What's in a Name? Physiologic and Pathologic Jaundice: The Conundrum of Defining Normal Bilirubin Levels in the Newborn

Abstract

Because at some point during the first week after birth almost every newborn has a total serum bilirubin (TSB) level that exceeds 1 mg/dL (17 μmol/L), the upper limit of normal for an adult, and ∼2 of every 3 newborns are jaundiced to the clinician's eye, this type of transient bilirubinemia has been called “physiologic jaundice.” When TSB levels exceed a certain value, the infant is often described as having “pathologic jaundice.” I would like to argue that these terms have limited utility and are often used incorrectly, with potentially damaging consequences. They should be abandoned. The physiology of the newborn differs from that of older children and adults in many ways. Newborns breathe 40 to 60 times per minute, and their hearts beat 120 to 160 times per minute. Their hematocrit levels are frequently >60%. In time, all of these values return to normal levels, as does their bilirubin level. However, we don't talk about physiologic tachypnea, tachycardia, or polycythemia, so why pick on jaundice? Some like the term “physiologic jaundice” because it has a reassuring sound for parents and physicians. Presumably, physiologic jaundice should apply to newborns whose TSB levels fall within a certain range, but what is that range? Because very few (if any) newborns have peak TSB levels <2 mg/dL, should an infant with a peak TSB of 1.5 mg/dL be considered abnormal or hypobilirubinemic? Unlike serum sodium levels, the range of normal TSB levels varies widely depending on the racial composition of the population, the incidence of breastfeeding, and other genetic and epidemiologic factors.1 There are also significant variations between different laboratories in their measurements of serum bilirubin.2 Term, healthy, North American, formula-fed infants have mean peak TSB levels between 5 and 6 mg/dL (86 and 103 μmol/L),3,4 whereas Japanese … Address correspondence to M. Jeffrey Maisels, MB, BCh, Department of Pediatrics, William Beaumont Hospital, 3601 W 13 Mile Rd, Royal Oak, MI 48073. E-mail: jmaisels{at}beaumont.edu