Bilirubin is a yellowish waste product created when the body recycles old red blood cells. In adults, the liver processes this substance for elimination, but in newborns, the system is less efficient, leading to a temporary buildup in the bloodstream. This buildup results in a condition known as hyperbilirubinemia, which often presents as newborn jaundice.
Why Newborns Produce Bilirubin
The temporary elevation of bilirubin levels in newborns, known as physiological jaundice, results from a dual mechanism of increased production and decreased clearance. Before birth, the fetus uses a unique type of red blood cell designed for the low-oxygen environment within the womb. After delivery, these fetal red blood cells are rapidly broken down and replaced, creating a significantly higher load of bilirubin waste compared to an adult.
A newborn’s liver is temporarily immature and cannot handle this sudden increase in bilirubin processing. The enzyme responsible for converting unconjugated bilirubin into a water-soluble form for excretion is called uridine diphosphoglucuronosyltransferase (UGT). Since UGT is not yet working at full capacity in the first few days of life, the rate of bilirubin clearance slows. This combination of increased production and decreased clearance causes the substance to accumulate in the baby’s tissues.
Interpreting Normal and High Levels
There is no single numerical value for “normal” bilirubin in a newborn, as the acceptable range changes dynamically every hour after birth. Healthcare providers use the Total Serum Bilirubin (TSB) measurement, which is the total amount of bilirubin in the blood, to assess a baby’s risk. The interpretation of this TSB value is entirely dependent on the infant’s age in hours, not just in days.
To determine if a bilirubin level is safe or concerning, doctors plot the TSB result on an hour-specific nomogram, which is a graph divided into different risk zones. These zones are categorized as low, intermediate, and high risk for developing severe hyperbilirubinemia. For a full-term, healthy baby, a TSB level under 5 milligrams per deciliter (mg/dL) in the first 24 hours is considered low-risk.
The threshold for requiring intervention rises over time, generally peaking around three to five days of life. For instance, a level of 10 mg/dL might be high-risk at 18 hours of age, but low-risk at 72 hours. Levels exceeding 15 to 20 mg/dL may require treatment, but the precise threshold is adjusted based on factors like the baby’s gestational age, the presence of risk factors such as blood type incompatibility, and the rate at which the TSB is rising. This tool helps differentiate between expected physiological jaundice and pathological hyperbilirubinemia that requires immediate action.
Identifying Jaundice and Screening Methods
The most common sign of hyperbilirubinemia is jaundice, which is a yellowish discoloration of the skin and the whites of the eyes. This visual change is caused by the deposition of bilirubin in the dermal and subcutaneous tissues. Jaundice follows a predictable pattern of progression, often appearing first on the face and head, then moving down to the chest and abdomen in what is called a cephalocaudal direction.
Visual assessment alone is considered unreliable for accurately estimating the bilirubin concentration. Therefore, all newborns are screened using objective methods, starting with the Transcutaneous Bilirubin (TcB) measurement. The TcB is a quick, non-invasive test where a device placed on the baby’s skin estimates the bilirubin level by measuring the intensity of the yellow color.
The TcB is an effective screening tool that reduces the need for invasive blood draws. If the TcB reading is above a certain threshold or if the baby has specific risk factors, the result must be confirmed with a definitive blood test. This confirmatory test is the Total Serum Bilirubin (TSB) measurement, which requires a small blood sample, usually obtained via a heel stick, and is considered the gold standard for accuracy.
Treatment for Elevated Bilirubin
The primary and most common treatment for significantly elevated unconjugated bilirubin is phototherapy. This intervention involves placing the newborn under special blue-green lights, often while wearing only a diaper and eye protection. The light energy penetrates the skin and works by changing the molecular structure of the bilirubin.
This process is called photoisomerization, where the light transforms the unconjugated bilirubin (which is fat-soluble) into water-soluble isomers. These transformed products bypass the immature liver’s conjugation process and are easily eliminated from the body through the baby’s urine and stool. Phototherapy is highly effective and safe, and it can be administered in a hospital or, in less severe cases, at home using specialized fiber-optic blankets.
In rare instances where bilirubin levels reach dangerously high concentrations and do not respond adequately to intensive phototherapy, an exchange transfusion may be necessary. This involves slowly removing small amounts of the baby’s blood and replacing it with donor blood to rapidly lower the circulating bilirubin concentration. This intensive measure is reserved for the most severe cases to prevent kernicterus, a rare but serious type of permanent brain damage caused by excessive unconjugated bilirubin crossing the blood-brain barrier.