In a newborn, the level of bilirubin that warrants treatment is not a single number; it climbs with the baby’s age in hours and is lower for more premature infants. This tool reproduces the shape of the NICE treatment-threshold graphs so you can quickly see where a measured total serum bilirubin sits relative to the phototherapy and exchange-transfusion lines.
How it works
For each gestational-age band, NICE defines two lines that rise over the first days of life and then plateau. This tool models each line as a linear rise from a birth value to a plateau ceiling reached around 96 hours, then constant thereafter:
threshold(age) = birth_value + (plateau - birth_value) x min(age, 96) / 96Greater gestational maturity raises both the birth value and the plateau, reflecting a higher tolerance for bilirubin. The exchange line always sits above the phototherapy line for the same gestation.
Clinical background: why bilirubin thresholds depend on age in hours
Bilirubin rises after birth as fetal hemoglobin is broken down and the newborn liver processes the resulting bilirubin load. Physiological jaundice typically peaks between 3 and 5 days of life in term infants, then resolves as hepatic glucuronyl transferase activity matures. In premature infants the peak is both higher and later.
The reason thresholds are expressed as a function of postnatal age in hours rather than days is precision: a bilirubin of 250 µmol/L at 24 hours of life is far more concerning than the same value at 96 hours, because at 24 hours it is rising rapidly from a low base and may cross the treatment line before the next routine measurement. Plotting on the age-specific chart captures this trajectory information that a simple cut-off number cannot.
Why gestational maturity shifts the threshold lower
Premature infants face two compounding risks. First, their livers are less mature and clear bilirubin more slowly, so the same bilirubin level represents a higher degree of processing failure than in a term infant. Second, and more critically, the blood-brain barrier is less intact in premature infants, so bilirubin penetrates more readily into brain tissue. The combination means that a level safe at 40 weeks could cause bilirubin-induced neurological dysfunction (BIND) at 28 weeks. NICE therefore applies lower thresholds as gestational age decreases.
Understanding the two action lines
Phototherapy threshold. Initiating phototherapy at this level prevents the bilirubin from climbing into the exchange-transfusion zone. Phototherapy converts bilirubin in the skin to water-soluble isomers that are excreted without conjugation, bypassing the liver. Intensive phototherapy (higher irradiance, maximum skin surface exposure) can lower bilirubin relatively quickly and is preferred when the level is close to but not yet at the exchange threshold.
Exchange transfusion threshold. At this level the risk of acute bilirubin encephalopathy becomes significant. Exchange transfusion — replacing the infant’s blood with donor blood — rapidly removes circulating bilirubin and the antibodies causing hemolysis if hemolytic disease is present. It is a more invasive procedure with its own risks, performed only when phototherapy is insufficient or the level is already at this threshold.
Interpretation and notes
A measured total serum bilirubin at or above the phototherapy line means phototherapy should begin; reaching the exchange line signals urgent senior review and preparation for exchange transfusion. The values used here approximate the published NICE graphs for common gestation bands and are intended only as a quick orientation. The definitive decision must always be made by plotting the value on the official NICE age-specific treatment threshold chart for the exact gestation, alongside clinical assessment and risk factors — including the presence of risk factors for haemolytic disease, neurotoxicity, or previous jaundice history. This is an educational tool, not a clinical decision system.