Bilirubin is a yellow pigment, a natural byproduct of the body breaking down old red blood cells. While the adult liver efficiently processes this substance for excretion, a newborn’s liver is often functionally immature immediately after birth. This immaturity allows bilirubin to accumulate in the bloodstream, a condition known as hyperbilirubinemia, which manifests as visible jaundice. Because high levels of this pigment pose a health risk, healthcare providers routinely screen infants using a non-invasive tool called a transcutaneous bilirubinometer (TcB). This quick screening method is the standard approach in hospitals to determine if a baby’s bilirubin level requires further attention.
How the Device Measures Bilirubin
The transcutaneous bilirubinometer works using multi-wavelength reflectance spectrometry. This handheld device is gently placed against the baby’s skin, typically on the forehead or sternum, where it emits a pulse of light. Bilirubin is a chromophore, meaning it absorbs light at specific wavelengths, particularly in the blue-green spectrum.
The device measures the light reflected back from the skin and subcutaneous tissue after absorption by bilirubin. Since other chromophores like melanin (pigment) and hemoglobin also exist in the skin, the measurement is complex. To account for these variables, the bilirubinometer uses multiple wavelengths of light. This allows its internal algorithm to mathematically separate the light absorption caused by bilirubin from that caused by other compounds.
The final reading, displayed as a transcutaneous bilirubin index, estimates the concentration of bilirubin within the skin tissue. This non-invasive measurement correlates highly with the actual bilirubin concentration in the blood, making the TcB meter an immediate screening tool. The devices are calibrated to approximate the Total Serum Bilirubin (TSB) level obtained from a blood sample.
Practical Benefits and Measurement Limitations
The primary benefit of TcB screening is its non-invasive nature, eliminating the need for painful heel-stick blood draws for every infant. The TcB reading is instantaneous, providing immediate information that allows clinicians to quickly assess the risk of rising bilirubin levels before discharge. This rapid, painless process significantly reduces the frequency of blood sampling, conserving nursing time and mitigating risks associated with frequent invasive procedures.
TcB screening is not a perfect diagnostic replacement for a blood test; it functions strictly as a screening tool. Its accuracy can be compromised under certain conditions. The device’s reliability is known to decrease as bilirubin levels climb higher. Therefore, TcB values approaching or exceeding 15 mg/dL, or those near the phototherapy treatment threshold, must always be confirmed with a Total Serum Bilirubin (TSB) blood test.
Factors Affecting Accuracy
Skin pigmentation affects the accuracy of the TcB reading. Melanin, the pigment responsible for darker skin tones, absorbs light and can interfere with the device’s spectrophotometric analysis. Although devices attempt to correct for this, TcB measurements may sometimes overestimate the true bilirubin level in neonates with darker skin.
The TcB reading also becomes unreliable if the baby is already undergoing phototherapy treatment. The therapeutic light bleaches the bilirubin near the skin’s surface, leading to a falsely low TcB reading that does not reflect the circulating blood level.
Clinical Follow-Up After Screening
After a TcB reading is taken, the result is immediately plotted onto an hour-specific nomogram, a graph that tracks bilirubin levels against a baby’s age in hours since birth. This allows staff to determine the infant’s risk zone for developing severe hyperbilirubinemia based on their current level and existing risk factors. If the TcB result falls into a high-risk zone or is near the intervention threshold, the next step is to confirm the reading with a blood test.
The Total Serum Bilirubin (TSB) obtained from the blood sample is the gold standard used to make all treatment decisions. This TSB level is then used with the nomogram to decide if phototherapy is necessary. Phototherapy involves placing the baby under specialized blue-green lights, which change the structure of the bilirubin molecule. This process, called photoisomerization, converts the fat-soluble bilirubin into a water-soluble form that the baby can excrete directly through urine and stool.
Once phototherapy is initiated, the TSB level must be monitored closely, often rechecked within 4 to 12 hours, to ensure the level is decreasing effectively. Continued monitoring of bilirubin is crucial because excessively high, untreated levels of the pigment can cross the blood-brain barrier and cause irreversible brain damage known as kernicterus. The entire protocol is designed to identify and treat hyperbilirubinemia in a timely manner, preventing this rare but severe outcome.