- At least one estimate of serum bilirubin. Bilirubin level estimated to be less than the 40th percentile ("low risk zone") based on the Bhutani nomogram. While a serum bilirubin in the low risk zone is the preferred method for assessing the bilirubin level, many pediatricians use transcutaneous measure of bilirubin as a screening tool for identifying "low risk" patients. For this reason, we will allow controls to be identified using transcutaneous measurements and collect serum bilirubin levels only as clinically indicated.