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D-Xylose Absorption
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Evoked Potential Studies
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Orbital Computed Tomography
Phenylalanine Screening
Plasma Ammonia
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Phenylalanine Screening

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.


  • To screen infants for possible PKU

Patient Preparation

  • Explain to the parents that the test is a routine screening measure for possible PKU and is required in many states.
  • Tell them that a small amount of blood will be drawn from the infant's heel but that collecting the sample only takes a few minutes.

Procedure And Posttest Care

  • Perform a heelstick, and collect three drops of blood-one in each circle­on the filter paper.
  • Reassure the parents of a child who may have PKU that although this disease is a common cause of congenital mental deficiency, early detection and continuous treatment with a low­phenylalanine diet can prevent permanent mental retardation.
  • Note the infant's name and birth date and the date of the first milk or formula feeding on the laboratory slip, and send the sample to the laboratory immediately.

Reference values

A negative test result indicates normal phenylalanine levels (< 2 mg/dl) and no appreciable danger of PKU.

Abnormal Findings

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.

Interfering Factors

  • Performing the test before the infant has received at least 3 full days of milk or formula feeding (false-negative)

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