Evaluation of the TB/pTP Ratio Using Refractometric Measurement of Plasma Total Protein in Neonates

Unbound bilirubin (UB) is the most direct indicator of bilirubin-induced neurotoxicity in neonates; however, its measurement may be unreliable under certain conditions. Although the total bilirubin-to-albumin ratio (TB/Alb) reflects bilirubin-binding capacity, frequent albumin measurement is limited by blood volume requirements and cost. Plasma total protein (pTP) measured by refractometry can be rapidly obtained using minimal blood volume and may serve as a practical surrogate. This study aimed to evaluate the relationship between pTP and albumin and examine whether the TB/pTP ratio correlates with UB levels.

We retrospectively analyzed neonates born at ≥ 35 weeks' gestation who were admitted to a neonatal intensive care unit between January 2023 and August 2025. Total protein and albumin were measured in the central laboratory, while TB, pTP, and UB were measured at the point of care. Agreement between TP and pTP was assessed using Bland-Altman analysis. Correlations among biochemical markers and the relationship between UB and the TB/pTP ratio were evaluated.

In total, 192 infants (694 measurements) were included. TP and pTP showed good agreement (mean difference, 0.04 g/dL). Both correlated significantly with albumin (r² = 0.73 and 0.57, respectively; p < 0.05). In non-acetaminophen-treated samples, UB was strongly correlated with the TB/pTP ratio (r² = 0.72, p < 0.05). In contrast, acetaminophen-treated samples showed higher UB values that deviated from those predicted by TB/pTP.

The TB/pTP ratio may serve as a simple and practical alternative to TB/Alb for managing neonatal hyperbilirubinemia, particularly when UB measurements are unreliable or unavailable.