Phenylketonuria

Swati Kolpuru
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Phenylketonuria - Diagnosis
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


Phenylketonuria Phenylketonuria 02/28/2001
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