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 (R2 =0.48; p<0.001), remaining significant (β for UB2 =-0.8; p<0.001. β for UB=1.1; p<0.001) after adjustment for gestational age, birth weight, postnatal age and albumin (Adj-R2 =0.72). UB contributes to the skin bilirubin deposition, as there are significant correlations between albumin and TcB (r=-0.202; p=0.01) and between PBA and ΔTcB (r=0.323; 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.