PKU (Phenylketonuria) on the NCLEX
Newborn screening rules, the diet for life, aspartame warning, and maternal PKU teaching.
PKU is the newborn-screening success story: a heel stick and a diet prevent intellectual disability. The testable details are the screening timing and what stays off the plate.
What it is and how it is caught
Autosomal recessive deficiency of phenylalanine hydroxylase: the infant cannot convert phenylalanine to tyrosine, and phenylalanine accumulates and damages the developing brain. Newborn screening by heel stick, ideally after 24-48 hours of protein feedings - screening too early, before the infant has taken milk, risks a false negative and gets repeated. Untreated signs: irritability, vomiting, musty or mousy odor to urine and sweat, eczema, fair skin and hair, then irreversible intellectual disability. Treated early, development is normal.
The diet, for life
Low-phenylalanine diet started in the first weeks and continued for life. Special phenylalanine-free formula provides protein; small measured amounts of regular milk or breast milk may be allowed to supply the little phenylalanine the body still needs. Avoid high-protein foods: meat, fish, eggs, dairy, nuts, legumes. Fruits, most vegetables, and special low-protein products form the diet. No aspartame - the sweetener contains phenylalanine. Teach label reading early. Blood phenylalanine levels are monitored regularly, and tyrosine is supplied by the formula.
Teaching and maternal PKU
Families need lifelong dietitian support; the diet gets socially hard in school years and adolescence, so plan for birthday parties and cafeterias, not just the kitchen. Maternal PKU: a woman with PKU must be in strict metabolic control before conception and through pregnancy. High maternal phenylalanine causes microcephaly, intellectual disability, and heart defects in the fetus even when the fetus does not have PKU. Genetics: autosomal recessive, so two carrier parents face a 1 in 4 risk with each pregnancy.
Frequently asked
Why repeat a screen drawn at 12 hours of life?
The infant needs 24-48 hours of protein feeding for phenylalanine to rise. An early sample can miss PKU.
Can the child ever stop the diet?
Current teaching is diet for life. Stopping in childhood or adulthood brings cognitive and behavioral decline.
Which drink is contraindicated?
Anything with aspartame, which breaks down to phenylalanine. Regular milk and high-protein shakes are also restricted.
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