Bilirubin is a yellowish compound that forms naturally in the body when old red blood cells are broken down. Hemoglobin, the oxygen-carrying component within red cells, is metabolized into this pigment. The liver is responsible for processing bilirubin so it can be eliminated from the body.
In newborns, the body’s systems for handling this compound are still developing, which often leads to a temporary accumulation. This buildup causes the skin and eyes to take on a yellow tint, a common and usually harmless condition known as newborn jaundice. Because excessively high bilirubin levels can pose a risk to the baby, healthcare providers closely monitor these concentrations in the first days of life.
Why Bilirubin Levels Rise in Newborns
The rise in bilirubin experienced by nearly all newborns is due to increased production and decreased clearance. Immediately after birth, a baby’s red blood cell turnover rate is much higher than an adult’s. This accelerated destruction of fetal red blood cells releases a larger amount of bilirubin into the bloodstream.
At the same time, the newborn’s liver is still immature and not fully efficient at processing this increased load. Bilirubin initially exists in an unconjugated, fat-soluble form that the body cannot easily excrete. The liver must attach the bilirubin to a sugar molecule—a process called conjugation—to make it water-soluble, allowing it to be flushed out in bile and stool.
The enzyme responsible for this conjugation, uridine diphosphate-glucuronosyltransferase (UGT), has low activity in the first days of life. This temporary deficit, combined with the high production rate, results in the accumulation known as physiological jaundice. Levels typically peak between the third and fifth day after birth.
Interpreting Newborn Bilirubin Thresholds
There is no single “normal” number for a baby’s bilirubin level, as the interpretation depends heavily on the baby’s age and their individual risk factors. For a full-term, healthy newborn, the average total serum bilirubin (TSB) level usually peaks around 5 to 6 milligrams per deciliter (mg/dL). This peak typically occurs between 72 and 96 hours of life before declining.
The threshold for when a level becomes concerning is dynamic and charted by pediatricians. Levels acceptable at 48 hours of age may require intervention if seen at 24 hours of age. For instance, 15 mg/dL might be the threshold for intervention in a 25- to 48-hour-old infant, but a higher level is tolerated at 72 hours.
The clinical decision is further modified by the presence of risk factors that increase the potential for neurotoxicity. These factors include prematurity (gestational age less than 38 weeks), significant bruising or a scalp hematoma from birth, blood type incompatibility, or a sibling who required phototherapy. A baby with these risk factors will have a lower bilirubin threshold for intervention than a healthy, full-term infant.
Bilirubin is measured using two main methods: transcutaneous bilirubin (TcB) and total serum bilirubin (TSB). TcB measurement is a non-invasive screening tool that uses a device gently pressed against the skin, often the forehead or sternum, providing a quick estimate. TcB is generally reliable for screening but cannot fully replace TSB, which requires a small blood sample.
If the TcB reading approaches or exceeds the age-specific cutoffs, a TSB blood test is required to confirm the level for clinical decision-making. TSB is considered the gold standard and is necessary when levels are high or when the infant is already undergoing phototherapy, as the light can make TcB measurements unreliable.
Measuring Levels and Clinical Management
Once bilirubin levels are measured and deemed concerning, medical professionals initiate steps to reduce the concentration. The primary intervention is phototherapy, which involves exposing the baby’s skin to specific wavelengths of light, typically in the blue-green spectrum. The light energy alters the structure of the unconjugated bilirubin molecule through a process called photoisomerization.
This change converts the fat-soluble bilirubin into water-soluble forms, such as lumirubin, allowing it to be excreted directly in the bile and urine. The effectiveness of phototherapy is maximized by ensuring the light source is close to the baby and that the largest possible surface area of skin is exposed. This treatment is highly effective and safe, significantly reducing the risk of a severe buildup.
Despite the effectiveness of phototherapy, monitoring is important because untreated, excessively high levels of unconjugated bilirubin can be toxic to the central nervous system. This can lead to a rare but severe form of brain damage called kernicterus.
In rare cases where the bilirubin level continues to rise rapidly despite intensive phototherapy, an exchange transfusion may be necessary. This procedure involves slowly removing a small amount of the baby’s blood and replacing it with donor blood, which quickly lowers the circulating bilirubin concentration. This is a rescue therapy reserved for the most serious cases to prevent neurological injury.