Transcutaneous Bilirubin Test: How It Works & What Results Mean

A transcutaneous bilirubin (TcB) test measures bilirubin levels in a newborn’s skin by shining light through it, no needle required. A small handheld device is pressed gently against the baby’s forehead or chest, and within seconds it displays a number that tells clinicians whether jaundice might be serious enough to warrant a blood draw or treatment. The test does not replace a blood test for making treatment decisions, but it has become a frontline screening tool in hospitals, clinics, and increasingly in home settings around the world because it is fast, painless, and cheap.

What the Device Actually Does

Bilirubin is a yellow pigment that builds up in a newborn’s blood and skin when the liver has not yet ramped up to full capacity. Almost every baby develops some visible yellowing in the first few days. The transcutaneous bilirubinometer works by sending specific wavelengths of light into the skin and measuring how that light is absorbed or reflected back. Bilirubin absorbs light at particular wavelengths, so the device can estimate how much bilirubin is sitting in the skin tissue based on the pattern of light that returns to its sensor.¹ The reading appears on a small screen, usually in milligrams per deciliter (mg/dL), and the whole process takes a few seconds with no discomfort to the baby.

The alternative is a total serum bilirubin (TSB) test, which requires a heel stick or venous blood draw, a trip to the lab, and a wait for results. That blood test remains the gold standard for deciding whether to start phototherapy or other treatment. But TcB screening can tell the clinical team whether the blood draw is even necessary, sparing a lot of babies an uncomfortable and costly procedure.

Where on the Body It Is Measured

The two most common measurement sites are the forehead and the sternum (the breastbone area). Both correlate well with actual blood bilirubin levels, but studies consistently find the sternum has a slight edge. In one study comparing both sites, sternal readings were about 1 mg/dL higher on average than forehead readings and showed a tighter agreement with blood test values.² A separate multi-site comparison also found the sternum and forehead had the strongest correlations, with the sternum performing marginally better.³

The interscapular area (between the shoulder blades) has also shown promise, particularly in early preterm infants, where one study found it had the best correlation of the three sites tested.⁴ In practice, the forehead is the easiest to access, especially when a baby is swaddled, which is why it remains widely used despite being slightly less precise. When used as an outpatient screening tool, research comparing the two main sites found the sternum had a higher ability to predict which babies would need hospital admission for treatment.⁵

How Close the Numbers Get to a Blood Test

No screening tool is perfect, and the TcB device is no exception. Across studies, the readings tend to run slightly higher or lower than the actual serum level. One large analysis found the average difference between TcB and TSB was about 0.8 mg/dL, with a correlation of 0.78.⁶ That is close enough to be useful as a screen but not close enough to base treatment decisions on without confirming with a blood draw when the number is in a concerning range.

A consistent pattern across devices and studies is that TcB tends to underestimate true bilirubin levels when those levels are high. One study found that transcutaneous measurements underestimated serum bilirubin regardless of the body site, and this underestimation was especially pronounced when the actual blood level exceeded about 15 mg/dL.⁷ That matters because 15 mg/dL is the range where clinicians start considering phototherapy in term newborns. A reading of 13 from the device when the true level is actually 16 could delay treatment if no one follows up with a blood test.

The flip side is that sensitivity is generally high, meaning the test is good at catching babies who truly do have elevated bilirubin. Both forehead and sternal measurements showed sensitivity above 94% in one study, though specificity was low, in the range of 19 to 31%.² In plain terms, the test catches nearly all babies with high bilirubin but also flags a fair number who turn out to be fine on blood testing. For a screening tool, that tradeoff is considered acceptable: you would rather have some unnecessary blood draws than miss a case of severe jaundice.

What the Numbers Mean in Practice

A single TcB number on its own tells you less than you might think. What matters is where that number falls relative to the baby’s age in hours. Bilirubin levels naturally rise in the first few days of life and then start to fall, so a reading of 10 mg/dL at 24 hours of age is more worrying than the same reading at 72 hours. Clinicians plot TcB values on an hour-specific nomogram, a chart that divides readings into risk zones, to decide the next step.

Research on these nomograms shows they work well for predicting which babies will go on to develop significant hyperbilirubinemia. In one study, a reading in the high-risk zone at 48 hours of life had 90% sensitivity for predicting later significant jaundice, while a reading in the low-risk zone at the same time point had a less than 1% probability of developing a problem.⁸ Another study confirmed that the risk-zone approach based on TcB gives a satisfactory prediction of which newborns will develop hyperbilirubinemia, with readings in the high-risk zone at 73 to 96 hours being especially predictive.⁹

Several decision rules have been tested for when a TcB reading should trigger a blood draw. These include comparing the reading to the 75th percentile on a standard nomogram, checking whether it reaches 70% of the phototherapy threshold for that baby’s age, or seeing whether it falls within 3 mg/dL of the treatment level.¹⁰ In all cases, the goal is the same: use the quick, painless reading to decide whether the baby needs a blood draw right now or can safely wait.

How Skin Color Affects the Reading

Because the device works by shining light through skin, anything that changes how skin absorbs or reflects light can throw off the measurement. Skin pigmentation is the most studied confounder, and the findings are more complex than a simple “darker skin gives wrong readings” summary.

A systematic in vitro study using a skin-mimicking model found that higher melanin levels led to larger underestimations of bilirubin by the device. At a true bilirubin level of about 250 µmol/L (roughly 14.6 mg/dL), the underestimation ranged from about 1.5 to nearly 7.7 mg/dL depending on the degree of pigmentation.¹¹ At lower bilirubin levels, the effect of pigmentation was within the device’s stated accuracy, but the gap widened as bilirubin climbed. Above about 10 mg/dL, the measurement deviation in darker skin colors exceeded the safety margin that clinicians typically apply in practice.¹²

Interestingly, a clinical study grouping neonates by skin color found a different direction of error: TcB tended to overestimate blood bilirubin more in darker-skinned neonates, though overall correlation remained strong across all color groups.¹³ The discrepancy between in vitro models and real clinical data likely reflects the complexity of living skin, which changes with blood flow, hydration, and subcutaneous fat thickness. The practical takeaway is that TcB is useful as a screening step for babies of all skin colors, but the threshold for confirming with a blood draw should be lower when there is any doubt about the reading’s accuracy.

What Happens During Phototherapy

Phototherapy, the blue-light treatment used for jaundice, poses a specific challenge for transcutaneous monitoring. The light breaks down bilirubin in the skin, which means exposed skin will show a falsely low TcB reading even if the blood level has not dropped as much. If the device is placed on skin that has been under the lights, the number will look reassuringly low and could mask ongoing hyperbilirubinemia.

The workaround that has gained traction is the “patch method”: a small opaque patch is placed on a spot of skin before phototherapy begins, shielding it from the light. The TcB reading is then taken on that protected patch of skin. Studies show this approach preserves the correlation between TcB and blood bilirubin throughout the course of phototherapy. One study found that on patch-covered skin, the good correlation between TcB and TSB continued at 24, 48, and 72 hours of therapy, while uncovered skin showed a significant loss of reliability at all those time points.¹⁴ The sternum patch site again showed the best agreement with blood levels compared to the forehead and back during phototherapy.¹⁵

Even with a patch, most guidelines still recommend confirming with a blood test before making any decision to stop or intensify phototherapy. The patch method extends the usefulness of TcB during treatment but does not eliminate the need for periodic blood monitoring in actively jaundiced babies.

Using TcB in Preterm Babies

Premature infants are at higher risk of bilirubin-related brain injury because their blood-brain barrier is less mature and their livers are even slower to clear bilirubin. They also tend to need more frequent monitoring, which means more heel sticks, more pain, and more blood loss from a tiny body. TcB screening in this group is particularly appealing because it can reduce that burden.

Studies in preterm infants, including those born as early as 23 weeks, have found that TcB correlates reasonably well with blood bilirubin across gestational age and birth weight subgroups.¹⁶ A review of TcB use in preterm infants between 23 and 34 weeks concluded it can be a reliable noninvasive screening tool that decreases painful procedures and blood loss.¹⁷

The limits of agreement are wider in premature babies than in full-term ones, though, and the precision gets worse as bilirubin levels rise. At lower bilirubin levels, TcB may underestimate the true value, potentially delaying needed treatment. At higher levels, it may overestimate, triggering unnecessary blood draws and parental worry.¹⁸ For this reason, the consensus is clear: TcB in preterm infants belongs in a screening role only, and treatment decisions should always be based on a blood test.

Fewer Needle Sticks, Lower Costs

The most immediate benefit for families is fewer blood draws. In the inpatient newborn nursery, introducing TcB screening has been shown to reduce the need for blood draws by more than a third.¹⁹ For parents watching their newborn get a heel stick, that reduction matters. In one outpatient quality-improvement initiative, the same pattern held: transcutaneous screening reduced blood draws while still catching the babies who truly needed follow-up.

The cost savings are real. A Canadian analysis estimated that a single TcB screen costs roughly $3.50 to $3.75, depending on the setting, compared to $15.80 for a blood test in the hospital and $50 to $65 for one done in a community setting, where specimens need to be transported to a lab. Over six months, one program saved nearly $20,000 in hospital-based screening and about $6,400 in community screening by using TcB as the first step.²⁰

Catching Cases That Eyes Alone Would Miss

Clinicians have traditionally assessed jaundice by looking at a baby’s skin under good lighting and pressing gently on the nose or chest to see the underlying color. This visual assessment is better than nothing but unreliable, especially in babies with darker skin. A 2025 study of universal TcB screening in a midwifery-led home care setting found that 28 newborns who needed treatment were identified by TcB screening but missed by visual inspection alone. Two of those babies had bilirubin levels above the exchange transfusion threshold, meaning they were dangerously jaundiced and could have been missed entirely without the device.²¹ The tradeoff was more heel sticks overall because some TcB screens triggered blood draws that turned out to be unnecessary, but the net benefit of catching those severe cases was considered to clearly outweigh the cost of the extra needles.

Home Visits and Community Settings

One of the more exciting developments is taking TcB out of the hospital. In many parts of the world, babies go home within hours of birth or are born at home, and follow-up for jaundice depends on a visiting nurse or community health worker noticing that the baby looks yellow. A feasibility study in Bangladesh trained community health workers to use transcutaneous bilirubinometers during routine home visits. Workers were comfortable using and maintaining the device, and families readily accepted the screening because it was painless and gave an instant result at home.²²

Home-based TcB monitoring as part of a structured nursing program has also been linked to reduced maternal anxiety. In one study, mothers in a home monitoring intervention group had notably lower anxiety scores on postpartum day 7 compared to a standard-care group, and parental satisfaction was significantly higher.²³ Jaundice is one of the most common reasons for early readmission and a source of considerable worry for new parents. Having a concrete number instead of “the baby looks a little yellow to me” appears to reduce that anxiety substantially, even when the number triggers a follow-up visit.

Device Upkeep and Infection Control

Transcutaneous bilirubinometers are durable devices, but they do require routine calibration and cleaning. A survey of Japanese neonatal units found that while most facilities disinfected their devices after each use, concerns about measurement accuracy and infection control persisted.²⁴ Most manufacturers provide a calibration tile or reference standard that should be checked at regular intervals. If the device is dropped, the optical window is scratched, or calibration values drift, the readings become unreliable. For a tool that is meant to prevent unnecessary blood draws, a poorly maintained device defeats the purpose because staff will stop trusting its readings and order blood tests anyway.

In practice, the devices are shared among many babies across a shift, making disinfection between patients essential. Most units use alcohol-based wipes on the sensor tip, and disposable caps or covers are available for some models. Facilities using TcB in community or home-visit settings face additional logistical challenges around keeping the device clean, charged, and calibrated when it is being transported between homes in varying conditions.

When TcB Should Not Be the Final Word

For all its convenience, TcB screening has clear guardrails. The reading should always be followed by a blood test if it falls near or above the treatment threshold for the baby’s age. It should not be used as the sole basis for starting or stopping phototherapy. And certain clinical situations make the transcutaneous reading especially unreliable:

After phototherapy on exposed skin: light-treated skin shows falsely low readings, as discussed with the patch method above.
Very high bilirubin levels: the device’s accuracy degrades above roughly 15 mg/dL, precisely the range where getting the number right matters most.
Extremely premature or sick neonates: wider variability and thinner skin can skew readings, and treatment thresholds are lower in this group, leaving less room for error.
Edema or bruising at the measurement site: fluid accumulation or blood under the skin changes how light is absorbed and reflected.

None of these limitations make the test useless in those situations. They just mean the result should be interpreted with more caution, and the threshold for ordering a confirmatory blood test should be lower. The test works best as a gatekeeper: a low reading in a well-appearing, full-term baby can confidently rule out a problem, while a borderline or high reading in any baby should always prompt a blood draw.