Early diagnosis is made when a high phenylalanine level and a low tyrosine level are detected during the screening of a newborn. If phenylketonuria runs in the family and DNA is available from the affected family member, amniocentesis or chorionic villus sampling with DNA analysis can be performed to determine whether the fetus has the disorder.

Newborn screening :

Screening for phenylketonuria by measurement of phenylalanine level on a dried-blood spot specimen, collected by heel stick and adsorbed onto filter paper is recommended for all newborns.

There are two screening methods that are widely used

• Guthrie inhibition assay
  
  
• McCamon-Robins fluorimetric test
  
  

The blood specimen for screening should be obtained at least 12 hours after birth. If done before 12 hours the test needs to be repeated regardless of the result by 2 weeks of age. Premature and sick infants should be tested at or near 7 days of age. Repeat testing is also required if the test results are positive.

False positive in the initial screening may be the result of

• Improperly prepared sample
  
  
• Liver immaturity
  
  
• Protein overload in newborns who are fed cow's milk
  
  All parents should be informed regarding the indication for testing, interpretation of test results and adequate counseling and referral if the test is positive. Dietary treatment should commence as soon as the diagnosis is confirmed
  
  

Carrier status identification :

Carrier status for phenylalanine deficiency can be determined in at- risk family members with a positive family history. Methods used are:

• Plasma phenylalanine concentration and phenylalanine/tyrosine ratio
  
  
• DNA analysis if the disease causing PAH mutation in the family is known
  
  
• Linkage analysis
  
  

TREATMENT

The only treatment for this disease is diet restricted in phenylalanine. Phenylalanine must be restricted starting in the first few weeks of life. Infants with PKU should be breast fed along with special formula which are phenylalanine free. A diet should be planned with adequate calories and amino acids to ensure adequate growth and development.

Certain vegetables, fruits, grains and other low phenylalanine foods should be added later. Regular milk, cheese, eggs, meat fish should never be allowed. Individuals with PKU must remain on a restricted diet throughout life. A restricted diet, started early and well maintained, makes normal development possible and prevents mental retardation.

FOLLOW UP

Individuals with phenylketonuria should be adequately followed so that phenylalanine blood levels are monitored and the importance of dietary restriction emphasized.

Blood levels are monitored weekly during infancy. After that they can be done every 3 months. Plasma amino acid concentrations are determined once or twice a year to ensure adequate tyrosine levels. 

Last created on 23-02-2001
Last updated on 28-04-2007