Transcutaneous bilirubin (TCB) measurement is a common, non-invasive method used in newborns to screen for jaundice. Jaundice, or the yellowing of the skin, results from the buildup of bilirubin, a yellow pigment produced when red blood cells break down. When a newborn’s liver is not yet fully efficient at processing this substance, bilirubin levels in the blood can rise, a condition known as hyperbilirubinemia. TCB screening provides a quick estimate of bilirubin levels, helping providers identify infants who require further testing or treatment.
How Transcutaneous Bilirubin Measurement Works
The transcutaneous bilirubinometer is a handheld device that uses light to measure the concentration of bilirubin deposited in the skin. This technology, called optical spectroscopy, directs specific wavelengths of light into the infant’s skin, typically on the forehead or sternum. The device analyzes the intensity of the light reflected back, correlating it with the yellowness caused by bilirubin content. This process gives an immediate, pain-free reading, avoiding the need for a heel stick blood draw for initial screening.
Transcutaneous bilirubin (TCB) provides an estimate, not a definitive diagnosis, of the total serum bilirubin (TSB) level in the blood. TSB, requiring a blood sample analyzed in a laboratory, remains the gold standard for diagnosing hyperbilirubinemia. TCB measurements are used as a reliable screening tool to determine which infants warrant the invasive, but more accurate, blood test. Using TCB significantly reduces the number of heel sticks for newborns while ensuring concerning bilirubin elevations are detected early.
Interpreting TCB Values by Age and Risk Factors
There is no single “normal” TCB value for a newborn; the reading must be interpreted based on the baby’s exact age in hours and individual risk profile. Bilirubin levels naturally rise in the first few days of life. For example, a TCB reading of 8 milligrams per deciliter (mg/dL) at 12 hours old represents a much higher risk than the same reading at 72 hours old. Providers plot the TCB or TSB value on an hour-specific risk nomogram, such as the Bhutani nomogram, to determine the infant’s risk zone and guide monitoring or intervention.
Biological and historical factors can shift an infant into a higher risk category, lowering the threshold for concern and prompting earlier intervention. Babies born prematurely, even those considered late preterm (35 to 37 weeks gestation), have less mature livers and a higher risk of complications. Major risk factors include jaundice appearing within the first 24 hours of life, blood group incompatibility, and a previous sibling who required phototherapy. Conditions such as cephalhematoma or significant bruising, which increase the breakdown of red blood cells, also elevate the baby’s risk.
Exclusive breastfeeding, particularly if the infant is not feeding effectively or shows excessive weight loss, requires closer monitoring. Genetic conditions like Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency also place an infant into a higher-risk category, even with moderate TCB values. A TCB result that falls into the high-intermediate or high-risk zone on the nomogram is a direct indication to perform a confirmatory total serum bilirubin (TSB) blood test.
When High Bilirubin Requires Intervention
When TCB screening indicates a value approaching or exceeding the high-risk zones, a blood test for TSB is required to confirm the level and inform treatment decisions. TSB values that cross the established phototherapy threshold, determined by the infant’s age in hours and risk factors, necessitate immediate intervention. The primary treatment for elevated bilirubin is phototherapy, which involves exposing the infant’s skin to specific wavelengths of blue-green light. This light changes the bilirubin molecules into forms that can be easily excreted by the body.
If the TSB level is extremely high or rising rapidly despite phototherapy, an exchange transfusion may be considered. This procedure, where a small amount of the baby’s blood is withdrawn and replaced with donor blood, is reserved for severe hyperbilirubinemia. This intervention prevents bilirubin from crossing the blood-brain barrier, which can lead to a severe form of brain damage called kernicterus. Medical staff closely monitor the TSB level during treatment, often every six to twelve hours, to ensure the therapy effectively reduces concentration.
Factors Affecting TCB Accuracy
While TCB measurement is an excellent screening tool, its accuracy can be influenced by several physiological and clinical factors. The presence of melanin, which determines skin tone, can affect how the light is absorbed and reflected, potentially leading to less accurate readings in infants with darker skin. Although TCB devices are designed to account for this variable, some studies suggest a tendency to overestimate the true TSB level in darker-skinned neonates. TCB is considered less reliable in very premature infants due to differences in skin maturity and composition.
The device’s accuracy is significantly compromised once an infant begins phototherapy treatment. The therapeutic light changes the bilirubin in the skin, causing a temporary bleaching effect that leads the TCB meter to underestimate the actual bilirubin level in the blood. Once phototherapy is initiated, TCB measurements are no longer used to monitor treatment effectiveness, and subsequent monitoring must rely on the TSB blood test. The TCB method is best utilized as a screening tool before phototherapy has begun or for monitoring infants with low to moderate risk levels.