phototherapy, occasionally requiring repeat phototherapy and prolonged hospitalization.

Current evidence regarding predictors of rebound hyperbilirubinemia remains limited, particularly in prospective NICU-based cohorts. Identification of infants at increased risk for clinically significant rebound hyperbilirubinemia may improve individualized monitoring strategies, optimize timing of discharge, and reduce unnecessary bilirubin measurements and hospital stay.

This prospective observational study will include neonates admitted to the NICU and treated with phototherapy for hyperbilirubinemia. Clinical and laboratory parameters associated with rebound hyperbilirubinemia will be evaluated. Special emphasis will be placed on delta total serum bilirubin (ΔTSB), defined as the difference between the phototherapy threshold recommended by current guidelines and the measured total serum bilirubin level at the time of phototherapy discontinuation.

Additional variables including gestational age, postnatal age at phototherapy initiation, feeding type, hemolytic risk factors, bilirubin kinetics, and phototherapy characteristics will also be analyzed. Rebound bilirubin measurements will be obtained within 24-48 hours after discontinuation of phototherapy according to unit protocol.

The primary objective is to determine predictors of clinically significant rebound hyperbilirubinemia requiring repeat phototherapy. Secondary objectives include evaluation of bilirubin rebound levels, hospitalization duration, and the association between phototherapy characteristics and rebound risk.

Inclusion Criteria:

• Neonates admitted to the NICU with neonatal hyperbilirubinemia Gestational age ≥35 weeks Requirement for phototherapy according to institutional protocol and current guideline-based phototherapy thresholds Receipt of standard and/or intensive phototherapy Availability of bilirubin measurements before, during, and after phototherapy Availability of rebound bilirubin measurement within 48 hours after phototherapy discontinuation Written informed consent obtained from parents or legal guardians, if required by the ethics committee

Exclusion Criteria:

• Major congenital anomalies Conjugated hyperbilirubinemia Neonates requiring exchange transfusion before completion of initial phototherapy Severe perinatal asphyxia Proven or suspected inborn errors of metabolism affecting bilirubin metabolism Significant congenital liver disease Neonates transferred to another center before completion of rebound bilirubin follow-up Missing or incomplete clinical or laboratory data Absence of rebound bilirubin measurement within 48 hours after phototherapy discontinuation Parents or legal guardians declining participation, if consent is required

The study population consists of neonates admitted to the NICU for neonatal hyperbilirubinemia and treated with phototherapy according to institutional protocols and current guideline-based phototherapy thresholds. Both standard and intensive phototherapy modalities may be used based on bilirubin levels, gestational age, and neurotoxicity risk factors.

Eligible neonates will be prospectively followed after discontinuation of phototherapy for evaluation of rebound hyperbilirubinemia and repeat phototherapy requirement. Clinical, laboratory, hemolytic, feeding-related, and phototherapy-related variables will be recorded and analyzed to identify predictors of clinically significant rebound hyperbilirubinemia.

Individual participant data (IPD) will not be publicly shared because the study involves sensitive neonatal clinical data obtained from a single-center NICU cohort. Data sharing may be restricted by institutional policies and ethical considerations regarding patient confidentiality and privacy. Aggregated and anonymized study results will be presented in scientific publications and presentations.