Transcutaneous bilirubinometry. The cephalocaudal progression of dermal icterus.

The transcutaneous bilirubinometer (TcB) was used to document the cephalocaudal progression of dermal icterus in neonates. The TcB index of a specific area was compared with that of the forehead (TcB ratio). Four surface regions were identified, placed in the following sequence with respect to decreasing TcB ratio values: (1) forehead and sternum; (2) elbow, upper part of the back, and upper part of the abdomen; (3) lower part of the back and knee; and (4) palm and sole. To examine the relationship of the skin region of different serum bilirubin levels, a model was created that allowed the correlation of the TcB index with the specific region's distance from the forehead. Area differences in TcB values at lower bilirubin concentrations were less marked than at high levels. While the mechanism of this phenomenon has not been elucidated, the role of biophysical properties of the skin remains to be explored.