Detection

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Detection

If detected at infancy, the destructive symptoms of PKU, such as mental retardation can be avoided. In America today, newborn infants are tested for PKU using two types of screening: the Guthrie inhibition assay and the McCamon-Robins flurometric tests. Both methods use blood spotted on filter-paper in order to measure phenylalanine levels. The blood used for these tests must be taken at least 12 hours after birth, and the American Academy of Pediatrics recommends that any test perfomed within 24 hours of birth should be repeated after 2 weeks.

The Guthrie Inhibition Assay was created by Robert Guthrie, a microbiologist from the University of Buffalo. In the test, a microbe (Bacillus subtills) is placed in a pan along with an inhibitor which prevents the microbe's growth. Blood from the baby's heal, is placed on filter-paper, and then placed in the pan and is allowed to stand overnight. If the blood contains enough phenylalanine, a sign of PKU, then the inhibitor is neutralized and a growth zone appears in the pan. If no growth is evident then the test is negative. The Guthrie Inhibition assay is a simple and inexpensive means for screening for PKU.


	The phenylalanine level in the blood of newborn infants is normally between 0.5 and 1.0mg/dL. Some states use 2.0mg/dL as a cut-off value for PKU, however many use 4.0mg/dL. However, in some cases the phenylalanine level could be as high as 6.0mg/dL in an infant, without the baby have PKU. This is due to a late maturation of enzyme needed to break down proteins in babies. Therefore, followup tests are recommended for all babies having a level higher than 3.0mg/dL. Intial PKU screening shows positive results in one percent of infants, but in 90% of the cases, there is a false-positive result. However, false-negative results are highly rare.

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