Jaundice is the most common transitional finding in the newborn period, occurring in 60–70% of term and ∼80% of preterm infants. An elevation of serum bilirubin concentration >2 mg/dL is found in virtually all newborns in the first several days of life. Jaundice becomes clinically apparent at serum bilirubin concentration of ≥5 mg/dL.
Bilirubin is the end product of the catabolism of heme derived primarily from the breakdown of red blood cell hemoglobin. The rate-limiting step in this production is the oxidation of heme to form a green pigment called biliverdin, a process controlled by the enzyme heme oxygenase. Each molecule of heme catabolized results in equimolar quantities of bilirubin and carbon monoxide. Other sources of heme include heme-containing proteins such as myoglobin, cytochromes, and nitric oxide synthase. Bilirubin exists in several forms in the blood but is predominantly bound to serum albumin; other compounds, such as drugs and metal ions, may compete with bilirubin for albumin binding sites. Elevated concentration of free unconjugated bilirubin can enter the central nervous system (CNS) and become toxic to neurons. The precise mechanism of toxicity is unknown.
Inside liver cells, unconjugated bilirubin is bound immediately to intracellular proteins, the most important one being ligandin. It is then converted into an excretable and soluble form through the process of conjugation that consists of the transfer of 1 or 2 glucuronic acid residues from uridine diphosphoglucuronic acid (UDPGA) to form a monoglucuronide or diglucuronide conjugate. Uridine diphosphoglucuronyl transferase (UDPGT) is the major enzyme involved in this process. Conjugation is impaired in newborns due to reduced UDPGT activity and a relatively low level of uridine diphosphoglucuronic acid. Conjugated bilirubin is water soluble and can be excreted in the urine, but most of it is rapidly excreted as bile into the intestine. Conjugated bilirubin is further metabolized by bacteria in the intestine and excreted in the feces.
Hyperbilirubinemia presents as either unconjugated hyperbilirubinemia or conjugated hyperbilirubinemia. The 2 forms involve different pathophysiologic causes with distinct potential complications. In contrast to unconjugated hyperbilirubinemia, which can be transient and physiologic in the newborn period, conjugated hyperbilirubinemia is always pathologic. See Chapter 100 for a discussion of unconjugated hyperbilirubinemia.
Conjugated hyperbilirubinemia is defined as a measure of direct reacting bilirubin of >1.0 mg/dL, if the total serum bilirubin (TSB) is ≤5.0 mg/dL, or more than 20% TSB. It is the biochemical marker of cholestasis and a sign of hepatobiliary dysfunction. Unlike physiologic unconjugated hyperbilirubinemia, typically known as “physiologic jaundice of the newborn,” it is important to emphasize that there is no physiologic conjugated hyperbilirubinemia. Chapter 57 provides information for rapid “on-call” assessment and management.
Conjugated hyperbilirubinemia affects ∼1 in every 2500 infants and is much less common than unconjugated hyperbilirubinemia.