WHAT'S KNOWN ON THIS SUBJECT: Phototherapy decreases bilirubin concentration in skin more rapidly than in blood. During and after phototherapy, transcutaneous bilirubin measurements are considered unreliable and therefore discouraged. WHAT THIS STUDY ADDS:Transcutaneous bilirubin underestimates total serum bilirubin by 2.4 mg/dL (SD, 2.1 mg/dL) during the first 8 hours after phototherapy. This gives a safety margin of ∼7 mg/dL below the treatment threshold to omit confirmatory blood sampling. abstract OBJECTIVES: To compare transcutaneous bilirubin (TcB) readings with total serum bilirubin (TSB) after phototherapy, estimating the range of TcB where confirmation through blood sampling can be avoided. METHODS:Preterm and term neonates receiving in-hospital phototherapy underwent TcB measurements (device JM-103, TcB) alongside routine TSB before and after treatment. We calculated time-dependent safety margins for transcutaneous readings to correctly assign 99% of infants not to receive phototherapy. RESULTS:Between August 2011 and December 2012, 86 newborn infants (47 preterm, 39 term) underwent a total of 189 parallel measurements. Mean difference (TcB 2 TSB) before treatment was 20.6 mg/dL (SD, 1.9 mg/dL). Within the first 8 hours after phototherapy, TcB levels were 22.4 mg/dL (SD, 2.1 mg/dL) below TSB. Thereafter the difference gradually returned to pretreatment values (21.8 mg/dL in 8-16 hours, 21.1 mg/dL in 16-24 hours, and 20.8 mg/dL after 24 hours), while variations remained stable over time (SD, 1.4-1.8 mg/dL). In the first 8 hours after treatment, TcB levels of 27.3 mg/dL below the individual phototherapy threshold allowed safe rejection of confirmatory blood sampling. After 8 hours, that safety margin was reduced to approximately 25.0 mg/dL. CONCLUSIONS: TcB measurements remain a valuable tool after phototherapy when time-dependent underestimation of TcB is being accounted for.