Also called the Guthrie screening test, this test is used to screen infants for elevated serum phenylalanine levels, a possible indication of phenylketonuria (PKU). Phenylalanine is a naturally occurring amino acid essential to growth and nitrogen balance; an accumulation of this amino acid may indicate a serious enzyme deficiency. This test detects abnormal phenylalanine levels through the growth rate of Bacillus subtilis, an organism that needs phenylalanine to thrive. To ensure accurate results, the test must be performed after 3 full days (preferably 4 days) of milk or formula feeding.
Procedure And Posttest Care
A negative test result indicates normal phenylalanine levels (< 2 mg/dl) and no appreciable danger of PKU.
At birth, an infant with PKU usually has normal phenylalanine levels, but after milk or formula feeding begins, levels gradually increase because of a deficiency of the liver enzyme that converts phenylalanine to tyrosine. A positive test result suggests the possibility of PKU. A definitive diagnosis requires exact serum phenylalanine measurement and urine testing. A positive test result may also indicate hepatic disease, galactosemia, or delayed development of certain enzyme systems.
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