Bilirubin is a yellow-orange pigment that is a natural byproduct of the body’s process of breaking down old red blood cells. In newborns, this process is accelerated because they have a higher number of red blood cells with a shorter lifespan. This produces more bilirubin than a newborn’s immature liver can initially handle.
The liver is responsible for processing bilirubin so it can be excreted from the body. A newborn’s liver enzymes may not be fully developed, leading to a temporary buildup of bilirubin in the blood, a condition called hyperbilirubinemia. This accumulation is why bilirubin levels are closely monitored in the first days and weeks of life.
What is Transcutaneous Bilirubin (TcB) Measurement?
Transcutaneous bilirubin (TcB) measurement is a non-invasive method used to estimate bilirubin levels in a newborn without a blood draw. This technique uses a handheld device, a bilirubinometer, which is gently pressed against the infant’s skin, typically on the forehead or sternum. The device works by directing a flash of light into the skin and measuring the intensity of specific light wavelengths that are reflected back.
The device analyzes the reflected light to determine the amount of yellow pigment from bilirubin in the skin and underlying tissue. This provides an immediate estimate, saving time compared to a lab test. Its primary advantages are that it is painless, reduces the risk of infection, and serves as an effective screening tool to determine if a formal blood test is necessary.
While TcB readings correlate well with blood levels, they measure bilirubin in the tissue, not the blood itself. The measurement provides a valuable estimation rather than a direct replacement for a total serum bilirubin (TSB) blood test, which is the gold standard for diagnosis. TcB screening helps reduce the number of invasive blood draws for newborns.
Typical TcB Bilirubin Levels in Newborns
There is no single normal TcB value for all newborns; levels are a dynamic range that changes based on the baby’s age in hours. Bilirubin levels rise in all infants for the first few days, peaking between the third and fifth day before declining. Healthcare providers interpret TcB readings using hour-specific nomograms, which are charts that plot bilirubin levels against the infant’s age.
These nomograms are constructed from data on thousands of newborns and are divided into percentile-based risk zones. A TcB reading is plotted on the chart to see which risk percentile it falls into for that specific hour of life. This allows clinicians to predict the likelihood of the bilirubin level rising to a point that might require intervention. A reading in a lower percentile suggests a low risk, while a reading in the 95th percentile indicates a high risk.
Factors such as gestational age also play a role. Infants born before 38 weeks of gestation often have higher bilirubin peaks than full-term infants because their livers are less mature. These charts are tools to guide clinical judgment, not rigid rules.
Understanding Elevated TcB Bilirubin Readings
An elevated TcB reading indicates hyperbilirubinemia, an excess of bilirubin in the blood. When bilirubin levels rise, the pigment gets deposited in the skin, mucous membranes, and the sclera (the whites of the eyes), causing a yellowish appearance. This visible yellowing is known as jaundice, and it appears on the face first and progresses down the body as levels increase.
While mild jaundice is common and often harmless, high levels of unconjugated bilirubin are a concern because it can be neurotoxic. If bilirubin levels become too high and are left untreated, the bilirubin can cross the blood-brain barrier and deposit in brain tissue. This can lead to a condition called acute bilirubin encephalopathy (ABE), with early signs including lethargy, poor feeding, and low muscle tone.
Without treatment, ABE can progress to cause brain damage known as kernicterus. The consequences of kernicterus can include athetoid cerebral palsy, hearing loss, and developmental delays. For this reason, healthcare providers take elevated TcB readings seriously, as monitoring and intervention are effective at preventing these outcomes.
Influences on Newborn Bilirubin Levels
Several factors beyond a newborn’s age can influence their bilirubin levels:
- Prematurity: Infants born before 38 weeks often have underdeveloped liver function, making it harder for them to process bilirubin effectively.
- Feeding Method: “Breastfeeding jaundice” may occur in the first week if a baby is not getting enough milk, while “breast milk jaundice” can appear later due to substances in breast milk that affect bilirubin processing.
- Birth Trauma: Significant bruising or a cephalohematoma (a collection of blood under the scalp) can increase bilirubin levels as there are more red blood cells to break down.
- Blood Type Incompatibility: Issues like ABO or Rh incompatibility between the mother and baby can lead to increased red blood cell destruction (hemolysis), causing a rapid rise in bilirubin.
- Genetic Factors: Glucose-6-phosphate dehydrogenase (G6PD) deficiency is a notable cause of severe hyperbilirubinemia due to increased hemolysis.
- Ethnicity: Newborns of East Asian descent, for instance, have a higher predisposition to elevated bilirubin levels.
- Inadequate Intake: Dehydration or poor caloric intake can slow the excretion process, allowing bilirubin to be reabsorbed in the intestines.
Responding to Concerning TcB Results
When a TcB screening shows a high reading, the standard response is to confirm the result with a total serum bilirubin (TSB) blood test. A TSB test is ordered if a TcB reading approaches or exceeds a specific threshold on the age-specific nomogram, ensuring that treatment decisions are based on the most accurate data.
Follow-up appointments with a pediatrician are an important part of managing bilirubin levels, especially for babies discharged from the hospital early. These appointments allow for continued monitoring to ensure the bilirubin level is decreasing as expected. Parents will receive guidance on feeding practices to ensure the baby is well-hydrated and having regular bowel movements.
If bilirubin levels reach a treatment threshold, the most common intervention is phototherapy. This treatment involves placing the baby under a special blue-spectrum light, which converts bilirubin into a water-soluble form that the body can excrete more easily through urine and stool. In rare and severe cases where phototherapy is not sufficient, an exchange transfusion may be considered to rapidly lower dangerous bilirubin levels.