Neonatal hyperbilirubinemia, commonly known as jaundice, is a frequent condition affecting many newborns in the first week of life. This yellow discoloration of the skin and eyes occurs due to an excessive buildup of bilirubin, a yellowish pigment produced when the body naturally breaks down old red blood cells. While the mother’s placenta removes this pigment in the womb, the newborn’s immature liver must take over this process after birth. Although usually harmless and temporary, excessive bilirubin levels can pose a risk, making timely and accurate screening necessary. Transcutaneous Bilirubin (TcB) measurement offers a non-invasive way to screen for elevated bilirubin levels, providing a rapid estimate without requiring a blood draw.
How Transcutaneous Bilirubin Measurement Works
Transcutaneous bilirubinometry uses a handheld device called a bilirubinometer for quick, non-invasive initial screening. The device works based on optical spectroscopy by directing a flash of light into the infant’s skin, typically on the forehead or sternum. It then measures the intensity of specific wavelengths of light reflected back. Since bilirubin absorbs light at certain wavelengths, the device uses this absorption to calculate the pigment concentration in the skin tissue. The entire process takes less than a minute and reduces the need for painful heel pricks.
The TcB reading is an estimate of extravascular bilirubin present in the subcutaneous tissue. This differs from the Total Serum Bilirubin (TSB) measurement, which is the gold standard for diagnosis and treatment decisions. TSB requires a traditional blood draw and measures the bilirubin circulating in the blood (intravascular bilirubin). While TcB is effective as a screening tool, any reading indicating a concerning level requires confirmation with the definitive TSB blood test.
Understanding Bilirubin Levels Based on Infant Age
There is no single static number that defines the “normal range” for transcutaneous bilirubin in a newborn. The interpretation of a TcB result is entirely dynamic, depending on the infant’s age measured precisely in hours of life. A bilirubin level considered safe for a 72-hour-old infant might be cause for serious concern in a baby who is only 24 hours old, as bilirubin levels naturally rise in the first days after birth.
Healthcare providers rely on specialized graphs called nomograms to plot the TcB or TSB result against the infant’s age in hours. The nomogram divides the results into risk zones (low, intermediate, and high) for developing severe hyperbilirubinemia. For example, a Total Serum Bilirubin (TSB) level of 8 milligrams per deciliter (mg/dL) might be high-risk if the infant is 25 hours old, but low-intermediate risk at 47 hours of age.
The risk level is also influenced by other factors, including the infant’s gestational age, blood group incompatibility (like Rh or ABO), and significant bruising from delivery. These factors can lower the threshold for when a bilirubin level is concerning. Because of these variables, every TcB measurement must be professionally assessed to ensure the most appropriate management plan.
Next Steps When Bilirubin Levels Are Elevated
When a transcutaneous bilirubin reading falls into an intermediate or high-risk zone, the first step is to obtain a definitive Total Serum Bilirubin (TSB) measurement. This TSB blood test confirms the exact concentration of bilirubin in the blood and serves as the basis for all treatment decisions. If the TSB confirms a significantly elevated level, the standard treatment is phototherapy.
Phototherapy involves placing the infant under specialized blue-green lights. The light energy penetrates the skin and is absorbed by the bilirubin molecules, triggering a structural change called photoisomerization. This process converts the fat-soluble bilirubin into water-soluble forms, such as Z-lumirubin. These modified products can then be excreted directly in the infant’s urine and stool without needing to be processed by the immature liver.
Parents should watch for signs of severe hyperbilirubinemia, which can indicate a progression toward acute bilirubin encephalopathy. These signs include extreme lethargy, difficulty waking the baby, poor feeding or sucking, a high-pitched cry, or the infant arching their neck and back (opisthotonos). Rapid recognition and treatment of these symptoms are necessary to prevent potential long-term neurological complications.
Limitations of Transcutaneous Bilirubin Screening
Although TcB is an excellent screening tool, its accuracy is compromised in specific clinical situations, meaning it cannot replace the TSB test entirely. A significant limitation occurs once an infant has begun phototherapy treatment. The therapeutic light bleaches the bilirubin in the skin, causing the TcB device to provide a falsely low reading in the areas exposed to the light. If a TcB reading is needed during treatment, it must be taken on a completely covered area of skin.
The accuracy of TcB also diminishes as the bilirubin level climbs higher. When TSB levels exceed approximately 15 mg/dL, TcB measurements tend to underestimate the actual concentration in the blood. For this reason, TcB is used primarily to rule out significant hyperbilirubinemia. Any reading approaching the treatment threshold requires immediate confirmation with TSB, as differences in skin pigmentation can also affect the correlation between the skin reading and the true serum level.