PHENYLKETONURIA (PKU)

PKU is caused by mutations in the phenylalanine hydroxylase (PAH) gene and results in excessive levels of phenylalanine which are detrimental to brain development. Birth prevalence is 1:14,000.35 Treatment with a phenylalanine-restricted diet is effective in preventing the long-term consequences of PKU but needs to be commenced early in life.
Babies with PKU typically have DBS phenylalanine levels >200 μmol/L (a typical NBS cut-off is ~150 μmol/L), and an increased phenylalanine/tyrosine ratio. Follow-up testing involves formal measurement of plasma amino acid levels.

Babies with PKU typically have DBS phenylalanine levels >200 μmol/L (a typical NBS cut-off is ~150 μmol/L), and an increased phenylalanine/tyrosine ratio. Follow-up testing involves formal measurement of plasma amino acid levels.

Some cases of classical PKU are responsive to tetrahydrobiopterin,39,40 the co-factor for phenylalanine hydroxylase, and follow-up may also involve assessment of this response. NBS also detects babies with mildly elevated levels of phenylalanine, termed hyperphenylalaninaemia, which may be caused by rarer defects in the biosynthesis or recycling of tetrahydrobiopterin. Assessment of urine pterin levels and response to phenylalanine and tetrahydrobiopterin loads are useful in classifying these babies.
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