Current consensus criteria indicating pathologic rather than physiologic levels of total bilirubin are the following:

1. Total bilirubin level over 5 mg/100 ml (88 µmol/L) in the first 24 hours.
2. Total bilirubin level over 10 mg/100 ml (171 µmol/L) during the second day of life, or an increase of 5 mg/100 ml per day or more thereafter.
3. Total serum bilirubin level more than 15 mg/100 ml in full-term neonates.
4. Total serum bilirubin level more than 12 mg/100 ml in premature neonates.
5. Persistence of jaundice after the first 7 days of life.
6. Conjugated bilirubin level over 1.5 mg/100 ml at any time.

A number of conditions in addition to hemolytic disease of the newborn may be associated with elevated bilirubin; some of these conditions include bacterial sepsis, cytomegalovirus or toxoplasma infection, glucose 6-phosphate dehydrogenase deficiency, and resorption of heme from cephalhematoma or extensive bruising. These are uncommon causes of jaundice. Other factors that may increase total bilirubin are the following:

1. Breast feeding. Although this subject is controversial, a number of studies have found an average increase of about 1.5 mg/100 ml and individual increases up to 2 mg/100 ml or even 3 mg in breast-fed neonates compared to bottle-fed neonates.
2. Race. In one study, Asian neonates had a 31% incidence of nonphysiologic total bilirubin levels after the first day, compared to 20% in Hispanics, 14% in Europeans, and 9% in African Americans.
3. Maternal smoking. This is associated with mildly lower infant bilirubin values.
4. Weight loss and caloric deprivation. These increase serum bilirubin values in both children and adults.
5. Prematurity. Neonates weighing less than 2,000 gm at birth are on the average about 3 times as likely to develop serum total bilirubin levels over 8 mg/1000 ml and those between 2000-2500 gm are twice as likely to develop such levels as infants with normal birth weight of 2500 gm or more. For bilirubin levels of 14 mg/100 ml, the likelihood is 8 times for the lowest birth rate category and 3 times for the intermediate category.

Total bilirubin in neonates is almost all nonconjugated, with the conjugated fraction being less than 0.5 mg/100 ml. Elevated conjugated bilirubin level suggests sepsis, liver or biliary tract disease, or congenital abnormality of bilirubin metabolism such as Rotor’s syndrome. However, the level of conjugated bilirubin partially depends on the combination of assay methodology, reagents, and equipment used. Proficiency test surveys such as those of the College of American Pathologists shows that most laboratories have similar results for total bilirubin. For conjugated bilirubin, however, results on a specimen with elevated nonconjugated bilirubin (such as a normal newborn) show normal conjugated bilirubin in some laboratories, significantly elevated conjugated bilirubin in another group of laboratories, and levels between those of the first two groups in a third group of laboratories. Most of the higher and intermediate values were performed on automated chemistry equipment. In my own laboratory, using a well-known automated chemistry analyzer called the Abbott Spectrum, we were in the group with false high values compared to the normal group and the group with intermediate elevated values. Our upper limit of the reference range for conjugated bilirubin on clinically normal adults is 0.4 mg/100 ml. In a study done on newborns with total bilirubin levels ranging from 2-25 mg/100 ml, the Spectrum reported conjugated bilirubin elevations up to 1.2 mg/100 ml when total (nonconjugated) bilirubin was elevated. Surprisingly, once the level of 1.2 mg/100 ml was reached, it would not increase further regardless of further increase in total bilirubin. Therefore, the Spectrum includes some nonconjugated bilirubin in its conjugated bilirubin assay up to a certain point. The majority of laboratories show this phenomenon to some degree. Therefore, for these labs, the reference range upper limit for conjugated bilirubin must be readjusted when the total bilirubin level is elevated.