What screening should the nurse inform a new mother will be delayed until her newborn is 36 to 48 hours old?

The screening that should be delayed until the newborn is 36 to 48 hours old is the one for phenylketonuria (PKU). This timing is important because PKU screening relies on the presence of sufficient metabolic byproducts in the newborn's blood that typically develop after the infant has begun feeding and metabolizing proteins. Conducting the test too early, before this metabolic process has commenced, can lead to false negatives, meaning that a condition may go undetected when it truly exists.

In contrast, other screenings are usually performed at different times. Screening for congenital heart defects can often be done shortly after birth using pulse oximetry. Jaundice is assessed visually and may be monitored continuously, and hearing loss screening can also be performed shortly after birth, typically before the infant leaves the hospital. Each of these screenings is designed to identify immediate concerns that could affect the newborn's health, while the metabolic conditions identified by PKU screening require a slightly longer period post-delivery to ensure accurate results.