Limitations and Opportunities of Transcutaneous Bilirubin Measurements

OBJECTIVE: Although transcutaneous bilirubinometers have existed for over 30 years, the clinical utility of the technique is limited to a screening method for hyperbilirubinemia, rather than a replacement for invasive blood sampling. In this study, we investigate the reason for this limited clinical value and address possibilities for improvement. METHODS: To obtain better insight into the physiology of bilirubin measurements, we evaluated a transcutaneous bilirubinometer that determines not only the cutaneous bilirubin concentration (TcB) but also the blood volume fraction (BVF) in the investigated skin volume. For 49 neonates (gestational age 30 ± 3.1 weeks, postnatal age 6 [4–10] days) at our NICU, we performed 124 TcB and 55 BVF measurements. RESULTS: The TcB correlated well with the total serum bilirubin concentration (TSB) ( r = 0.88) with an uncertainty of 55 µmol/L. The BVF in the measured skin volume ranged between 0.1% and 0.75%. CONCLUSIONS: The performance of our bilirubinometer is comparable to existing transcutaneous devices. The limited clinical value of current bilirubinometers can be explained by the low BVF in the skin volume that is probed by these devices. Because the TcB depends for over 99% on the contribution of extravascular bilirubin, it is a physiologically different parameter from the TSB. Hence, the standard method of evaluation that compares the TcB to the TSB is insufficient to fully investigate the clinical value of transcutaneous bilirubinometers, ie, their predictive value for kernicterus. We suggest that the clinical value may be improved considerably by changing either the method of evaluation or the technological design of transcutaneous bilirubinometers. * Abbreviations: BVF — : blood volume fraction PLs — : prediction limits TcB — : cutaneous bilirubin concentration TcHb — : cutaneous deoxygenized hemoglobin concentration TcHbO2 — : cutaneous oxygenized hemoglobin concentration TSB — : total serum bilirubin concentration