Relationship between transcutaneous bilirubin and circulating unbound bilirubin in jaundiced neonates

Background: Transcutaneous bilirubin (TcB) consists of the skin-deposited bilirubin. Free bilirubin represents the protein-unbound bilirubin (UB) that is able to pass into the tissues. We aimed to describe the relationship UB-TcB and study the passage of UB into the skin. Methods: We prospectively enrolled 194 neonates and we measured TcB, UB, serum bilirubin and albumin. Multiple sites TcB measurement was performed, bilirubin-albumin equilibrium constant and plasma bilirubin avidity (PBA) were calculated. Result: TcB has a similar correlation with UB and TSB. There is a quadratic relationship between UB and TcB (R-2 = 0.48; p < 0.001), remaining significant (beta for UB2 = 0.8; p < 0.001 beta for UB = 1.1; p < 0.001) after adjustment for gestational age, birth weight, postnatal age and albumin (Adj-R-2 = 0.72). UB contributes to the skin bilirubin deposition, as there are significant correlations between albumin and TcB (r = -0202; p = 0.01) and between PBA and Delta TcB (r = 0323; p = 0.017). Conclusion: TcB assay does not seem to directly replace UB measurement. However, TcB and UB are linked by a quadratic relationship: UB contributes to the skin bilirubin deposition but it is not the only bilirubin species measured by transcutaneous bilirubinometry. (C) 2016 Elsevier Ireland Ltd. All rights reserved.