Master's in Genetic Counseling (MGC)

Phenylketonuria (PKU)

Phenylketonuria (PKU) is an autosomal recessive disorder of phenylalanine metabolism. Phenylalanine, an essential amino acid present in dietary protein, accumulates in the body due to the absence of phenylalanine hydroxylase, one of the enzymes responsible for its conversion to tyrosine.

Clinical Features

Infants with PKU may be asymptomatic for many months. However, with unrestricted intake of phenylalanine, untreated infants will begin to manifest developmental delay, microcephaly, progressive mental retardation, motor retardation, seizures, hyperactivity, eczema, and decreased growth rate. Most untreated PKU patients will have an IQ of less than 50. (OMIM)

Laboratory Tests

Screening for phenylketonuria is performed by determining the level of phenylalanine in the dried filter paper blood spot. Elevated levels of phenylalanine may be suggestive of PKU. A genetic/metabolic consultation is recommended for diagnostic testing and management of the disorder.

Treatment

With early detection and treatment, the symptoms of PKU may be prevented. Treatment should be started as soon as possible after birth once elevated phenylalanine levels have been confirmed and should be continued throughout life. Dietary restriction of phenylalanine, except for the minimal amounts needed for normal growth and development, is the mainstay of therapy. This can be achieved by the consumption of a special formula, along with a diet low in phenylalanine but adequate in other nutrients. A trained nutritionist should be consulted to manage the diet. Periodic monitoring of blood phenylalanine levels is important for successful management of this disorder. For specific treatment guidelines, please consult with a genetic/metabolic specialist.

Screening Practice Considerations

At least 24 hours of feeding with breast milk or a protein containing formula is necessary before the infant is tested. This will allow the levels of phenylalanine to rise in affected infants to a value that will allow for detection of those individuals. False negative results may occur in infants who have been taken off protein feedings or infants on antibiotics. As with any screening test, false positive results may also occur. Prompt confirmatory testing is required even if there is suspicion of a false positive result.

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