Transcutaneous bilirubinometry: bilirubin kinetics of the skin and serum during and after phototherapy.

Skin bilirubin (transcutaneous bilirubinometry, TcB, reading) and serum bilirubin kinetics were studied in 20 full-term hyperbilirubinemic infants during and after phototherapy. TcB readings at the chest site decreased after 2 h of phototherapy from 28.0 to 21.3 (25% of initial TcB reading, p less than 0.001) and thereafter continued to decrease in a nonlinear fashion through the first 24 h of phototherapy. However, TcB readings after 12 h of phototherapy were about 50% of the initial readings (p less than 0.001), declining minimally during the second 12 h of phototherapy, suggesting saturation kinetics. While serum bilirubin levels remained unchanged during the first 4 h of phototherapy, thereafter they showed a rapid decline to 12 h of phototherapy. The rate of decline decreased during the second 12 h of phototherapy, but serum bilirubin levels still decreased significantly from 16.9 to 14.9 mg/dl. Although TcB readings showed saturation kinetics during the second 12 h of phototherapy, the serum bilirubin levels were shown to decrease significantly. This suggests that the main site of action of phototherapy may be intravascular at this stage, rather than the skin and subcutaneous capillary bed as in the early stage of phototherapy.