The transcutaneous bilirubin (TcB) test is a non-invasive method used for screening bilirubin levels in newborns. It measures the yellowness of a baby’s skin to estimate the amount of bilirubin present. This test offers a quick and painless alternative to traditional blood draws, making it a preferred initial assessment tool. The TcB test provides an immediate indication of bilirubin levels, aiding in the early identification of infants who might require further evaluation for jaundice.
The Science of Bilirubin and Jaundice
Bilirubin is a yellowish substance that forms when red blood cells naturally break down. This substance is typically processed by the liver and then excreted from the body. Newborns, however, are particularly susceptible to elevated bilirubin levels because their bodies have a higher rate of red blood cell destruction compared to adults.
The immature liver of a newborn has a limited capacity to process and excrete bilirubin efficiently. This imbalance leads to its accumulation in the blood, causing the skin and whites of the eyes to appear yellow, a condition known as neonatal jaundice. While mild jaundice is common and often resolves without intervention, very high levels of unconjugated bilirubin can cross the developing blood-brain barrier. Such accumulation can cause severe and irreversible neurological damage, a condition termed kernicterus. Monitoring bilirubin levels is important to prevent these serious complications.
How the Transcutaneous Bilirubin Test Works
The transcutaneous bilirubin test uses a specialized handheld device. A healthcare professional gently presses the device’s probe against the baby’s skin, typically on the forehead or sternum. The device then emits a flash of light into the skin and analyzes the light that is reflected back.
This technology is based on optical spectroscopy, which measures how much yellow light is absorbed by the bilirubin in the skin. The device’s algorithms use this information to estimate the bilirubin concentration. The entire process is painless for the infant and provides a reading on the screen within seconds.
Interpreting the Results
A transcutaneous bilirubin (TcB) reading estimates bilirubin levels, serving as a screening tool, not a definitive diagnosis. The result is plotted on a specialized chart called a nomogram. This chart considers the baby’s age in hours, allowing healthcare providers to assess the risk level for developing significant hyperbilirubinemia.
Nomograms have zones indicating low, intermediate, or high risk. If a baby’s TcB reading falls above a certain threshold for their age, it suggests elevated bilirubin levels and warrants further investigation. In such cases, a confirmatory Total Serum Bilirubin (TSB) test is required for a precise measurement. This approach helps identify infants who may need interventions like phototherapy.
Accuracy and Limitations
The transcutaneous bilirubin (TcB) test correlates well with Total Serum Bilirubin (TSB) levels, particularly at lower and moderate concentrations. This makes TcB an effective screening tool, reducing the need for painful heel stick blood draws and minimizing infection risk.
Despite its benefits, the TcB test has limitations. Factors such as the baby’s gestational age and birth weight can influence readings, making it less reliable in preterm infants or those weighing less than 2500g. Skin pigmentation can also impact results, though modern devices account for varying skin tones. If a baby has already started phototherapy, the TcB reading may be less accurate due to the skin bleaching effect. The TcB test identifies infants who require further evaluation through a blood test.