Phototherapy thresholds are determined by hour-specific bilirubin nomograms based on which factors?

• A. Birth order and parental age.
• B. Gestational age and birth weight.
• C. Postnatal age and sex.
• D. Apgar score.

Answer: (B)

Phototherapy thresholds are set using hour-specific bilirubin nomograms that adjust for how mature the baby is and how much body mass they have. That means the nomograms are stratified by gestational age and birth weight. These two factors reflect bilirubin handling: a more mature liver and a larger infant can conjugate and eliminate bilirubin more efficiently, so they can reach higher bilirubin levels before needing therapy. Conversely, preterm or very low birth weight infants have immature bilirubin clearance and are at greater risk, so therapy thresholds occur at lower bilirubin levels.

Birth order and parental age don’t influence bilirubin processing; postnatal age is the axis of time on the nomogram, but the factors used to define the threshold curves themselves are gestational age and birth weight; Apgar score isn’t used to set these thresholds.