Guthrie Test for Phenylketonuria

Norm of Guthrie Test for Phenylketonuria

Negative or <3 mg/dL.


Usage of Guthrie Test for Phenylketonuria

Neonatal screening for phenylketonuria (PKU).


Description of Guthrie Test for Phenylketonuria

The Guthrie test is a bacterial inhibition assay in which elevated levels of phenylalanine cause growth of the bacterium Bacillus subtilis around the blood sample. If growth occurs around the bacterium, the diagnosis of PKU is suggested; further blood testing is needed to confirm the diagnosis.
Phenylketonuria is an autosomal recessive inherited disorder with a frequency of 1 in 10,000 live births. This condition is caused by a deficiency of the enzyme phenylalanine hydroxylase, which leads to an accumulation of phenylalanine and deficiency of tyrosine in the blood. Resultant symptoms include neurologic conditions such as mental retardation, microcephaly, seizures, and hyperactivity, as well as eczema, growth retardation, and a musty odor. If treatment with a restricted phenylalanine diet is begun early, these symptoms can be minimized and even prevented.
PKU testing is performed in the newborn period on a metabolic screening filter paper. Although states vary in the panel of conditions they test, all states test for PKU.


Professional Considerations of Guthrie Test for Phenylketonuria

Consent form NOT required.

  1. Obtain supplies: 2.0–2.5 lancet, alcohol wipe, gauze pads, gloves, and blood collection form (filter paper) with completed information.



  1. Warm heel for 3–5 minutes with warm, wet cloth or heel warmer with foot lowered below the heart to increase blood flow.
  2. Clean heel with alcohol wipe and dry with sterile gauze.
  3. Puncture the lateral third of the plantar surface of the heel. Wipe first drop of blood away with gauze pad and, when another large drop of blood collects, lightly touch the filter paper, making sure to fill the designated circle completely. Fill in all circles on the filter paper in the same manner.


Postprocedure Care

  1. Place pressure on the heel for 1–2 minutes, and then cover with sterile gauze or a bandage.
  2. Allow the form to dry on a flat surface for a minimum of 4 hours and mail to the laboratory within 24 hours.


Client and Family Teaching

  1. Inform parents that the sample is sent to the state laboratory and that they will be informed of an abnormal result within 2 weeks.
  2. When positive results are found, call the family immediately, repeat the test, and institute a low-phenylalanine diet with consultations to a nutritionist and geneticist.


Factors That Affect Results

  1. Method of sample collection: Samples must be carefully collected. The heel should not be squeezed excessively, blood spots on the filter paper should not be touched, the sample should be air-dried, and the sample should not be applied with a capillary tube.
  2. Factors that produce false-negative results:
  3. a. Protein intake: Child must have had 24 hours of protein feedings (either breast milk or formula) in order for the test to be valid.
  4. b. Antibiotic use: Test should be repeated if child had been taking antibiotics when the sample was collected.
  5. Factors that produce false-positive results: goat milk intake, total parenteral nutrition, severe illness, layering of blood on test paper.


Other Data

  1. Test should be performed, if possible, before a transfusion.
  2. Each circle on the filter paper needs to be completely filled.
  3. If the baby is <48 hours old at the time of collection, a repeat sample should be obtained within the first week of life.
  4. See also Neogram Amino Acids and Acylcarnitines Test.